The sun slowly rose from behind a mountain range overgrown with dense forest. Under its rays, dew drops glistened on the leaves of trees and shrubs, and hundreds of insects and other small animals left their shelters where they waited out the night. The sky gradually turned a pure lilac-pink color. There is not a cloud on it – everything promises a perfect summer day.
The birds began to sing and flutter shortly before dawn, and now their voices are heard in the forest in full force. Some of the birds are looking for food among the branches, while others move to the lake shore to drink. There they find themselves as if in the snowy storm. Birds are surrounded by thousands of insects. Fragile and delicate mayflies, obeying unknown but clear to them signals, chose this day for the most important mission in their short life. Modestly looking nymphs crawl out on coastal rocks and reed stalks by the hundreds, and molt, turning into graceful winged creatures. They quickly spread their wings and fly up, hastening to the mating flight. Swarms of these insects celebrate their rebirth, and immediately die in the thousands, barely having time to lay eggs. Female mayflies complete the life cycle of their species as quickly as their parents, and their parents’ parents, and millions of generations of their ancestors. They move under the water and lay eggs among the strands of green algae, which overgrow densely the rocks in shallow water. After that, they die, and their bodies serve as food for many fishes. The water literally boils in the places of the mating flight of mayflies: dozens of fishes scurry under the surface, greedily devouring insects. But there are still too many mayflies to eat them all, and some of the insects will leave the offspring unhindered.
In the age of man, such a performance became a rarity: water pollution led to the fact that mayflies began to disappear from human-populated areas. But with the decline of the human epoch, the biosphere for millions of years immobilized, resolved and destroyed the toxines with which people poisoned it. The unique ability of the natural environment to self-purify has allowed such gentle creatures as mayflies to succeed again in the arena of life. The favorite habitat of mayflies is clean cool water, just like in the lake they have chosen. After a brief moment of triumph, millions of lifeless bodies of these insects carpet the water. The waves cast banks of dead mayflies ashore, and for several days the birds simply do not manage to peck them all. Even forest animals come ashore and devour the nutritious free treats. The lake could easily feed all the larvae that underwent metamorphosis in these few days. It could feed as many more of these insects, and even more. The fact is that the flight of these insects takes place on Lake Baikal – the greatest freshwater body of the planet. Over the millions of years that have passed since the human disappearance, this lake has greatly expanded and has become larger than some seas.
Usually lakes exist for a very short time. In most cases, they are remnants of a riverbed, or traces of a retreating glacier. Such lakes fill with sediments, which rivers bring into them, in a few millennia. But Baikal is a long-lived lake. This lake has a completely different origin: the bed of Lake Baikal is a rift and is continuously expanding. Since the extinction of humankind, rifts have significantly changed the face of the Earth. In the 25 million years that have passed since the extinction of mankind, rifts have split Africa and split off from it a huge microcontinent – the Zinj Land. The Red Sea rift had closed due to the movement of Africa towards Europe, and this circumstance turned Arabia into a tectonically active area. Baikal also continues to increase: in Neocene, the lake is much larger than it was in the memory of mankind. In Neocene, the lake became almost 500 kilometers wider – in the human era and in subsequent times, its shores diverged by 2 centimeters per year. It resulted in the formation of a colossal freshwater reservoir in the center of Asia – even more majestic than Baikal was in the memory of mankind. Perhaps millions of years later, a new ocean will form in this place. But it will not even be in the Neocene, but tens of millions of years after the end of this epoch. The changes that the lake had undergone over millions of years have affected the climate of the surrounding area: it is warmer and more humid in the vicinity of the lake. The taiga retreated higher into the mountains, and the shores of the lake were overgrown with broadleaf forests similar to the forests of the Far East.
In the age of man, the unique fauna of Lake Baikal suffered from human actions, but in Neocene, nature returned its waters to amazing purity and transparency. In Neocene, Baikal still belongs to oligotrophic lakes – its biological productivity is relatively low, and the existence of many animal species in it is rather due to the gigantic size of the lake itself. The low sedimentation rate is the key to the “eternal youth” of the lake at its considerable age. In Neocene, Baikal is absolutely not going to become shallow and disappear; it continues to hold firmly the priority as the oldest freshwater body of the planet.
The life of the lake has changed in great degree under the human influence. In the historical era, a number of attempts were made to introduce various freshwater fish species into the lake. Most of them were unsuccessful, but in Neocene part of the ichthyofauna of the lake is represented by descendants of introduced species. Some of the descendants of the alien species introduced to the lake died out during the ice age, when the water temperature in the lake decreased under the influence of the glacier. During the period of maximum glaciation at the boundary of Holocene and Neocene, the edge of the glacier approached almost to the shores of the lake, and every summer a large amount of ice melt water entered the lake. Even in the warm and humid Neocene, the fauna of the lake bears the imprint of a long-past ice age – there are only few descendants of heat-loving southern species introduced by man.
The shores of Lake Baikal are rocky, and dense thickets of higher aquatic vegetation are found only in the mouths of rivers flowing into the lake. The shallow waters of the lake make up a very small part of its area, so the basis of the biological productivity of the lake is plankton.
Many species of birds inhabit the shores of Lake Baikal. Some of them are typical aquatic inhabitants, others are casual guests only occasionally coming to the lake from the surrounding forests. At the coast of Lake Baikal, birds search for food – small animals inhabiting shallow water. Therefore, lake residents should be very careful to avoid pointed beaks.
In the bright sun, coastal rocks are covered with a slimy green coating – lower algae expand on them. But in some places, damage is noticeable on the algae film – someone small diligently scraped them off the rocks, leaving a long narrow path. While birds fly over the stones, the creature that left these traces does not appear. But when the birds leave this part of the coast, this animal continues its activity.
A pointed muzzle with small shining eyes and thin wattles sticking down and to the sides appears from under a flat stone. It freezes for some minutes, assessing the situation outside, and then begins to move forward. A small short-bodied fish with a wide short tail and slimy skin swims out from under the stone. It is one of the inhabitants of the coast, the Baikal lipscraper. Hiding in the shade, the fish swims up to a large stone, on which surface a thick layer of green algae has grown. The gray color with irregular green stripes makes the Baikal lipscraper invisible against the background of rocks and algae, and this circumstance can give the fish a few moments to escape from a predator.
The surf sways the short green hairs and the body of the fish swimming over the algae and choosing a place to feed. Having landed on a rock, the Baikal lipscraper sticks to its surface with its mouth, and begins to scrape algae. At the time of feeding, the fish stops moving its operculi, so it has to interrupt feeding from time to time in order to “catch its breath”. After detaching from the stone, the fish ventilates its gills with rapid movements for some seconds, and then attaches itself to the stone again. Behind it a slightly intermittent path of the cleaned surface of the stone is left. Algae represent not too nutritious food, and some of it will not be digested anyway, so the lipscraper should feed most of the day. The fish creeps on the stone almost near the surface of the water, and it can be dangerous – here the lipscraper can fall prey not only for herons or grebes specialized for feeding on fish, but also for any random predator. Therefore, the shadow of flying bird forces the lipscraper to stop feeding and hide under a stone, even though it was just a goose harmless for it. When the danger was over, the lipscraper cautiously leaned out of its shelter. After making sure that nothing threatens it, the fish starts feeding again.
It is not only flying birds that make the lipscraper interrupt feeding. From time to time, the fish stops and clicks loudly several times in a row. This sound is clearly audible underwater and is a formidable warning to competitors. Each individual knows perfectly well where the border between the individual territories lies, and notices when relatives invade its possessions. A specific nutrition source that needs to be restored requires a change in behavior – so, the Baikal lipscraper turned into a militant individualist. It does not intend to rush into battle when seeing a relative – usually a fish that has got into another one’s territory is ready to hide at the first sign of aggression from the territory owner. After another series of clicks, which the adult fish emitted, its small relative rushed away from under the stone. Such individuals usually live on the edge of the territory of the dominant fish, patiently waiting for the rightful owner to grow old or be eaten by predators. But not all young fish manage to live up to such a moment – individuals having no permanent territory die from predators more often than “stay-at-home” ones.
The flapping of wings is heard in the air, and a small heron with red cross-striped plumage lands on a stone. It lightly touched the surface of the water with its foot while doing this and a splash of water caused the Baikal lipscraper to stop feeding and rush to the burrow. The heron only managed to notice how a fish scuttled under the stone on which the bird landed. This bird is a skilled and patient fish hunter. It is careful, but the fish is even more careful. The lipscraper hears the scratching of the bird’s claws on a stone, and hides, having attached by its mouth to the roof of the burrow. Rocks and water conduct sounds well, and the fish feels the presence of a predator without even seeing it.
The Baikal lipscraper does not live in a hole alone – a special kind of symbiotic scud also lives in its shelter. The lipscraper does not hunt amphipods – its mouth is not adapted to eat such a large solid prey. But it tolerates the presence of this crustacean, although it would immediately drive scud of any other species out of the burrow. The symbiotic scud is colored quite brightly and catchily: it is decorated with a white longitudinal stripe along the red-brown background of the body. The crustacean cautiously approaches the fish, rises on walking legs and emits a few short clicks with the front pair of legs. This signal is clear to the lipscraper. The fish carefully leaves the roof of the burrow, turns back and sinks to the bottom. Scud crawls up to the fish and starts cleaning its body. With the front pair of legs, on which brushes of fine hairs grow, the scud gently cleanses the skin of the fish, rolling the slime and debris stuck to it into lumps. The crustacean immediately eats these lumps without interrupting the cleaning of the fish’s body. With light movements of the front pair of legs, the scud cleans the gills of the lipscraper. The crustacean carefully extracts a small parasitic worm from under the operculum of the fish, and immediately eats it. The lipscraper does not touch the much-needed symbiote, and only during the care of the clutch, the symbiotic scud loses the favor of the lipscraper – the crustacean can easily eat the eggs and fry of this fish. When the fry swim away, the lipscraper becomes favorable to the crustacean again.
The heron flew off to look for easier prey, and the lipscraper soon swam out of the hole to continue feeding. It has to feed almost continuously to get enough nutrients from the algae. While the fish feeds on algae somewhere on its territory, the scud remains in the lipscraper’s hole as a nominal host. In addition to parasites and wast from the fish’s body, it eats excrement containing a lot of undigested organic matter. Scud also helps the fish to keep cleanliness in the burrow. Clinging to the stone with its front thoracal legs, the crustacean actively waves its swimming and abdominal legs – in this way it sweeps garbage out of the lipscraper’s burrow. Scud jealously guards the burrow from relatives, scaring them with loud clicks. Some of its relatives live near the lipscraper’s burrow, but hide among the stones. Perhaps when the owner of the burrow and the host fish dies, one of them will take its place. However, the probability of this event is very little – the life expectancy of “homeless” scuds is noticeably less than that of those who have found a reliable permanent shelter.
The mutual benefit that symbiont animals receive from each other allows them to survive more successfully. Therefore, in addition to symbiotic scuds and lipscrapers, there are many other living beings in the lake that have embarked on a path of symbiosis. Among these animals there are the unique Baikal sponges. In the human era, sponges in Lake Baikal were represented by the endemic Lubomirskiidae family, three endemic genera and six species. In Neocene, the diversity of sponges increased due to the expansion of the lake and the increase in the area of shallow waters. The sponges of Lake Baikal entered symbiosis with unicellular brown and green algae, forming a peculiar analogue of sea corals – these creatures are slow-growing water-filtering organisms that form porous colonies. In the surf zone, colonies of sponges have a denser structure and form cortical growths on rocks, but at a depth of 2 to 5-6 meters, their colonies resemble coral or organ pipes. In the upper layer of the body of sponges, microscopic algae form clusters covering the corneous skeletons of these animals. From time to time, the amoeboid cells of the sponge’s body swallow algae, and in this way the sponge receives additional nutrition.
The similarity of sponge colonies to coral reefs is not limited to their symbiosis with algae. The faveolate colonies of sponges and their internal cavities are inhabited by various species of animals – scuds and nymphs of insects – dragonflies and mayflies. Fishes also find shelter here, so the sponge thickets of Lake Baikal are a place where life is in full swing. Baikal water is clean, and mayflies are found in the lake in huge numbers. Every year at the beginning of summer the massive summer mayfly swarming takes place. A mayfly spends several years at the bottom of the lake in a nymphal stage. Among the Baikal mayflies there are species whose nymphs feed on microscopic algae; there are predators and eaters of dead animals also.
The abundance of small animals attracts predators to the thickets of sponges. Dragonfly nymphs roam the sponge colonies. Flattened or having elongated bodies, they carefully examine the surface of the colony with the help of a long labium, which is called “mask” in their case. When the dragonfly nymph crawls on the surface of the sponge colony, the mayfly nymphs shoot out from their shelters and quickly swim into the neighboring ones. In addition to them, small sponge-dwelling scuds live in colonies. These small crustaceans no longer rely on speed, but on the reliability of the shelter. They gnaw long tunnels through the sponges, and when the dragonfly nymph crawls past, they hide in the depths of the sponge. A dragonfly nymph can easily catch a mayfly nymph by grabbing it with a “mask” when it leans out of its hiding place. On the contrary, the scud hidden deep in its tunnel is inaccessible to the dragonfly nymph. However, there are predators at the sponge “reefs” of Baikal that are much more dangerous for various small animals than a dragonfly nymph.
A large flatworm of yellowish-brown color slowly crawls along the surface of the sponge. Its body is decorated with black cross stripes, but no fish shows interest in it. Even dragonfly nymphs, sensing the smell of this animal, try to crawl away from it. It is quite understandable – the slime covering the body of this worm is poisonous. This inhabitant of Lake Baikal, called the long-throated planaria, crawls through colonies of sponges in search of any animals that it can catch and swallow. Scuds, nymphs of mayflies and small dragonflies, fish fry – they all may fall its prey. The long-throated planaria is perfectly equipped for hunting small animals. When crawling around the colony, the worm sometimes seems to freeze in place for a long time. However, this is only an illusion. Having stopped, the worm fumbles with its extensible pharynx in the cavities of the sponge and devours on the spot small animals that settle in it. Stretching a lot, the pharynx of this worm can independently penetrate deep into the cavity of the sponge and determine the location of prey.
Pairs of thin antennae protrude from the surface of the sponge – this is a colony of sponge-dwelling scuds. These small crustaceans have drilled holes in the sponge’s body and constantly live in them now, becoming recluses in their own homes. They feed only on those dribs and drabs of organic matter that get into their burrows with the water flow. They also eat algae growing on the surface of the sponge. Sponge-dwelling scuds are very delicate and fragile creatures with thin body covers. Their worm-like bodies are well adapted for living in narrow long burrows. With the help of short legs, crustaceans crawl along their own passages and along the inner cavities of the sponge, easily moving both head and abdomen forward.
The approach of the planaria is first felt by individuals settled at the edge of the colony. Sensing the smell of the worm, the scuds one by one hide in their burrows, uttering an alarm signal – a quiet click. These animals never leave the sponge, so they cannot escape from the worm by flight, and can only hope for the depth of their tunnels. In some seconds, the entire colony of sponge-dwelling scuds hides in the depths of the sponge. At the same time, the long-throated planaria is slowly creeping up on their colony. For some minutes, the worm analyzes the odors coming from the burrows, and then turns out the pharynx and pushes it into one of the burrows. The pharynx of the planaria, like an independent organism, crawls along the scud’s tunnel. It carefully touches the walls, trying not to miss large holes in which prey can hide. Chemoreceptors covering the edges of the pharynx are more important to the worm than the eyes – they determine the location of prey. Sensing that the enemy has already intruded the colony, the scuds hide deeper in their burrows. But the burrows of some individuals are not deep enough, or do not have emergency exits through which they can escape from the all-pervading planaria’s pharynx. One scud has already crawled down to the bottom of the hole. It has an emergency exit in its burrow, but it is much higher, and... The way to it is already cut off by the throat of a long-throated planaria, which rummages in the dwelling of a crustacean. A tragic denouement is inevitable. Some minutes pass, and the pharynx of the worm already touches the antennae of the crustacean. Having sensed the smell of prey, it grabs the prey, and pulls the scud into the mouth. Feeling that the prey is caught, the worm begins to pull carefully its pharynx out of the scud’s burrow. Having dealt successfully with such difficult task, the planaria crawls away from the scud colony.
After the predator’s visit, one burrow turned empty, but it is not for long: perhaps any young sponge-dwelling scud will soon settle there, or one of the old-timers of the colony will crawl into this hole. After the planaria hunt, the scud colony is covered with viscous poisonous slime. Although the planaria itself has crawled away a long time ago, sponge-dwelling scuds do not immediately stick out of shelters: the slime retains the smell of a predator, and scuds trust their sense of smell more. When the slime of the planaria gradually dissolved in the water and the frightening smell disappeared, the sponge-dwelling scuds began their activities. They leaned out of their tunnels and froze waiting for the edible particles that the water current brings. Scuds undoubtedly eat part of the food from the sponge, which serves as their home. But instead, scuds eat small invertebrates that settle in the body of the sponge. They also clean the sponge from dirt – the crustaceans roll the dirt brought by the water into lumps, and the flow of water coming out of the sponge takes the garbage out.
The water in the central part of the lake is amazingly clean and transparent. But it has a downside: fewer living creatures live in the water of the central part of Lake Baikal, because they cannot find enough substances necessary for development here. Near the shores, the upper layers of water have a greenish tint – rivers carry a large amount of mineral and organic substances into the water. First of all, they are used for the growth by microscopic green algae and diatoms. They form the basis of many food chains of Lake Baikal and only they ensure the development of zooplankton in the upper layers of water. Microscopic algae cells represent a very specific type of feed. Algae grow and renew quickly, but only animals with the necessary adaptations for their feeding mode can use them for food.
In calm sunny weather, life flourishes in the upper layers of the water. Rotifers and other microscopic creatures can be found in every drop of water. Next to them, there are hard-shelled cells of diatoms, encased in a shell of silica, and bizarre-shaped cells of unicellular green algae, creatures that stand on the edge between animals and plants. Strong jaws and tenacious claws are useless for catching these creatures. Therefore, they are consumed by a creature that has its own plankton trap. It is another type of Baikal scud – a filtering barmash. This crustacean has a deep and short body, and the edges of its shell form a groove on the ventral side, along which water constantly moves, driven by the movement of the abdominal legs. The crustacean passes water through the thoracal legs folded together and densely covered with hairs, which have lost the function of walking. When enough food particles accumulate on the hairs, the filtering barmash stretches its leg to the mandibles and scrapes off the accumulated food. Due to an almost inexhaustible source of food, the filtering barmash forms large congestions in the upper layers of water. With the help of movements of abdominal legs, this crustacean not only filters food, but also swims an abdomen first. If it does not need to swim anywhere, the crustacean hangs about in the water column, spreading its legs and antennae to the sides.
The filtering barmash makes up a significant part of the lake plankton, and its congestions attract many species of pelagic fish of the lake. But it is not the only inhabitant of the water column of Lake Baikal. The neighbors of the scuds are small thin worms, pelagic oligochaetes of various species. They do not filter small food, but grab larger food particles one by one. Wriggling like a snake, worms quickly swim next to crustaceans. Small pelagic species of oligochaetes can cause some inconvenience to the filtering barmash – they occasionally steal food from it.
One filtering barmash diligently passes water through the legs, and a greenish coating gradually appears on the hairs growing on the inside of its legs. There are not only green algae, but also diatoms, and various rotifers in its catch. They are pressed by the flow of water to the legs of the crustacean and cannot get anywhere. Pelagic oligochaete, wriggling its pinkish body, swims closer to a crustacean floating in the water. This worm is attracted by the smell of food gathered by the scud. After swimming near the edge of the crustacean’s body, the worm attaches itself to its shell, and its front end begins to fumble among the mass of algae cells accumulated on the hairs of the crustacean’s legs. In fact, the worm robs the crustacean, but it is not a specially planned action at all – the animal was attracted by the smell of food and found its source. The worm is especially interested in rotifers, and scud has gathered a rich catch: small rotifers pass through its filter apparatus, and large ones are caught. That is what the worm gathers. But such a delicacy is rarely given for free: when the scud stretches its leg to the maxillipeds, the worm gets stuck between the hairs on the legs of the scud. And the maxilliped accidentally tears the worm’s body in half. This is not as harmful for the worm as it seems at first glance: a relatively small piece of its body turned out to be in the mouth of the crustacean, and a much larger part of it simply swims away. A torn worm regenerates the lost organs in some days. This ability even helps pelagic oligochaetes to reproduce: once every few days, the body of a large worm simply divides itself into two parts, each of which turns to a full-bodied organism. This feature helps worms to survive, living next to fish fry and other carnivorous plankton inhabitants.
Lake plankton, as well as ocean plankton, has its own predators and prey. Moreover, one of the most bizarre planktonic predators, dangerous for the filtering barmash, belongs to the number of worms. It is one of the many species of Baikal planarians, although at first glance this live being looks more like some kind of unearthly creature. The flat transparent body of rounded outlines resembles a small jellyfish, but is not so mobile – it only floats in the water column. A long outgrowth hangs from the underside of the body – it is the former pharynx, the main weapon of planarians, which has lost its original function in this species. In addition, at the edges of the animal’s body there are moving outgrowths, crowned with thin movable tentacles. The most unexpected thing about its anatomy is that each outgrowth has its own normally functioning mouth opening to the worm’s digestive system. This strange creature deservedly bears the name multimouthed polypostoma (“polyp mouth”). Unlike the marine pelagic flatworms known in Holocene epoch, this creature is very passive. Polypostoma swims only at a very early age, when its structure is similar to that typical for planarians. The grown-up animal becomes a true plankton inhabitant: it just floats in the water column. The worm does not pursue prey. It stretches a lot of pharynxes to the sides, and waits for potential prey to touch it. This is followed by an instant capture of the prey by the tentacles surrounding its mouths, and it is transferred to the branched digestive cavity of the worm. Due to its hunting apparatus, the polypostoma can snatch several animals at the same time. Its prey items are most often small oligochaetes and young filtering barmash scuds, as well as fry of various fish.
Scuds occasionally eat fish fry, but usually the opposite happens – the fish eats them. In the warm season, when the upper layers of the water are warming up, schools of small fish swim near the surface of the lake. The famous Baikal omul has become extinct a long time ago, and in Neocene, small cyprinid fishes make a basis of the biomass of pelagic fish of Lake Baikal. They are omnivorous, but pelagic scuds represent the basis of their diet. Schools of cyprinid fish do not dive to a great depth: most of their prey is found near the water surface. The filtering barmash and other planktonic animals form large congestions, and it is not difficult for fishes to feed. They just swim through a congestion of crustaceans, and swallow them one by one. But scuds feel the presence of enemies and try to escape. Crustaceans fold their antennae under the shell, covering their thoracal legs with them and swim, pushing off with their hind thoracal legs and helping themselves with their abdominal legs. But all the same, fish are faster than scuds, and those crustaceans that managed to escape from one fish can easily get in the mouth of its relative moving at a distance. The population of the filtering barmash is saved from extinction by two circumstances: the high rate of reproduction of the crustaceans, as well as the presence of natural enemies in fish.
A predator is watching the cyprinid fish hunting. It is not hiding – there is nothing that could serve as a shelter far from the shores. However, its presence does not disturb the feeding fishes. A round eye with a golden iris and a round pupil turns slightly, watching the small fishes hunting scuds. The predator estimates the possibility of an attack, and it seems that the hunt may end successfully. The fishes are not alarmed at all, and all their attention is focused on the scuds trying to escape. Fishes are watched by one of the main pelagic predators of the lake – a large pelagopike. This descendant of the northern pike settled in the central part of the lake and became a peculiar analogue of the barracuda. The pelagopike’s body is about two meters long, very narrow and elongated, and the fins are pointed and sickle-shaped. This fish is not adapted to a long exhausting chase, but it is an excellent specialist in short and deadly accurate rushes. Sea barracudas can gather in schools, but their Baikal counterpart is strictly solitary predator at any age. The pelagopike is not visible to its prey from afar – its bluish-gray “marble” coloration merges so well with the background of the water column that the fish can attack its prey as if out of nowhere. Watching the fish, the pelagopike remains motionless. It helps during the hunting – water fluctuations from a swimming predator will not alarm the prey. Pelagopike’s whole body is tense in wait for the rush, but it avoids sudden movements. Although the fish is not hidden by anything from the view of possible prey, it knows how to approach the prey without causing alarm. Although the pelagopike’s body is still stretched out motionlessly, the predator is slowly moving towards the school of small fish. The pectoral fin on the side of the body facing away from the prey moves quickly and pushes the fish’s body forward. The distance between the predator and the prey shortens, and soon the distance to the prey shortens to only some meters. Pelagopike suddenly “comes to life” and attacks. With a flick of its tail, it accelerates quickly and cuts in the school of feeding prey. The school instantly disperses and gathers already behind the pelagopike’s body. To turn around quickly is a problem for the pelagopike, and the second attack follows formally, rather than for the hunting: fishes see the predator; their attention is focused on its movements, so during the second attack the fishes have time to swim away in all sides.
This time the pelagopike was unlucky: not a single fish was caught by the predator. Prey fishes were saved by the reaction rate and exceptional transparency of the water. Baikal water in the Holocene epoch had a transparency of up to 30 meters, and in Neocene it completely restored its pristine purity. Therefore, it is difficult for the pelagopike to get close to the prey unnoticed – in the upper layer of water, especially in good sunlight, it is easy to notice the pike before it approaches the distance of an exact rush. However, the predator does not starve: pelagopike has many ways of hunting in store. The things impossible to do during the daytime may be easily done in dusk or... using the hiding. It is very difficult to find shelter away from the coast, but this way of hunting is much easier and more productive. After the snow melts on the mountain peaks, the raging rivers carry tree trunks into the lake. Before drowning, such trunks float around the lake for a long time, attracting various inhabitants of the upper layers of the water. A predator can easily hide near such a trunk, and the prey seems to be looking for the predator intentionally – small fish willingly swim up to a floating tree trunk and hide in its roots. One such trunk slowly drifts across the surface of the lake. The wood is already soaked with water, and only some branches and a large root stick out above the surface. Pelagopike swims up to the trunk and hides in its shade. It is more difficult to see it so, but the predator itself sees small fish frolicking near the trunk in the sunlight. Small fishes search for shelter and food among the branches and roots, but their hopes are not always justified. This time the attack of the scaly predator is swift and successful, and only some prey’s scales swirling in the water indicate that one more little tragedy, very common to nature, has taken place. Pelagopike is a large predator, dangerous for fish. In its habitats, no one fish can escape from its pointed teeth. However, it is not the largest predator of the lake.
As if loud sighs and snorts are heard in the air above the lake. Splashes mix with the measured noise of the waves, and fountains of spray and water vapor shoot up into the air. A school of unique inhabitants of Lake Baikal swims to the surface – these are Baikal hard-beaked dolphins, the largest and smartest inhabitants of the lake. These animals are the descendants of beluga whales that got into the lake during the last glaciation. Millions of years ago, beluga whales successfully settled in Lake Baikal, and in Neocene their descendants became the top predators of the lake.
A school of Baikal hard-beaked dolphins includes several females of about the same age, the dominant male, which is noticeably larger than females, and the juveniles. The snow-white coloration that distinguished their ancestors among the many species of cetaceans of the Holocene epoch is not so important for life in the lake, where hard-beaked dolphins have no large enemies. But the gray-blue coloration that these animals have in Neocene is more useful because it hides the presence of these cetaceans from their prey, fish. The male is slightly lighter than the females, and his head is almost white. He also has several white spots on his stomach. Swimming at the head of the herd, he occasionally rotates around his axis, displaying these spots to the females. This behavior is not a game, but a way of maintaining of hierarchical relationships, which is important for a conflict-free existence within the school.
Strong friendly ties bind the members of the school, although hostile relations between individuals are also possible. But the power and authority of the head of the clan quickly stop conflicts and strengthen the integrity of the clan. In winter, this clan was a part of a large school and wintered in the central part of the lake, which freezes later and is covered with thinner ice. With the onset of spring, when the ice melted, the clan separated from the large school without any losses. Moreover, it was joined by another adult female from the clan that broke up in the winter after the death of the dominant male.
Baikal hard-beaked dolphins are not as fast as the sea dolphins of the Holocene epoch. They have rounded pectoral fins and large heads with thick melons. Like beluga whales, their ancestors, these dolphins have almost no beak. But in males, and occasionally even in females, the front teeth are very large and massive. They form a kind of battering ram, with which the adult male breaks the ice and bites the edges of the air-hole, supporting its existance in frosty weather. Also, males strike each other with these teeth – the skin of an adult male bears scars, traces of such fights.
These cetaceans are very smart and quick-witted; they are able to obtain various kinds of food. In summer, in calm weather, these dolphins look for food near the shore, eat snails and bivalves, and catch benthic fishes. However, they spend most of the year away from the shore and hunt pelagic fish of Lake Baikal. The behavior of Baikal hard-beaked dolphins is mostly acquired and very flexible. Different individuals forage more or less successfully, and, if necessary, they learn from each other. The chool is a coin box of collective experience passed down from generation to generation. In young individuals, learning is especially successful. Recently, three calves were born in the clan, and while the adults are feeding and resting, they perceive the world in games. Young animals differ from adults in darker coloration – they are almost black. A “nurse” female swims near them, ready to help or protect them if necessary.
A tree floating on the surface of the water is a rare and interesting find for young dolphins. All three calves swim up to the tree and begin to explore it. They enjoy scratching their backs against it, swimming right under the tree trunk. Their attention is attracted by branches sticking out in the water. The calves bite the branches with their still toothless jaws, and try to break them. The movement of branches and the bustle of dolphin calves frighten several fishes hiding in the branches of a floating tree. One calf took chase after them, but the fishes are faster, and it soon returns. Meanwhile, another dolphin calf tore off a strip of bark, and a new game began. Young animals try to snatch this piece of bark from each other’s mouths, chase the owner of this object, and within some minutes the bark passes from mouth to mouth for several times. Finally, it is torn to pieces and left. However, there are still lots of interesting things left near the floating tree, and the young dolphins continue exploring it. One of them swims along the trunk and finds something more interesting than bark or branches. An adult pelagopike hid behind a tree trunk and clung to the bark. It is longer than any of the dolphin calves, but weighs much less than any of them. Therefore, when young dolphins scare it, pelagopike does not attack them, but does not rush to escape. It only swims away from them for some meters, and freezes in the water. Fish brain is too small and primitive to understand that now it has become the object of the young dolphins’ game. They quickly catch up with the fish, and “cat and mouse” game begins. Pelagopike is fast and could easily swim away from dolphins. But dolphins, even young ones, are more resilient. They try to catch the pelagopike by the tail, and if they succeed, they pull the fish by the tail fin. The pelagopike gets bored of such games very soon and swims some meters away from the dolphin calves that become frolicsome, and then just dives into the depths. The dolphin calves at first try to chase it, but then turn back one by one. They do not want to move away from the school, and the “nurse” accompanying them swam forward, and pushes now one of the baby dolphins to the surface with her round head. The dolphin calves obviously don’t want to leave such a toy, but the “nurse” insistently pushes them to the surface of the water. Soon the adults return: their voices sound in the water of the lake. Baikal hard-beaked dolphins use a diverse set of sounds for communication, and baby dolphins quickly recognize the voices of their mothers. When all clan members are together, the dolphins swim away.
Pelagopike got off lightly after the games of the dolphin young: the tip of the upper lobe of its caudal fin is torn off and the dorsal fin is slightly bitten. If the game had continued, the dolphins could have “played” it to death. However, it is a small loss – the bitten fins will grow back soon. The voices of dolphins are getting silent – their school has swum away. The tree trunk is still swaying on the surface of the water, and the pelagopike rushes to it again to arrange an ambush.
Pelagopikes are gradually passing to a completely pelagic lifestyle. Although they spawn in early spring in the shallow waters of the lake, their eggs, carried by the current, successfully develop in the water column. Pelagopike fry grows in plankton, and adult fishes turn into fast-moving and merciless predators of the central part of the lake. Young fishes of this species are more timid and cautious: they have more enemies. A young pelagopike less than half a meter long swims near the surface of the water, catching planktonic scuds. They are quite large enough for the fish, although it is increasingly difficult for it to catch such small prey. But this is easily possible when adding some patience – the fish simply freezes at a shallow depth, and waits until a suitable scud appears right in front of its jaws. Then it is enough to open mouth only, and the crustacean is caught. Also, this fish does not disdain to eat its own smaller relatives – this is how the fishes themselves regulate the number of the species and actually prevent overpopulation of the lake. Several individuals of pelagopike less than three centimeters long swim near the surface and feed on scuds. They do not yet have the color characteristic of adult fish – the bodies of such small fishes are transparent, and a dark marble pattern of dots just began to appear on them. When the young pelagopike caught its tiny relative, the others only rushed to the sides during its attack. From the first seconds of its independent life, pelagopike is a loner relying only upon its own force. It depends only on the fish whether it will be well-fed and whether it will escape from enemies, which are too numerous in the young individuals.
The young pelagopike swallowed the captured fry, and was completely engaged in this activity. It didn’t pay attention to the fact that a partly rotted tree was floating some meters away from it. Therefore, it was a complete surprise to the fish that a wave pushed it sideways. And after that, the teeth of a larger relative closed on its body, instantly interrupting the life of a young pelagopike, which turned from a predator into prey in a matter of seconds. A large adult fish with a bitten tip of the caudal fin slightly opened its jaws, snatched its prey by the head, and swallowed it with a little effort. Now it can stop hunting for about a week, while digesting its prey. With a majestic swing of its tail, the adult pelagopike slid into the depths. Neither relatives nor hard-beaked dolphins will annoy it here. It is not afraid of their calves, but an adult dolphin could easily crush its head with one bite and tear its body to pieces before eating it. Therefore, the fish rushes to the depth, where life flows much calmer.
Baikal is noticeably different from other lakes not only in Neocene, but also in Holocene. In the lakes of the tropical zone, the lower layers of water are lifeless: due to the insufficient mixing of water, they are poisoned with hydrogen sulfide. The same situation had taken place in the Black Sea of the Holocene epoch. Baikal is located in an area dominated by seasonal climate, and this circumstance saved the lake from slow degradation. Fresh water has the highest density at a temperature of +4°C. After cooling on the surface, it sinks to the bottom and water comes from the depths in its place. Due to it, the depth of the lake does not turn into a stagnant hydrogen sulfide abyss. The water in Lake Baikal is also mixed due to vertical migrations of fishes and other inhabitants of the lake. During the Holocene epoch, all the depths of Lake Baikal were inhabited by a variety of fish and invertebrates.
Night is a time of changing actors on the stage of life. Daytime inhabitants hide in shelters or form congestions and passively drift in the water column. On the contrary, the nocturnal inhabitants of the lake at this time arrange enchanting performances.
The pelagopike, who had fallen into a state of slight rigor caused by satiety, did not immediately feel how the water around it as if came to life. Suddenly, something soft and flexible touched fish’s side, and then the pelagopike’s brain recorded the movement of numerous living creatures around it. Creatures that might seem like the souls of dead fishes emerge from the dark depths. They are pale like phantoms; their movements are sluggish and lack the tone characteristic for most fishes living in the water column. These creatures swim, having spread out their fragile fins with a delicate transparent membrane between the fin rays. Their elongated heads and flabby bodies are covered with thin slimy skin. Tiny eyes stare blankly at the world around them – vision is completely useless where these ghosts of the fish kingdom live. These inhabitants of the depths are Baikal false oilfishes, the most numerous species of deepwater fish of the lake. They make such a journey every night – it is a vital necessity. At the surface of the lake, false oilfishes eat planktonic animals, because they have nothing to eat in the depths.
Fish with fragile bodies slowly swim upwards. The musculature of false oilfishes is not suitable for prolonged activity, so the fishes get tired quickly. After making a few tail movements, the false oilfish spreads its fins and freezes for some seconds, drifting in the water column. Nightly ascent from the depth is a very dangerous and long procedure. However, such a journey is a vital necessity.
The eyes of the false oilfishes would not be able to see the prey even in sunlight, but this circumstance does not prevent the fishes from satisfying their appetite fully every night. False oilfishes have a very sensitive sense of smell, and a dense network of lateral line channels is formed on the head of such fish. Due to these features of anatomy, the fish easily distinguishes the movement of small invertebrates and fish fry in the water. This is more than enough for hunting.
In the dark, the false oilfishes get into the congestion of filtering barmash, a planktonic scud. At night, the crustaceans descend to a depth of about 10 meters – this is a great journey for such tiny creatures. In the dark, filtering barmash is inactive – it filters microalgae with less intensity, and simply spreads its limbs and antennae, floating in the water column. Bumping into each other, the crustaceans make a sharp leap to the side, and continue floating in the water. Fry of various fish species swim next to the scuds, sluggishly moving their fins. Congestions of small animals represent actually a set table for nocturnal predators.
The fragile and pale false oilfish seems too delicate to be a predator. However, the long jaws and thin pointed teeth show that it is capable of killing. When planktonic inhabitants touch the skin of fish with their antennae or fins, false oilfish transforms. It folds its pectoral fins and begins hunting. When the scud touches the head of the false oilfish with the tip of its leg, the fish reacts immediately: it opens its wide mouth, and the ill-fated crustacean appears sucked into it by the flow of water. Then the fish closes its jaws and releases water through its teeth. They are hooked and very hard, so a scud trapped in the mouth of a fish will not get out. The jawbones of the false oilfish are very thin and flexible, and are connected with a stretchable transparent membrane. Therefore, when the fish gently moves its jaws, returning them in initial position carefully, it is seen how the caught scud is rushing in its mouth, trying to get out. But the fish swallows it, and continues the search of the next prey.
Some false oilfishes swim with their mouths slightly open, and when planktonic animals touch their jaws, they abruptly close their mouths. This method of hunting requires less energy, but is less productive – sometimes prey escapes, finding way between hooked teeth.
Fragile fishes scurry in the upper layers of the water like ghosts. Their numbers in Lake Baikal are very high, and one school can spread on the surface for several hundred square meters. And yet the false oilfishes are creatures somewhat alien to the upper layers of the water. They came from the safe depths into a world where predators reign, and not all of them are destined to return to the saving darkness of the lake. Not all pelagopikes managed to hunt successfully during the day or at dusk, and they use the opportunity to eat well while the false oilfishes stay on the surface. One of the nocturnal false oilfish hunters is an adult pelagopike a little more than one meter long. It failed to catch enough prey during the day, but now it is literally surrounded by easy, affordable and satisfying prey. Sensitive vision helps pelagopike, although at night it receives most of the information using the lateral line. In the weak light of the night luminaries, the bodies of the false oilfishes are barely noticeable, but the waves spreading in the water from their movement make the picture extremely clear for the perception of the predator. Pelagopike makes a short sharp rush and catches up with one of the false oilfishes. The fish did not even fight back when the sharp teeth of the pelagopike pierced it, and instantly died when the predator clenched its jaws. After swallowing the prey, the pelagopike rushed to the next prey, then overtook another false oilfish, and then one more... The fragile inhabitants of the depths are unable to defend themselves from the predator. Their salvation lies only in the fact that the false oilfishes spend most of the time at depths, where pelagopikes do not get in. Otherwise, they simply would have been exterminated, or they would not have evolved into such fishes as they are.
At night, the starry sky is not only reflected in the waters of the lake. The water itself as if turns into a miniature lookalike of the starry sky. The depths of the lake flash with millions of small yellowish lights that slowly move in the water: at the scene of life other inhabitants of Lake Baikal appeared – tiny, but numerous fireworms. During the day they hide in shallow water areas, curling up there into snarls, and at night they rise in swarms to the surface of the water and eat organic particles. Fireworms are especially numerous near the river mouths, where the water contains more organic particles.
Millions of glowing worms swim up to the surface of the lake. Their congestions near the shores and in shallow water areas form luminous spots on the surface of the night lake. Half-asleep fishes, swimming through these clusters, cause flashes of light, and behind them a dark trail of worms extinguished from fright stretches. When the troublemaker swims away, the worms glow with renewed vigor.
Not far from the surface of the water, a thick cluster of worms formed a large glowing ball. Fireworms have found an abundant food source. They are eating a dead false oilfish emerged to the surface. During a night hunt, a certain pelagopike severely wounded this fish, but it escaped from its mouth and soon died from wounds. Pelagopikes do not chase wounded prey, and now the body of the false oilfish has become food for fireworms. Thousands of these miniature wriggling creatures swim around the body of the fish torn by sharp teeth. Some of the worms penetrate into the wounds on the body of the fish and pinch off pieces of its musculature with their mouths, and other individuals pick up rags of tissues that float in the water. The biomass of fireworms is very large, and the reproduction rate is high. Therefore, fireworms easily maintain the purity of water during their “night shift”, despite the fact that some of them will inevitably be eaten. When the worms swim away, only a soft cartilaginous skeleton covered with rests of skin will remain from the body of the dead false oilfish.
Fishes sleep while swimming slowly in the water column. Some species change their body color at night – they turn pale, or, on the contrary, become almost black. Fishes can afford a really deep sleep. However, there are animals in the lake, for which water is an alien and hostile environment, although it is their habitat. These are hard-beaked dolphins – secondarily aquatic animals. Having lost their connection with the land, their ancestors remained connected with the air medium. Therefore, cetaceans, having mastered life in the water, have lost the ability to sleep entirely. Their brain sleeps in parts – at any given time, only a part of the brain is resting, but other parts of the brain control the behavior of the animal. Therefore, cetaceans do not lose control of themselves and their surroundings during the sleep.
A family group of dolphins swims in Lake Baikal water at night. They know from experience that nocturnal inhabitants of the lake are much easier to catch than agile fishes active in the daytime. Their sonars show the presence of a cluster of small living creatures ahead – it is a school of false oilfishes who have risen to the surface of the lake. Even half asleep, dolphins enjoy the opportunity to eat. Baikal hard-beaked dolphins are not very fond of false oilfish – this fish is not as tasty as other fishes. The meat of the false oilfish is loose and soaked in fat, not too pleasant to the dolphin’s taste. However, these fishes are easy to catch – they do not resist the attacks of predators.
A group of dolphins penetrates into the school of false oilfish. Although the dolphin’s brain does not work fully at night, the areas of the cortex that remain active are quite enough to control the animal’s hunting. Dolphin sonars send ultrasonic signals into the water in whole series. Each animal “sees” the fish surrounding it, and begins hunting. Sensing the approach of large predators, the false oilfishes make feeble attempts to escape – they rush to the sides and freeze in the water column, hoping that the predator will swim by. Predators from among the fish, like pelagopike, might not have noticed the false oilfish, floating motionlessly in the dark in the water column. Fishes receive a significant part of information through the organs of the lateral line, and an object that does not cause water movement is most often uninteresting to them. On the contrary, Baikal hard-beaked dolphins “probe” water with ultrasound, and the false oilfish, floating motionlessly in the water column, represents an easy prey. The dolphin’s strong jaws crush the fragile fish, and the animal swallows it whole without slowing down. False oilfishes rush from side to side in a panic – they smell blood and a special “fear substance” that is released from the damaged skin of their relatives. Some fishes swim away into the depths – this is the best defense tactic. Dolphins stay close to the surface, and from time to time the splashing and the noise of exhalation and inhalation of these animals can be heard in the air. They have simply surrounded a small part of the school of false oilfishes, and methodically devour the fish. However, no matter how voracious they are, they will never eat all the oilfishes. These fishes are numerous in the lake, and the population of the Baikal hard-beaked dolphin does not exceed about five thousand individuals – the lake is simply unable to feed a large number of these animals.
In the morning, the false oilfishes leave the surface layers of water, and swim down to the depth where sunlight does not penetrate. Their daily migrations are on a colossal scale, and represent the most important source of organic matter entering the depths of the lake. False oilfishes carry in their stomachs prey caught near the surface – fireworms and scuds, as well as fry of other fish and their own young. Some of this prey will turn into droppings, set on the lake bottom and be eaten by the most omnivorous inhabitants of the depths, and some will be digested in someone else’s stomach – with a lucky hunter.
In the morning, the hard-beaked dolphins wake up – their brains begin to work in full force. These residents of Lake Baikal do not have a high swimming speed, so they do not want to miss out on a plentiful and easily accessible breakfast of false oilfish. With the first rays of the sun, these fishes begin to descend into the depths, and by the time the sun rises over the mountain ranges surrounding Lake Baikal, they already reach the depths of about fifty meters. Young hard-beaked dolphins do not yet know how to dive to a great depth, so they stay on the surface. There are no predators in the lake that can kill them, so all the adults of the group dive for feeding, and nobody stays to look after the baby dolphins. When adult dolphins return from feeding, they will easily find their offspring using echolocation. And the baby dolphins, if they have moved away, will find their parents by their voices, which are clearly audible in the water.
Dolphins dive for the false oilfishes swimming away to the depth and easily overtake a fish school. In the gloom of the depths, animals orient themselves by echolocation. The family heritage of dolphins replaces their vision here, and the “sound image” of prey in the gloom means to them much more than visual one. In addition, sound is an effective weapon. One of the dolphins recognized a cluster of oilfishes, and gave a sound signal to its relatives. Thanks to the advanced social behavior, dolphins have developed a fairly rich sound language with which animals send information to each other. Hearing its signal, relatives from its family group also direct to a fish school. Cetaceans attack false oilfishes, “firing” very powerful ultrasonic rays. The melon, which gives the head of Baikal hard-beaked dolphin a characteristic rounded shape, serves not so much as a battering ram for breaking ice in winter, as a peculiar sound “lens”. It focuses the scattered sound signal into a powerful ray, which serves as an excellent hunting weapon. Within a few minutes, the depths of the lake are resounded with the “shots” of dolphins. Hunting gives results quickly – within a few minutes each dolphin managed to stun and swallow about two dozen fish. Stunned by the ultrasonic ray, the false oilfishes shudder their bodies convulsively and slowly move their fins. It seems that if dolphins continue beating fish, they can exterminate a significant part of the school. But circumstances balance the chances of predator and prey. Dolphins need fresh air, so they hurriedly pick up prey, and rush to the surface, leaving a school of false oilfishes. Fragile fishes continue their journey into the depths, but several individuals will no longer be able to do it. One of the fishes turns upside down – it’s dead. Nearby, several stunned fish twitch and cannot follow the school. They remain in place, and their uncertain movements, as well as the smell of blood of fishes swallowed by dolphins, attract other predators. One large pelagopike woke up, and it feels the smell of blood. It is still dark in the depths of the lake, but the sense of smell tells the fish the location of the prey. Therefore, the pelagopike unmistakably picks up one of the stunned fish at full speed. Rapidly turning around, it snatches one by one several fishes left after hard-beaked dolphins’ hunting. Soon another pelagopike appears out of the darkness, and the last stunned false oilfish disappears into its throat.
Echolocation abilities allow dolphins to “see” animals whose body conducts sound differently than water. Hard-beaked dolphins surface after hunting in a congestion of filtering barmash, which moves to the surface of the water in the morning. Dolphins easily notice scuds, but do not eat them – these crustaceans are too small for them. Dolphins perceive them from afar as some kind of scattered “interference” for the echolocation signal. But when the animals swim through their congestion, they feel the presence of crustaceans. There are several hairs on the chin and near the eyes of dolphins – it is a legacy inherited from land-dwelling ancestors. The scuds accidentally cling to them, and the dolphins realize that very tiny creatures live next to them.
Not all Baikal animals can be "seen" by a hard-beaked dolphin with the help of echolocation. When the dolphins scanned the water with sound rays in the depths of the lake, they did not pay attention at all to the watery lump of slime floating in the water. The sound just passed through this slime, and not a single animal noticed the distortion of the sound signal. Meanwhile, it was not a lump of slime, but a living creature that is actually a food competitor of a dolphin. The chaotic movement of thousands of oilfishes, the powerful swings of dolphins’ tails and strong sound vibrations frightened this creature. But when everything calmed down around it, the lump of slime moved, slowly unrolls and turned into a translucent planaria of a huge size for these worms. This planaria is poorly visible in the semi-darkness of the depths of the lake, and therefore rightly bears its name – “Baikal phantom”. The worm, about 30 centimeters long, lazily unfolds its body and turns into an elegant creature of fragile appearance. However, the appearance of the “Baikal phantom” is deceptive: it is a predator dangerous to the inhabitants of the depths of the lake. The “Baikal phantom” swims waving the edges of a wide flat body, and its pharynx is retracted and does not interfere with movement. On the shores of Lake Baikal, even after the strongest storm, it is impossible to find the body of this worm: “Baikal phantom” belongs to pelagic species. It never leaves the depths, and is not found near the shores.
“Baikal phantom” is a representative of planarians, and its mental abilities do not differ from those characteristic of these worms in general. The brain of this worm represents two tiny ganglia of nerve cells at the anterior end of the body at the bases of the tentacles. Primitive behavior is quite enough for this predator to catch successfully a false oilfish or a pelagic scud – its main prey. The tentacles on the head of the “Baikal phantom” are well developed, and there are numerous receptor cells on them. Therefore, the worm easily distinguishes the odorous “traces” that animals leave behind them. A school of false oilfishes, swimming into the depths of the lake, left a distinct trace behind them, and the “Baikal phantom” rushed after the fishes. Of course, this worm cannot compare in speed with either a pelagopike or a Baikal hard-beaked dolphin. But false oilfishes are also very slow swimmers, and the worm may well count on a successful hunt.
On the surface of the water, a school of false oilfishes encountered various predators. Many fish died, and some were injured. One young false oilfish swims into the depths, having a shallow wound on its side – a pelagopike touched it with the tip of its fin and only cut the skin. But where there are predators, such a wound can be fatal. Blood oozes slightly from the wound and the “Baikal phantom” is especially attracted by the smell of this particular individual. The worm rushes after the fish school. Soon its body begins to touch the fins and skins of the fishes. The false oilfishes sense the approach of this predator and rush to the sides – if the fish is small, the worm may well attack it. But the “Baikal phantom” is not interested in healthy individuals. The worm clearly discerns the smell of blood, and gradually approaches the wounded fish. It feels the movement of the water coming from the wriggling worm, and also tries to escape from it. But the fish can’t swim away from its own smell. Several sharp movements of the overtaken fish increased the flow of blood from the wound, and the worm received an even clearer tip to the prey.
The chase cannot last for long – both the predator and the prey get tired. Pursued by the “Baikal phantom”, the false oilfish moves slower and slower – it rests longer and longer, and its rushes are rarer and shorter. Following the fish, a transparent slime-covered predator moves. The wounded false oilfish had felt the worm’s touch on the caudal fin, and had made another short rush, after which it froze in the water column powerlessly, having stretched its fins.
The “Baikal phantom” pounced on its prey like a hawk. It caught up with a young false oilfish and covered the fish with its flat slimy body. The edges of the giant planaria’s body closed around the prey, enclosing it in a translucent “cocoon”. For the first seconds, the captured false oilfish tries to escape – it twitches and tries to leave the “embrace” of the worm. Its movements are clearly visible through the body of the “Baikal phantom”. It may be dangerous for the worm – its body is too tender and it is easy to tear. Therefore, the worm tries to kill its prey as quickly as possible. The cells on the underside of its body began to secrete slime profusely. The slime of the “Baikal phantom” covers the body of the captured fish from all sides, glues its fins and clogs into the gills. The resistance of the fish weakens, it suffocates. The edges of the worm’s body as if stick together around the false oilfish and the worm waits until the fish dies. It will last not for long – after a few minutes, the last convulsions of the prey stop, and the worm can start eating. Like all planarians, the “Baikal phantom” does not know how to bite off pieces of the prey’s body. But the absence of a hard skeleton gives the worm the opportunity to swallow it whole. The worm’s pharynx is easily stretchable, and it can easily swallow whole prey half its own size long.
When the false oilfish died, the “Baikal phantom” weakened its “embrace”, and the body of the prey appeared floating in water. But it was almost immediately seized by the edges of the flexible pharynx of the worm. Swallowing large prey is not an easy task. Secreting slime plentifully into the pharyngeal cavity, the worm as if “pulls” it on the prey and gradually moves the body of the false oilfish into the gastric cavity. After about twenty minutes, it managed to complete this task. For a while, the prey will be visible in the middle part of its body as an oblong bulge, but the worm will gradually digest it. Despite its apparent voracity, the “Baikal phantom” has a very moderate appetite – such a large prey will give it a supply of nutrients for about a month of full life.
In nature, the death of a live being from natural reasons is extremely rare. Perhaps only Baikal hard-beaked dolphins or large pelagopike individuals can die without falling anyone’s prey. The rest of the animals are very likely to end their lives in the stomachs of various predators.
Dead animals, droppings and the remains of prey from the inhabitants of the upper layers of the water sink down to the bottom of the lake. Here, at a depth of over one and a half kilometers, cold and enormous pressure reign. In some places, there are hot springs at the bottom – this is a manifestation of the tectonic activity of the Earth’s crust, due to which Lake Baikal appeared. The lake is gradually expanding. There is a layer of thin silt at the bottom of the lake. Over millions of years, the lake could turn into a swamp, and then into the flood plain only. Such a fate befell many lakes in the north of Eurasia, known in the age of man. But Lake Baikal belongs to oligotrophic lakes, and the sediment accumulation on on its bottom is very slow. In addition, the expansion of the lake due to geological processes additionally extends its life.
Over millions of years of evolution, life has mastered even such an extreme habitat, although the diversity of deepwater animals of Lake Baikal is only a faint semblance of diverse life in shallow waters and in the upper layers of water. Very few species of living organisms have managed to adapt to living in the depths of the lake. Food chains in the abyssal zone of the lake are short. The only kind of food here is already existing organic substance. Animal corpses fall from above, and bacteria and small invertebrates live on the bottom in a thick layer of silt – these are the only food resources for the inhabitants of the depths. At the bottom of the lake there is a unique special world of predators and scavengers.
The silt covering the bottom of the lake is very loose and in this respect it is similar to quicksand – it is easy to fall into its thickness without any special adaptations for movement. The surface of the bottom silt is like an open book: traces of various animals are imprinted on the silt, by which it is possible to find out how they live. Among the tracks, long, but shallow grooves are visible – they were left by worms crawling right under the surface of the silt. They swallow silt and pass it through their intestines, digesting bacteria and the tiniest particles of organic matter. Such grooves may be interrupted by a small hole – most likely, a certain inhabitant of the abyss had found and ate the worm. Sometimes there are short tubes at the bottom, made of a mixture of silt and slime. Their inhabitants are also worms. The silt near these tubes is streaked with random grooves, and the even trails interrupt – the worms are clearly predators, and the grooves are left by their prey, which they grabbed, having stuck out from the tube. Sometimes the traces on the silt become indistinct – the fish, swimming over the bottom, scatters the silt with a swing of its tail.
A special trace on the silt layer is very similar to the trace left by a miniature car wheel: two rows of small strokes stretch along its edges, and between them deep grooves make up a pattern similar to a “herringbone”. But in Neocene, there are no objects in nature that are the creation of human hands. Such traces are left by one of the species of deepwater crustaceans – the wormlike bathyasellus. It is a highly derived representative of the isopods. It looks more like a millipede than a woodlouse – it has an elongated flexible body covered with a very thin and delicate shell. The walking legs of the bathyasellus are short, and the front pair of legs has a special structure. They are covered with hairs that form combs and serve for silt scooping. Bathyaselluses belong to the number of the most peaceful inhabitants of the bottom of Lake Baikal. They spend most of their lives eating silt. When the crustacean moves slowly through the silt, raking it alternately with one or another leg, a characteristic “herringbone” pattern remains. The prints of walking feet complement the similarity with the trace of a car wheel. The bathyasellus does not fall into the silt due to the fan-shaped bristles on the legs, which increase the support area.
Food of these crustaceans is literally underfoot, and there is actually no intraspecific food competition. Bathyaselluses pass silt through the intestines, extracting dribs and drabs of organic matter – bacteria and fungi – from it. The population of bathyaselluses is the most numerous in the depths of the lake, and predators often depend on the well-being of these crustaceans.
The largest predator of the depths of the lake is constantly in search of prey: its length is more than a meter, and it constantly needs food. Populations of bottom animals are very sparse, and in order to get enough food, a predator must cover a distance of several hundred meters per day, relying on the sense of smell and touch in the search. The predator “walks” along the bottom of the lake, only slightly touching the silt, and leaving a double row of shallow transverse grooves on its surface. Despite the obvious preference for this method of movement, this creature is a fish. The completely blind deepwater fish of Lake Baikal, the bathylota, has almost zero buoyancy due to a very large and fatty liver. The abdominal fins of the bathylota are shifted to the throat, and the fish “walks” along the bottom, leaning on them, or rather, slightly touching the surface of the silt with their tips. Sometimes it helps itself with a lazy swing of its tail or a movement of its pectoral fins. A long wattle grows at the very tip of the lower jaw of the bathylota. When a fish is searching prey, the wattle as if lives its own independent life. It feels the silt in front of the fish. For a blind fish, this wattle is like a guiding thread. The tip of the wattle is rich in chemoreceptors, and the fish can detect the presence of buried living creatures by simply sticking the wattle into the upper layer of silt.
With light pushes of the abdominal fins, the bathylota moves over the bottom like an airship. But suddenly the receptors in the wattle caught a smell indicating the recent presence of a crustacean – bathyasellus – on this part of the lake bottom. The bathylota stops and probes the silt several times with a wattle. The fish shakes its head from side to side, while the wattle explores the silt intensely. Finally, the fish chooses the proper direction – it feels in which side the smell of the crustacean is stronger, and begins to chase the prey. Bathyasellus has to be careful – with its length of about two centimeters, it is a tasty food for deepwater predators. Therefore, the most important thing for it is to feel the impending danger in time and hide. The crustacean is frightened by the approach of the bathylota – it senses the movement of water from a cautiously swimming predator with the help of receptors located on its antennae. With a sharp movement, the bathyasellus bends its body, bounces aside a few tens of centimeters, buries itself in the silt with a jerk and freezes. Clouds of fine silt rise in the water, but it does not prevent the bathylota for orientating – the silt does not hide the smell of the crustacean. In addition, the fish clearly caught the waves running from the moving bathyasellus with the help of its lateral line organs. The presence of the crustacean is not a secret now, and bathylota begins to search for it. Before the attack, the fish freezes, and only the wattle twitches from side to side. Bending the long trunk, the bathylota begins to search the bathyasellus hidden in the silt with the help of a wattle. The chemical sense serves the fish right – a few minutes later it finds a crustacean frozen in the silt by smell. When the wattle of the bathylota touches the tip of the bathyasellus’ antenna, the frightened crustacean jumps out of the silt and makes one more jump. The exact rush of the bathylota followed instantly, and the bathyasellus disappeared into its mouth. With a few sharp movements of the gill covers, the fish throws out from the oral cavity the silt, which it accidentally scooped up with the crustacean. A tail swing of the bathylota erased the traces of a successful hunt from the surface of the silt. The fish swam through the clouds of silt raised by the movements of its tail, and disappeared into the darkness.
Someday this bathylota will die of old age, will be eaten by a stronger relative, or will die from parasitic diseases. Then, its body will turn to the food for numerous benthic invertebrates of the lake, including bathyaselluses. The mineral substances that made up the body of the bathylota will dissolve in the water, and due to the mixing of the water layers, they will move to the surface of the lake. Here they will be used for the growth of microalgae, which are at the base of the food chains of the lake ecosystem. So the circle of life will be closed, and everything will happen again.

Long-throated planaria (Macrobentoplana prolongopharynx)
Order: Planarians (Tricladida)
Family: Planariidae (Planariidae)
Habitat: Baikal, coastal shallow waters, colonies of sponges.

Picture by Alexey Tatarinov

In the productive shallow zone of Lake Baikal, sponges of the Lubomirskiidae family, endemic to this lake, have a very important position. They are peculiar analogues of coral reefs – the bodies of sponges have a cellular structure with many cavities, where various small animals live – fishes, scuds and mollusks. Such a variety of prey attracts a variety of predators to the sponge “reefs” of the lake – mainly fishes and scuds. Among the predators of the Baikal sponge “reefs”, a large benthic planaria of bright color occupies a special position. Planarians are very characteristic for Lake Baikal, and even in the human era, unfavorable for the fauna of the lake, they were very diverse. In Neocene, their evolution continued, and new interesting species of these worms appeared in the lake.
Among the colonies of sponges, one large Baikal planaria lives – a long-throated planaria, a large worm about 10 cm long and about 3 cm wide. The body of this worm has a dense consistency and is covered with poisonous acrid slime – this is how the worm protects itself from fishes and crustaceans that live among sponges. To prevent a senseless (and dangerous for smaller animals) attack, the worm has a bright warning coloration – its body is yellow with a brownish tinge, covered with a pattern of black specks merging into cross stripes. Contrasting coloring will be well remembered by a predator decided to attack ершы worm. The long-throated planaria is active during the day and, due to chemical protection and bright coloring, crawls on the sponges and does not hide.
This worm is a predator, and is perfectly equipped for successful hunting. The eyes of the long-throated planaria are relatively large, and the worm can distinguish the contours of close objects. Behind the eyes, there are thin movable tentacles, which surface is covered with chemoreceptors – chemical sense plays a much greater role in the life of this worm than vision. And the main hunting device of this animal is a very long and stretchable pharynx located on the underside of the body. The pharynx can stretch to a length twice the size of the worm. Sensitive cells are located on its edges. The long-throated planaria crawls along the sponges, and with an elongated throat fumbles in the cavities of the sponge in search of small animals that settle in it. The worm may not see its prey directly, but its pharynx searches independently for small animals hiding in the cavities of the sponge with the help of chemoreceptors.
Like all planarians, the long-throated planaria is a hermaphrodite. During the summer, each individual lays large cocoons containing up to 20-30 eggs in the sponge cavity. By autumn, young worms emerge from them; they reach the size of an adult the next year.
In case of damage, the body of a long-throated planaria can fall apart with the help of sharp muscle contractions. Pieces of the animal’s body remain viable and easily regenerate into full-blooded individuals.

Multimouthed polypostoma (Polypostoma polystoma)
Order: Planarians (Tricladida)
Family: Planariidae (Planariidae)
Habitat: Baikal, the central part of the lake, the upper layers of water to a depth of 20-30 meters.

Picture by Amplion

In isolated habitats, the active process of adaptive radiation takes place – many specialized species differing from each other in ecology descend from a few ancestors. In this case, the descendant species in due course of evolution “diverge” into various ecological niches, and competition between them decreases.
Baikal is actually an isolated habitat – it is far from the oceans and isolated from other lakes and swamps by mountains. Few rivers flow into it, and in the Neocene only the Angara still flows out from it. In such conditions, the fauna of the lake has retained a high degree of endemism. A significant increase in the area of the lake in the Neocene allowed an even greater number of endemics to arise.
In the Neocene, planktonic organisms represent the main producers of organic matter in the lake and the first link in the food chains. Baikal zooplankton is formed of various pelagic crustaceans and swimming annelid worms. Along with peaceful herbivorous forms, various predators are found in zooplankton. One of them is a peculiar pelagic planaria with numerous mouths. The body length of this small worm is about 3 cm. The main mouth of this worm has disappeared, and the long pharynx serves to maintain balance, acting as a kind of drift-anchor. The length of the pharynx is about 7-8 cm. Its tip, on which the mouth should be located, is covered with numerous hairs. Instead of a mouth disappeared forever, this worm has developed many secondary mouths at the edges of the body. These secondary mouths are located on movable stalks about 5 mm long and are surrounded by thin movable tentacles. They are similar to hydroid polyps, and for this feature the planaria is named “multimouthed polyp-mouth”. The tentacles help the planaria to capture small prey, but, unlike the tentacles of polyps, they do not have stinging cells.
The body of the polypostoma is almost completely transparent. The slime covering it gives the worm’s integument an iridescent sheen, and by this feature the worm can be easily seen in sunny weather if it swims near the surface of the water. The body of the polypostoma has rounded outlines, and the head is almost not pronounced. On the edges of the head two short triangular tentacles grow, between which there are two pairs of very small eyes.
The polypostoma feeds on small planktonic organisms – pelagic worms and young scuds. This animal is slow-moving, and it simply drifts in a congestion of plankton, waiting for potential prey to touch the tentacles surrounding the secondary mouths. The tentacles are equipped with many nerve endings and small muscles, so they immediately grab prey and shove it into the pharynx.
If the worm senses the approach of a larger animal, it defends itself by shrinking into a tight ball and secreting slime abundantly. A slime coating hides its smell and has a pungent taste, so most predators do not touch this worm. Only adult pelagic scuds occasionally attack the polypostoma and bite off its tentacles and edges of the body. However, due to the ability to regenerate, this worm easily recovers from damage.
With the onset of the cold season, this worm stops feeding, and its secondary mouths significantly degenerate: the pharynx shortens, and the tentacles dissolve. The worm does not prevent the cold from penetrating into its body – it overwinters, freezing in ice. In early spring, eggs begin to form in the body of the polypostoma. These worms begin to reproduce when ice is still floating on the surface of the lake. On the underside of the ice floes, the worms twist together (sometimes up to 5-6 individuals take part in mating at the same time) and mutually fertilize each other. The eggs laid (about 100 pieces) are in a dense slimy cocoon, which the adult holds at the tip of the pharynx, which has turned into an “anchor”. A month later, young individuals emerge from eggs and keep sit on the abdominal side of the body of the parent individual for several weeks and feed on small planktonic organisms (rotifers and young worms) that adhere to the slime of the parent. Later, they switch to a pelagic lifestyle, and they develop secondary mouths, and the pharynx turns into the “drift-anchor”.

Baikal phantom (Baicaloptera spectrum)
Order: Planarians (Tricladida)
Family: Planariidae (Planariidae)
Habitat: Baikal, depths of 30-50 meters, far from the shores.
In the age of man, Lake Baikal was known for a rich fauna of worms, among which planarians had a special place. In the human era, these worms were remarkable in their large sizes and unusual coloring. Most of the Baikal planarians were endemic to the lake.
In the Neocene, planarians continue to inhabit the lake. They have mastered various ecological niches, and some Baikal planarians have turned into unusual creatures. One Baikal planaria evolved from a crawling creature into a graceful and fast swimmer. It is found in the water column at a depth of up to 30 meters, where sunlight penetrates, and never swims down to the bottom. The body of this worm reaches 30 cm in length and about 20 cm in width; it is white in color and semitransparent – that is why the worm was named “Baikal phantom”. The body covers of the worm are so transparent that the branched digestive system and thick nerve and muscle fibers can be easily seen through them.
The body of the Baikal phantom has rounded outlines, with a slightly shaped head part. On the upper side of the worm’s head, short sensitive tentacles are developed, slightly similar to the tentacles of snails. At the base of the tentacles large nerve ganglia are located, clearly visible in the thickness of the worm’s body. The Baikal phantom has numerous eyes – there are about a dozen of them along the front edge of the head. However, the vision of this species is poorly developed – the Baikal phantom can only distinguish light from darkness. In addition, the entire surface of the worm’s body is covered with many chemoreceptors, and the animal easily detects the presence of predators or prey by smell.
The Baikal phantom can swim quickly, undulating the sides of the flattened body. Despite the jelly-like body consistency, this worm is an active predator. The Baikal phantom hunts not only various pelagic invertebrates, but also catches small slow-moving fish. A mobile muscular pharynx grows on the lower side of the Baikal phantom's body; it is able to extend strongly and stretches wide enough to swallow a small fish whole. The worm actively chases its prey and catches it, enveloping it with its whole body. The edges of the worm’s body close around the prey, and the animal simply waits until the prey suffocates, after which it swallows the prey whole. This worm grabs small prey with its throat and immediately swallows it alive.
Having eaten enough, the Baikal phantom passively floats in the water column. In case of danger, the worm greatly contracts its body, turning into a slimy ball about 5-6 cm in diameter. If a predator attacks a swimming worm, the Baikal phantom easily looses its body parts, and regenerates them rather soon. Even if the worm is simply torn into several parts, it will regenerate easily – an independent organism will grow from each part of the body. Another means of protecting at the Baikal phantom is its unpleasant taste. Its slime contains acrid substances that scare off fish and predatory invertebrates.
The Baikal phantom is a hermaphrodite species. An adult one lays over 300 small eggs at once. They are covered with a common shell, and the adult animal bears these cocoons on its body, gluing them to the underside of the body behind the throat. Incubation of the clutch lasts a long time – for about six months, so within two years the worm makes no more than three clutches. However, due to the care of the offspring, the survival rate of the clutches of the Baikal phantom is very high. The juveniles of this worm also lead a pelagic lifestyle. Young Baikal phantoms live almost at the very surface of the water and feed on crustaceans and young fish. As they mature, young worms gradually migrate to the deeper layers of water. They become adults and reach their maximum size in the second year of life. The life span of this worm can reach ten years, but the regeneration processes “rejuvenate” these animals, and the repeatedly regenerated worm reaches the age of 20 years or more.

Baikal fireworm (Pyronais lucens)
Order: Haplotaxids (Haplotaxida)
Family: Detritus worms (Naididae)
Habitat: shallow areas of Lake Baikal, lower reaches of rivers flowing into the lake.

Picture by Biolog

In the human era, Lake Baikal belonged to the oligotrophic lakes – its biological productivity was relatively low compared to other lakes in Eurasia. The lake has a relatively narrow productive shallow water zone (in the Holocene epoch, more than 80% of the lake’s surface was occupied by areas of depths of more than 250 meters), and a significant part of the biomass is composed of freshwater plankton. Far from the shores, plankton is formed mainly of pelagic crustaceans, but closer to the river mouths and shallow areas of the lake, the role of the dominant planktonic species passes to a kind of endemic species of swimming oligochaetes.
The Baikal fireworm is a descendant of small swimming detritus worms (Nais) widespread in fresh waters during the Holocene epoch. The body length of this animal is about 10-15 millimeters. The worm’s body is pinkish – there is a small amount of hemoglobin in its blood. Since the fireworm lives in the oxygen-rich water of Lake Baikal, it does not need to develop additional respiratory organs, as in related species living in oxygen-poor reservoirs. On each segment of the worm there are two pairs of bundles of short bristles. They serve for swimming, and with their help worms can link into large congestions in the water column.
This worm species is one of the main sanitation workers of the lake: it feeds on rotting organic matter. Baikal fireworms gather tiny organic particles carried out by rivers and suspended in the water column. They also gather in large numbers on the corpses of various animals and on rotting parts of plants.
The name of this worm species was determined by one unique feature: the Baikal fireworm has bioluminescence organs – it is a phenomenon unique for freshwater animals. However, there are even frogs among freshwater luminous animals in the Neocene fauna. The bioluminescence organs are located on several segments at the posterior end of the body, and represent rudimentary gills. The gills of the fireworm have lost the function of respiration, since Baikal water is rich in oxygen, and the animal gets enough oxygen through the epithelium. But they form bubbles filled with glowing bacteria, and can be drawn into the thickness of the worm’s body, or bulge out. When the worm wants to stop glowing, it draws the luminescence organs into special sacs lined with black pigment cells (because of this, the back end of the worm’s body looks darker).
The Baikal fireworm has small eyes of primitive structure that allow it to recognize the glowing of its relatives, but are unable, however, distinguish the contours of objects.
During the day and in mucky weather, fireworms gather in places protected from strong water movement at the bottom of the lake and among vegetation. At this time, they curl into small balls and do not glow. Light signals serve worms to communicate with their relatives at night, during the period of greatest activity. After dark, when the activity of predators decreases, fireworms leave their shelters and swim to the surface of the water. Here they gather food particles. Disturbed worms fade away, therefore, by the reaction of their congestion at night, fish are clearly visible from the surface of the water. Near an abundant food source (for example, dead fish), worms form dense and brightly glowing clusters.
Worms that glow brightly at night represent an easy prey for nocturnal predators. They are often devoured by pelagic crustaceans and aquatic insects. But the Baikal fireworm survives thanks to two important qualities: the rate of reproduction and the ability to regenerate. It easily restores damaged areas of the body, and can even multiply, dividing spontaneously into several fully functional organisms with the help of the body fission. This phenomenon known as paratomy was usual among related worm species in the human era. Due to this feature, the Baikal fireworm multiplies rapidly and restores its numbers after its flocks are eaten by fish and crustaceans. Due to this form of reproduction, the biomass of worms doubles every week.
Glowing bacteria live in small numbers in the worm’s blood as symbionts, so all worms formed as a result of asexual reproduction get microscopic allies necessary for life. When they fully develop the posterior end of the body with glowing organs, they glow along with intact individuals.
By winter, the Baikal fireworm passes to sexual reproduction. This species is a hermaphrodite and lays eggs in cocoons with a dense shell. For reproduction, worms find sponges with a porous structure by smell, and drop cocoons over them. Eggs develop in the thickness of the porous body of the sponge, protected from most predators. The young are hatched in the spring – a necessary condition for the development of embryos is a strong cooling of the egg for several weeks.

The idea of the existence of such a worm species was proposed by Nem, the forum member.

Sponge-dwelling scud (Lubomirskiincola assiduus)
Order: Amphipods (Amphipoda)
Family: Gammarids, scuds (Gammaridae)
Habitat: Baikal, coastal shallow waters, colonies of sponges of the Lubomirskiidae family.
Symbiotic relationships benefit representatives of species that enter symbiosis. Therefore, they are widely present in nature. In the relatively isolated ecosystem of Lake Baikal, many species of living creatures have established symbiotic relationships.
In the endemic Baikal sponges growing at shallow depths, small crustaceans belonging to the scud family are often found. They represent a separate species – a sponge-dwelling scud – which is so adapted to life in sponges that it has lost the ability to live anywhere else except sponges. It is a very small crustacean about 15 mm long. Life in the holes made in the sponge’s body has left its mark on the appearance of the crustacean. It turned into a short-legged creature with a worm-like body, unable to swim. The body covers of the sponge-dwelling scud are very thin and delicate, and the body is pale and translucent. Only the head is covered with a slightly harder cover.
This animal spends almost whole adult life in burrows that it drills in the sponge’s body. Therefore, its contacts with the outside world are minimized. The eyes of the sponge-dwelling scud are poorly developed, and vision is very poor – it can only distinguish light and darkness, and detect the movement of objects. However, it receives most of the information about the surrounding objects due to a highly developed chemical sense. The antennae of this crustacean are long – they are approximately equal to the body length of the crustacean. There are many receptors on the antennas. At rest, the crustacean keeps near the entrance to the burrow, exposing only the antennae.
The sponge-dwelling scud chews narrow burrows through the sponge colonies perpendicular to the surface of the sponge body and reaching its internal cavities. The tunnels of these crustaceans can permeate the entire body of the sponge. Each individual knows the smell of its own burrow and easily finds it if it was forced to leave the dwelling. This animal settles in assemblies numbering up to several dozen individuals. Young individuals prefer to settle near adults – they find colonies of crustaceans of their kind by smell, and build a dwelling near them, or occupy empty burrows. Some of the juveniles on the edge of the colony die from predators.
An adult animal never leaves the sponge, and gets everything it needs, living in its own burrow. Sponge-dwelling scud feeds on organic substances that are absorbed by the sponge. It gathers food using three front pairs of legs. These legs are covered with bristles along the edge, and catch food particles. When enough food accumulates on them, the animal rolls it into a ball and swallows it. This species is omnivorous. In addition to organic suspension from water, the sponge-dwelling scud eats parasites of sponges that settle in the loose body of the animal.
After digesting the food, the animal crawls to the opposite end of the burrow, which opens into the inner cavities of the sponge’s body and dumps the droppings into the water. The flow of water carries it outside, and thus cleanliness is maintained in the hole. This crustacean throws larger wast out of the burrow with strong movements of its legs. Anchored in the burrow, with strong movements of the abdominal legs, it creates a current that takes out the waste. This behavior also serves to ventilate the burrow – this is how females having offspring in the burrow often behave.
Sensing danger, this crustacean emits alarm signals – clicks of its forelimbs. Hearing someone else’s alarm, each animal repeats it and hides in a hole. The alarm of the entire colony is clearly heard in the water as a crackling.
Sponge-dwelling scud reproduces up to five times a year during the spring-summer season. The male of this species is smaller than the female. He is more active than the female and can leave his burrow. With the help of the sense of smell, he finds females ready for fertilization. Without leaving the sponge, he moves through its cavities, crawls into the burrows of females and fertilizes them. Competition for females arises between males living in the same sponge – each male pushes the opponent away from the female, blocking the entrance to the burrow with his own body.
The young stays in the brood pouch, and then lives in the female’s burrow for a few days. After the first molt, the young animals develop hairs on their hind thoracal legs, and young scuds get the ability to swim. They leave the sponge with a current of water, and live for several weeks in shallow waters as part of zooplankton. After two molts, the young individuals settle on the sponges and begin to make their own burrows, or occupy ready-made empty burrows or other cavities of the sponge body. At the age of two months, young crustaceans reach sexual maturity, and individuals of the first spring generation have time to give the first offspring by autumn.
The lifespan of this scud does not exceed two years.

Filtering barmash (Barmash plumophorus)
Order: Amphipods (Amphipoda)
Family: Gammarids, scuds (Gammaridae)
Habitat: Baikal, the upper layers of water away from the shores.
In the Holocene, the copepod crustacean Epischura (Epischura baicalensis) formed the basis of Baikal zooplankton. This crustacean species ensured the amazing purity of water of the lake – it got its food by filtering the water. But the Epischura had one weak spot – this crustacean was very sensitive to water pollution. Therefore, its number decreased in the age of man, and later this crustacean was substituted by representatives of amphipods. In the human era, scuds were known to locals as “barmash”. In Neocene, a special kind appeared among these crustaceans, that extracts food by filtering water. It has become the dominant zooplankton species.
Filtering barmash is a small glass-transparent planktonic crustacean. Its body length is about 7 mm. The body of this crustacean is relatively deep and short; the front four pairs of walking legs only slightly protrude beyond the very elongated edges of the shell. This animal spends its entire life in the water column, not approaching the shores of the lake. But this crustacean clearly tends to areas opposite the mouths of rivers – there the favorable conditions for algae growth form. During the year, these animals make vertical migrations – in winter they stay at a depth of about 200-300 meters and are inactive, and in summer they swim up into the food-rich near-surface layers of water – to a depth of 3-10 meters. This scud is quite passive – it swims due to the movement of water that occurs during the food filtering, but most of its life it just floats in the water column. To facilitate floating in the water, it developed long lateral outgrowths on the antennae, giving them the appearance of feathers. The hind pairs of legs are also covered with hairs – they help to keep balance. Pressing its legs and antennae to its body, the crustacean slowly swims.
The filtering barmash is feeding by filtering water through four pairs of front walking legs. They are densely covered with hairs on the inside, and represent an excellent “sieve” for the smallest phyto- and zooplankton (rotifers and protozoans), as well as for other organic particles. The water flow from the abdomen to the head is provided by the continuous movements of the abdominal legs covered with hairs. From time to time, the animal scrapes off the plankton accumulated on the bristles of the limbs with its mandibles.
The filtering barmash forms a great biomass due to rapid growth and rapid generational change. An adult individual does not live for long: individuals hatched at the end of winter and the beginning of spring live no more than six months, and animals born at the end of summer live until the next spring. During her lifetime, the female produces up to 500 eggs, making 5-6 clutches.
For the filtering barmash a hypogenesis is characteristic – from the egg, young individual hatches, similar in appearance to an adult crustacean. In summer, clutch develops quickly – for about two weeks; winter clutch develops longer – for up to 3 months. The sex of this amphipod species is not genetically determined. The young are born agamous, in the warm season (at the temperature of the upper water layers above +10°C) they turn directly into a parthenogenetic females and begin to reproduce at the age of about 20 days. In autumn, winter and early spring, when the water is cold, young individuals initially become males and fertilize the females of the previous generation. Then, after two molts, they turn to females and begin to produce eggs parthenogenetically.

Symbiotic scud (Syngammarus ichthyophilus)
Order: Amphipods (Amphipoda)
Family: Gammarids, scuds (Gammaridae)
Habitat: Baikal, coastal shallow waters. The symbiont of the fish Baikal lipscraper.
Lake Baikal has a surprisingly rich fauna of amphipods. Among them there are species occupying a variety of ecological niches – bottom-dweling and planktonic, shallow-water and deep-water, herbivorous, omnivorous and carnivorous ones. There are no decapods in Baikal, which allowed the amphipods to evolve freely.
In the Neocene, Baikal scuds are as diverse as in the human era. Diverse species of these crustaceans in the process of evolution have developed various survival tactics. Among the Neocene scuds of Baikal, a species appeared that repeated the evolutionary step of some shrimps of Holocene epoch: it entered into a symbiotic relationship with one local fish, the Baikal lipscraper.
Symbiotic scud is a small species of burrowing crustaceans having a body length of about 15 mm. The antennae of this crustacean are relatively long – they are one and a half times the body length. The body of the crustacean is deep and shortened with well-developed legs. Brushes of thin flexible hairs are developed on the front pair of legs. Their role in the life of a symbiotic scud is very great: with their help, the crustacean scrapes food particles from the body of the fish or from the substrate.
The eyes of the symbiotic scud are well developed, and vision plays a considerable role in its life. With the help of vision, the symbiotic scud recognizes the host fish, as well as its own relatives. This species has developed an eye-catching contrasting striped coloration – a thin white longitudinal stripe stretches along the red-brown background on each side. This scud prefers to live alone in a lipscraper’s hole or in another shelter. The crustacean, which is not ready for reproduction, is aggressive towards its relatives and violently attacks them if they appear in the immediate vicinity of its shelter. Almost with the same ferocity, the symbiotic scud attacks amphipods of other species that have at least a distant external resemblance to its relatives.
In addition to good eyesight, this crustacean has a keen sense of smell – there are many chemoreceptors on its long antennae.
The symbiotic scud prefers for life the burrows of the Baikal lipscraper, which represent a reliable shelter from predators. The presence of fish provides the animal with good nutrition. The crustacean cleanses the body and gills of the fish from parasites and slime flakes and bites off dead tissue on wounds. It also feeds on fish droppings containing a lot of undigested organic matter. When the symbiotic scud is going to offer the services of a cleaner to the fish, it informs about its intention with a click. The scud makes sounds with the help of an outgrowth on the last segment of the front leg, which is abruptly inserted into the recess on the large and flattened penultimate segment.
Usually, the population of Baikal lipscrapers in the coastal shallow waters of Lake Baikal is not enough for the entire population of this species, and there is a fierce struggle for the every burrow inhabited by this fish. Therefore, part of the population of symbiotic scuds is forced to survive independently – the animals themselves find shelters and get food. Free-living individuals eat insect larvae and dying parts of plants.
The male of this species is a “nomade”: he does not have a permanent shelter and is always in search of females ready for fertilization. It is smaller than the female, but with more developed front walking legs and a bright color – the background of its body may be blood-red, and the white stripe on its side is wider than that of the female. During the day, the male hides in random shelter, and at night leaves it and crawls in shallow water, searching by smell for females ready for fertilization, which live sedentary. Males are very aggressive towards each other, and cannibalism is often found among them – adults eat young animals that have not reached puberty.
The female gives 2-3 broods per year, bearing offspring in a brood pouch on the abdomen. She throws off the young, ready for independent life, outside the burrow to prevent the accidental eating of her offspring by the lipscraper. The juveniles immediately hide between the rocks and lead a solitary life. Young symbiotic scuds scrape sedentary invertebrates from rocks. Sensing the approach of a storm, individuals living freely hide deeper into the ground. Juveniles can crawl into the ground to a depth of about half a meter.

Wormlike bathyasellus (Bathyasellus vermiformis)
Order: Isopods (Isopoda)
Family: Asellidae (Asellidae)
Habitat: Baikal, lake bottom, muddy ground.
The depths of a number of lakes and seas of the Holocene epoch were lifeless – due to unsufficient mixing, the lower layers of water were poisoned with hydrogen sulfide, and only a few species of bacteria could live there. Unlike them, Baikal in the Holocene epoch was full of diverse life from the surface to the bottom. In Neocene, the situation has not changed – animals live even at the very bottom of the lake. The living conditions at the depths are very specific – there is a huge water pressure, cold and very scarce food resources. But at the same time, the animals that have settled at the bottom of the lake have virtually no competitors.
The thick layer of silt covering the bottom of Lake Baikal is the home and nutrition source for bacteria and microscopic lower fungi. They are eaten by a small worm-like animal with a thin shell and short legs – the wormlike bathyasellus, one of the representatives of the Baikal deepwater crustaceans. This representative of the isopods is a descendant of Asellus baikalensis species, which lived in Lake Baikal during the human era.
Due to the pass to life in the depths of the lake, the structure of this crustacean underwent profound changes compared to its ancestor. Bathyasellus has an elongated worm-like body (the length of the crustacean is about 25 mm with a body width of no more than 3 mm), covered with a very thin, soft and translucent shell. Through the shell, the insides of the animal are visible – the intestines stuffed with silt, the heart and large nerve nodes. Two pairs of antennae grow on the head of the bathyasellus – one pair is long, almost equal to the length of the body, and the other one is short, about a third of the body length. The sense of touch and chemical sense give the animal fairly complete information about the surrounding world. Bathyasellus is blind, and only barely noticeable pigment spots on the sides of its head remain from its eyes.
Almost all the limbs of the bathyasellus lack any specializations and look almost equal to each other. Their only special feature is the presence of fan-shaped bristles on the end segments, which help with the movement of the animal on the silt surface. In addition, bathyasellus can swim with the help of a rear pair of walking legs – they are elongated, forked and covered with hairs at the edges. The disturbed crustacean can swim about half a meter. While swimming, it presses its walking legs to its sides. Also, with the help of the rear pair of legs, the bathyasellus is able to dig into the silt – the animal stirs up the silt and plunges into it vertically, head up.
The front two pairs of walking legs differ in shape from the others – they are short and wide, covered with bristles on the inside. The most accessible source of food in the depths of the lake is silt – more precisely, the microflora inhabiting it. It forms the basis of the diet of the wormlike bathyasellus. The crustacean feeds by scooping up silt with its forelimbs.
Bathyasellus is a diecious animal. The male of this species is smaller than the female. He swims more often and more actively, looking for females and fertilizing them. Seasonality in reproduction in this crustacean is not pronounced. On average, every three to four months, a female of this species gives offspring. The female differs from the male by a wider posterior part of the body. She bears offspring in a brood pouch, which is located at the base of the rear pairs of walking legs. The fertility of bathyasellus is low – about 20 young animals in the brood. The length of the animal that left the parent brood pouch is about 4 mm. Young crustaceans immediately begin to hunt nematodes and rotifers living in the mud. On the two front pairs of walking legs, they have long pointed spikes, with which the animals hold their prey. As the spikes grow, they become thinner and more numerous, turning into an apparatus for silt scooping. When the intestine reaches the required length, young crustaceans begin to feed in the same way as adults.
Young bathyaselluses reach sexual maturity at the age of 3 months.

Baikal lipscraper (Brachycobitis pseudobotia)
Order: Cyprinid fishes (Cypriniformes)
Family: True loaches (Cobitidae)
Habitat: Baikal, coastal shallow waters.

Picture by Alexander Smyslov

Baikal is a rift, so the shores of the lake are relatively steep. The coastal shallow water zone is very narrow and rocky, and thickets of higher aquatic vegetation are extremely rare. Therefore, the shallow-water fauna is relatively poor, despite the considerable size of the lake itself. Storms reaching great intensity make additional difficulties for life in the coastal zone. However, the life exists in the shallow waters of Lake Baikal.
The coastal rocky shoals were inhabited by a small fish, a descendant of the spined loach (Cobitis taenia), which lived in Lake Baikal during the Holocene epoch. This fish feeds on the lower aquatic vegetation, scraping it off the surface of the stones, for which it is named as Baikal lipscraper.
The fish is well adapted to inhabit the surf zone of the lake. The body length of the lipscraper is about 10 cm. The body of the ancestor of this species was long and flexible, but such a physique is very unfavorable when living in the surf zone, where there is a high probability of injury from waves. The body of the Baikal lipscraper is short, relatively tall and muscular. The skin of the fish is scaleless, but covered with abundant slime. The caudal fin of the fish is shovel-shaped and has very hard rays. In case of danger, the fish buries itself in the sand and pebbles with the help of its tail. But usually it hides from predators in a permanent shelter. The Baikal lipscraper builds a well-fortified dwelling for itself – with the help of its tail, the fish digs a narrow long hole under a massive flat stone and constantly renews it. This is a reliable shelter not only from predators, but also from bad weather: during a storm, the Baikal lipscraper hides in a hole and attaches with its mouth to the stone roof of the hole. Scuds of a particular species often settle in the burrows of this fish.
The mouth of this fish is modified into a sucker and is surrounded by short sensitive wattles. The lipscraper feeds on lower algae and sedentary invertebrates, scraping them off rocks. This fish lives in shallow waters well illuminated by the sun, where it is easy for local feathered predators – herons and grebes – to spot it. To protect against them, the body of the fish has camouflage coloration – gray with transverse greenish stripes forming a “marble” pattern. Male and female have the identical coloration.
Algae and sedentary animals represent a kind of food that requires some time to resume. Therefore, like all fish species that feed on such food, the Baikal lipscraper is a territorial fish. Each individual occupies a part of the shore with an area of about 10 square meters, and protects it from encroachments of relatives. Since it is unsafe to use bright color signals in shallow water, the lipscraper warns relatives about its rights to the territory with the help of sounds. The fish meets the intruder with a series of abrupt clicks. If the stranger does not swim away, the owner of the territory emits a long series of sounds resembling the grasshopper’s chirping. After such a signal, the owner of the territory expels the trespasser with head rammings.
With the onset of winter, all territorial conflicts are forgotten: the fishes leave the shallow waters and swim away for the winter. At a depth of about 10 meters, they hide deep into the cracks between the stones, where they winter in large groups, numbering up to 20-30 fish together. At this time, the body of the lipscraper is covered with particularly slippery slime with high protein content. If a predator grabs a wintering fish, even a half-asleep fish has an opportunity to escape – it will simply slip out of the predator’s teeth.
When the lake is completely free of ice, overwintered fishes return to shallow waters. Usually, each fish easily finds its last year’s plot by smell and landmarks. Upon the return of fishes from wintering, the boundaries of the plots may be revised: not all fishes return, and in some places new neighbors appear. Hungry and weakened after wintering fishes avoid a direct collision, and prefer to frighten neighbors with sounds that are clearly audible from the shore at this time. When the weather becomes clear and it gets warmer, algae begin growing on the rocks. Fishes quickly regain their physical shape and prepare for spawning. Males attract females to their site by uttering single clicks. This fish spawns in pairs in a burrow, and immediately after spawning, the male drives the female away. He guards the clutch by fanning it with his fins, and cleans it of debris. At this time, the male does not even swim out to meet the trespassers of his territory, limited only to warning sounds. When the offspring hatches from the eggs, the male carefully cleans the young from the remnants of the egg shells and continues protecting it until the young fish begin to swim. After the fry leave the parent burrow, the male continues to lead a habitual life.
Young fish live mainly in the border territories of adult fish. They do not make permanent shelters and, in case of danger, hide in the cracks between the stones. In the second year of life, the Baikal lipscraper becomes capable of reproduction.

Pelagopike (Pelagoesox sphyraenoides)
Order: Pikes and allies (Esociformes)
Family: Pikes (Esocidae)
Habitat: Baikal, an area of open water far from the coast, depths from the surface up to 50 meters, some individuals descend to a depth of 100 meters.

Picture by Alexander Smyslov

In the human era, the northern pike (Esox lucius) was one of the characteristic native species of Lake Baikal. Its unpretentiousness and ability to live in various conditions allowed this fish to survive the epochs of anthropogenic pressure and severe glaciation. The descendant of the pike in the Neocene epoch is the pelagopike, a relatively large specialized predator, an inhabitant of the water column. This type of fish is at the stage of active development of the middle part of the lake, and is the largest fish in remote areas of the lake. The pelagopike’s body is adapted to fast swimming and rapid rushes for prey. It is arrow-shaped and muscular, a bit like the body of a barracuda. The pelagopike is distinguished by a strong but light build: a fish up to 2 meters long weighs about 8-10 kg. The fins are pointed; the unpaired fins are shifted back to the tail. The fish has long jaws and relatively large eyes, giving an almost complete circular field of view. The scales are small and rough (this feature allows it to suppress small swirls of water that slow down the body during rapid movement), covered with a layer of slime. The skin of a fish out of water has a prominent iridescent sheen until the mucus dries up. The color is silver with a marble bluish-gray pattern, masking the fish against the background of the water column. Deepwater individuals are colored darker than the inhabitants of the upper layers of water.
Pelagopike is an active predator. It hunts for small fish (no more than 20 cm long), and adults of this species often eat their own young. This feature of behavior constrains the number of species at a certain level, allowing to avoid overpopulation of the lake. In summer, the pelagopike can attack the chicks of waterfowl. Cautiously stalking to the brood under water, the fish catches the intended prey’s legs with one rush and drowns it. Adult waterfowl of small species can also fall its prey.
This predator is a solitary fish that tolerates its relatives only during the spawning, which occurs early in spring, as soon as the ice melts near the shores, and the underwater vegetation begins to give new shoots. Reproduction is the only stage of the life cycle that makes the pelagopike approach the shore. Spawning is paired, but dozens of breeding pairs of these fish accumulate in places suitable for breeding (shallow waters with underwater vegetation). During spawning, the male and female “stand” in the thickets for a long time, exposing part of their backs with fins out of the water. From time to time, the fish splash their tail and make sounds resembling grinding. During the climax of spawning, the male strongly presses against the female from below with his whole body, literally pushing her out of the water. At this time, the female spawns the eggs – there are up to 300-350 thousand eggs in the clutch of a normally developed adult one.
In the process of reproduction in this species, a clear transition from phytophile to pelagophile is noticeable. Eggs of pelagopike are not sticky, and their specific gravity is close to the specific gravity of water. Part of the eggs is washed away by waves and subsequently develops successfully in the water column, although here a significant part of them is devoured by pelagic scuds and other planktonic animals. Fry hatch in about a week. Juveniles soon begin to swim and switch to a pelagic lifestyle away from the coast. They live in the water column at a depth of up to 2 meters, feeding on planktonic crustaceans, swimming worms and juveniles of other fish species. Some fry die during storms, but due to the great fecundity of fish, these losses do not harm the population. One-year-old pelagopike adolescents about 10 cm long are active cannibals. This fish reaches sexual maturity in the third year of life, although active growth continues until the age of five. Life expectancy reaches 60 years or more.

Blind bathylota (Bathylota caeca)
Order: Gadiformes (Gadiformes)
Family: Codfishes (Gadidae)
Habitat: Lake Baikal, deep water layers, lake bottom.
Many lakes in tropical areas have lifeless depths poisoned with hydrogen sulfide. Against their background, Baikal seems to be a fortunate exception – this deepest lake on the planet is suitable for life from the surface to the bottom. At the bottom of this lake, a peculiar freshwater deep-sea fauna has developed, among which the blind bathylota, a large predatory fish, occupies the place of the top predator. The name of the fish means “deepwater burbot”: this species is a descendant of the burbot (Lota lota) – one of the common fish of Eurasia in the human era.
In fact, the bathylota is twice a result of the influence of the ice age. For the first time, burbot, its ancestor, moved to life in fresh water during the Quaternary glaciation. The second, Holocene-Neocene glaciation caused short-term adaptive radiation of the cold-loving burbot in Lake Baikal, and the settling of one of its descendants in the cold depths of the lake. Living in conditions of eternal darkness and cold, the blind bathylota is active all year round. This fish never rises to the surface and is found in Lake Baikal from a depth of 250-300 meters to the very bottom. This fish prefers horizontal sections of the bottom for life, so it settles at the rocky “terraces” of the coastal slopes of the lake, as well as at the very bottom.
The body length of the blind bathylota reaches 120 cm. In its appearance, this fish somewhat resembles the deep-sea Macrurus fish, known in the human era. The body of the bathylota is larger and more massive in the front part, but sharply tapers to the tail, which is about two-thirds of the total length of the fish. Individuals from the “terraces” of the coastal slope are distinguished by a lead-gray color with a small number of blurred spots on the back and sides, and specimens from the bottom of Lake Baikal are pale, as if chemically discolored, with translucent skin through which the insides are visible.
The head of the bathylota is large in relation to the body. It is flat, with a wide mouth and completely reduced eyes. Individuals from the bottom of the lake have two darker spots on their heads in the places where the ancestor of the bathylota had eyes, but in fish from coastal “terraces” these spots are absolutely indistinguishable against the background color of the body.
The ventral fins of the bathylota are shifted forward on the throat, and their bases are attached to the lower jaw. Such a peculiar feature of the anatomy is associated with the way the fish moves. The bathylota swims reluctantly, but is able to move quite quickly, “walking” on the ventral fins along the bottom and pushing them alternately or simultaneously. Despite its large size, the fish moves easily in this manner: its neutral buoyancy is provided by a large fatty liver, which makes up about a quarter of the body weight.
The unpaired fins of the bathylota are reduced: the dorsal, caudal and anal fins merged together, bordering the fish’s tail with a narrow strip. The pectoral fins, on the contrary, are relatively broad and have fleshy bases. With their help, the fish swims from one “terrace” to another and arranges a short chase for prey, leaving the bottom.
In the depths of Lake Baikal, food is very scarce compared to the surface layers of water, so all the inhabitants of the depths of the lake are non-specialized and indiscriminate predators or scavengers. Bathylota is no exception among them. It is a successful active predator, but if possible, the bathylota easily switches to carrion feeding. The vision of this fish is completely lost, but it is completely compensated by other senses – tactile and chemical sense. The chin wattle, characteristic for codfishes, is well-developed, fleshy and mobile, about 10 cm long. The fish searches for prey at the bottom of the lake with the help of smell and touch, and with the help of lateral line organs detects the presence of moving objects in the water. The sensitive surface of the nostrils of the bathylota is significantly expanded due to the leathery valves developing above them. Bathylota’s food includes worms, crustaceans, small fish and small carrion of any kind. This fish is a cannibal, and most of the juveniles of this species die in the mouths of adult fish. The phenomenon of cannibalism makes it possible to restrain the number of bathylota at a relatively constant level without harming the populations of other inhabitants of Lake Baikal.
The seasonal cycles in the bathylota from the bottom populations are not expressed, and it can spawn at any time of the year. Individuals from the coastal slopes feel the change of seasons and prefer to spawn in winter. This fish is a solitary predator, so a meeting with an individual not ready for reproduction can be life-threatening. Ready to spawn, males emit a special odorous substance, and females search them, being guided by it.
Bathylota is characterized by a pronounced parental care. The male of this fish is larger than the female, because he has parental responsibilities: he bears eggs in the mouth cavity. The clutch of the bathylota consists of 500-600 thousand very small eggs, which the male incubates for 3-4 weeks without feeding at this time. The young immediately after hatching leaves the male and leads an independent life. The first days of life, young bathylotas passively drift in the current, which takes them to the upper layers of the water. Here the larvae grow and gradually turn into actively swimming fry. At the age of about three months, young bathylotas begin to descend into the depths of the lake. At this time, they often fall prey of larger cannibal relatives. Sexual maturity occurs at the age of about 10-11 years, with a length of 60-70 cm.

Baikal false oilfish (Xenocomephorus fragilis)
Order: Percoid fishes (Perciformes), suborder Gobies (Gobioidei)
Family: Odontobutidae (Odontobutidae)
Habitat: Lake Baikal, water column away from the coast, depth about 100-700 meters. It can migrate to the water surface at night.
In the human epoch, one completely undesirable settler – a Chinese sleeper (Perccottus glenii) – appeared in Lake Baikal. Thanks to human activity, this migrant from the Far East spread widely across the fresh water bodies of Eurasia, and the highest ecological plasticity allowed him to survive easily the global ecological crisis at the boundary of Holocene and Neocene. As a result, there are many descendants of this unpretentious fish in Eurasia. The descendants of Cinese sleeper displaced the endemic species of deep-water sculpins (Abyssocottidae) from Baikal, and even mastered life in the water column. Before them, the unique freshwater deep-sea fish of Lake Baikal were Baikal oil fish (Comephorus) – delicate fish of fragile build with long wing-shaped fins. The descendants of the Chinese sleeper gradually displaced the oil fish in the lake and formed their own species of pelagic fish, similar in ecology to the oil fish.
The Baikal false oilfish vaguely resembles the true Baikal oilfish, but this similarity is the result of convergent evolution. It is a nearly blind fish about 30 cm long, of which a flattened head with long jaws makes about a third. The small eyes of the false oilfish, directed upwards and slightly to the sides, can distinguish light from darkness, and recognize the outlines of large objects. This fish spends the day in the depths of the lake, and never sees sunlight. Only during storms on Lake Baikal, dying fish remain on the water surface during the day – this fish really suffers from seasickness and becomes helpless. But such fishes quickly fall prey of various predators.
The body of the Baikal false oilfish is covered with a thin grayish-pink skin with a thick layer of slime and reduced scales. The musculature of the fish is weak, flabby and loose, soaked in liquid fat. The false oilfish swims slowly and spends most of the time passively floating in the water column. The abundance of fat in the body gives this fish additional buoyancy.
The pectoral fins of the fish are very wide and fan-shaped, with a thin delicate membrane. The fish floats in the water column, stretching them wide, and only if necessary folds them and swims like other fish, moving its thin tail equipped with a rounded tail fin.
False oilfish keeps in schools numbering up to 100-200 fishes. During the day, these fishes hide from numerous near-surface predators in the water column, at a depth of 200-500 meters, where none of the large predators of the lake can dive. But at night, false oilfish schools swim up almost to the very surface of the water for feeding. This fish does not tolerate warm water, so in the summer it stays at depth even at night. But in winter and spring, the false oilfish swims up to the very surface of the water, under the ice.
This fish has not changed the food preferences of its ancestor: the false oilfish is a predator and feeds on a variety of small animals. It usually eats crustaceans, pelagic worms and fish fry. The jaws of the false oilfish consist of many pairs of bones connected by an elastic membrane and extensible ligaments. During the capture of prey, they stretch out, forming a kind of tube, sucking a small animal into the mouth of the fish. There are many thin spikes growing on the branchial arches, directed into the pharynx and helping to hold slippery and smooth prey. In the front part of the jaws long thin hook-shaped teeth grow. After capturing and swallowing the prey, the false oilfish neatly folds its fragile jaws, avoiding their damage.
The ventral fins of the false oilfish, shifted to the throat, have a special structure. They consist of split rays and almost lack membranes. Only at the base of the rays there is a small patch of the membrane. The male has longer ventral fins than the female – their tips reach to the base of the caudal fin.
The “true” Baikal oilfish was a viviparous fish, but its Neocene analogue reproduces in a way more traditional for fish, by spawning eggs. Reproduction of the Baikal false oilfish takes place at the end of summer or the beginning of autumn. The ancestors of the false oilfish were coastal bottom-dwelling fishes and bred by spawning eggs in shelters. However, their descendant is not connected to the bottom of the lake at any stage of the life cycle. The false oilfish female spawns a large amount of small eggs, which she pastes in a bunch on the belly and abdominal fins of the male. At the same time, the male ejaculates soft roe, fertilizes the clutch, and then bears and protects it for three months. Shortly before the spawning season, the male grows thin hair-like outgrowths on the rays of the abdominal fins, which serve to hold the eggs. Males with eggs unite in separate flocks, and hover slowly in the water column, without giving away their presence with unnecessary movements. To protect themselves from enemies, they produce a bitter substance that permeates their muscles, and their slime gets an unpleasant smell.
Offspring develops slowly, and hatch only by the beginning of winter. At this time, males with clutches stay in the upper layers of the water and the young hatching from the eggs immediately begin to feed on small crustaceans. The fry of these fish lives in the plankton for the first six months of life. Young false oilfish reaches the size of an adult fish in the third year of life, but begins spawning at the age of two years. Holocene-era Baikal oilfish died after the first spawning, but the Neocene false oilfish reproduces several times during its lifetime.

Baikal hard-beaked dolphin (Delphinapterops odontorostrum)
Order: Cetacea (Cetacea)
Family: Narwhals (Monodontidae)
Habitat: Baikal, surface water layers away from the shores.

Picture by Alexey Tatarinov
Colorization by Carlos Pizcueta (Electreel)

Throughout most of the Cenozoic, cetaceans were specialized aquatic mammals occupying one of the upper stages of the food pyramid. However, human economic activity has affected their existence in a negative way. By the twentieth century, large whale species were subjected to predatory extermination, and throughout the entire subsequent period of human existence, their populations remained very small. Dolphins were seriously affected due to irrational fishing during the period of anthropogenic pressure, and their populations also declined. The “planktonic catastrophe” at the boundary of Holocene and Neocene caused the extinction of most cetaceans, summing up the natural result of the trends that were established in the human era. Only a few species of dolphins survived from the entire order. They survived in fresh waters, which productivity did not decrease during the global ecological crisis. In Neocene, the number of Cetacea order remained as small: having adapted to life in fresh water, the dolphins of Neocene lost the ability to live in the sea, and therefore the order is represented by only a few species with a very limited range. Neocene is the time of decline of this order.
A lucky accident helped one polar cetacean species to survive. Glaciers advancing from the north blocked the flow of the Yenisei and pushed some of the marine life into the mouth of the river. Along the edge of the glaciers, lakes with cold fresh water from the river runoff and meltwater of the glacier had formed on the West Siberian Plain, where many cold-loving fish lived – salmon, whitefish and some others. Some of the marine animal species that were pushed into fresh water became extinct, but some species managed to adapt to life in fresh water. Among the captives of the glacier, there was one cetacean species – beluga whale (Delphinapterus leucas). This cetacean species even in the human era was remarkable by the ability to live more or less long in fresh water. Thanks to this feature, beluga whales easily adapted to life in fresh water and populated the upper reaches of the Yenisei and its tributaries. These cetaceans soon reached Angara, from where they got directly into Lake Baikal. Beluga whales have literally found a new homeland in Baikal. They quickly displaced the small Baikal seal and firmly took the place of the top predator of the lake ecosystem.
The Neocene descendant of the beluga whale, the Baikal hard-beaked dolphin is a medium-sized cetacean. The female of this species is up to 3 meters long, the male is about a meter longer. These animals are smaller than their marine relatives and have retained some of their juvenile traits in appearance – this is due to the fact that the resources of the lake are quite limited, despite its size. The thick skin of the Baikal hard-beaked dolphin is gray-blue in color (the adolescent beluga whales have approximately the same color of the skin). The pectoral fins of the animal are wide; there is no dorsal fin. In the absence of large natural enemies, the Baikal hard-beaked dolphin has become a relatively slow-moving cetacean species.
A large fat melon grows on the forehead of this animal, especially strongly developed in males. Its rear part is penetrated by a system of air sacs that allow animals to emit a variety of sound signals. Communicating with each other, animals use sounds similar to creaks, clicks and chirps of various frequencies. The fat melon also helps dolphins to use sounds, but its purpose is different. This is a kind of “sound lens”: it focuses the sound and directs the sound wave in a narrow beam forward from the head of the animal. The Baikal hard-beaked dolphin stunts fish with “shots” of ultrasound – so did its distant relatives in the seas of the Holocene epoch.
Sexual dimorphism in this species is expressed not only in size. The development of teeth in males is remarkable: one pair of very thick conical teeth grows in the front of both jaws. Their roots are brought together, and the teeth form a kind of “beak”. With its help, the animal chews through thin ice. Also, this dolphin breaks through the ice with a blow of the forehead: the skin on the head is very thick and insensitive. The teeth are used by males during the mating season – the animals inflict bites and blows on each other. The ramming blow of the “beak” could break the ribs of a land-dwelling animal of similar size, but due to the thick hide, males do not receive noticeable damage from this method of establishing hierarchy.
In winter, the Baikal hard-beaked dolphin stays in the middle part of the lake, which does not have time to freeze. However, some individuals live in coastal areas, avoiding large congestions of relatives. This is due to the fact that there is clearly not enough food in the central areas of the lake in winter, and the animals wintering in these places lose weight more by spring. Coastal residents are forced to constantly break through the ice during the winter and keep the holes, through which the whole school breathes.
The Baikal hard-beaked dolphin is a social animal. An adult male has a harem of 4-5 adult females. In winter, such harems can unite into schools numbering up to 30 individuals – this makes it easier to maintain a network of breathing holes. But all the same, individuals of the same harem stick together even within a large school, and do not allow strangers into their “society”. Young bachelors gather in schools of the same age, young females also form separate groups.
Pregnancy in these cetaceans lasts about 8 months. In the first half of summer, a female gives birth to one large cub having almost black skin. This color unmasks it against the background of water, so for hunting, harem females leave their offspring in the care of one of them. By winter, the cubs acquire the color of an adult and reach a length of about 2 meters. A young animal becomes capable of reproduction at the age of 3 years. Life expectancy is about 40 years.