Siberian "Freshwater Australia"

There are tens of thousands of lakes on our planet. Most of them are rather small and shallow, young, with the age usually not exceeding 100—1,000 years, and inhabited by usual, widely spread forms of animals and plants. However, some 10 lakes are well known to all the biologists studying the diversity and evolution of freshwater forms of life. These lakes are called ‘ancient’ to emphasize, first of all, their ancient origin and long history.

The most famous among the ancient lakes are: Baikal (Siberia), Tanganyika, Victoria, Malawi and other lakes of the Great African Rift Zone, Biwa (Japan), Ohrid (Macedonia, Albania), Caspian sea (Eurasia), Khubsugul (Mongolia), and Titicaca (southern America). The second in importance characteristics of these lakes, after the age, is an enormous and extraordinary diversity of their fauna and flora. It is noteworthy that in terms of biodiversity Lake Baikal is the world leader among the ancient lakes.

Creation of Uniqueness

The Baikal depression is the central part of the Baikal Rift Zone stretched for more than 2,000 km from Lake Khubsugul to southern Yakutia. Today the lake consists of three — southern, central, and northern — ponds divided by natural barriers.

Baikal is the region of high neotectonic activity. Last volcanoes died away here only a few thousands years ago. Over the last 150 years more than ten earthquakes with the magnitude varying from 8 to 11 took place here. One of the most severe was the Tsagan earthquake that occurred in the middle of 19th century in the Selenga River delta: the area of about 200 square kilometers went underwater overnight. Geologists note that the lake hollow widens and deepens annually for 20 and 6 mm, respectively.

The geological history of Baikal can be presented as a sequence of lakes-predecessors that existed throughout the Cenozoic. Following the data of paleolimnological studies, Baikal got its contemporary form ‘only’ a few millions of years ago. Apparently the entire superdeep lake was formed as a result of the recent tectonic activity less than one million years ago.

Thus, one can assume that the uniqueness of Baikal fauna and flora is caused by a long-term continuous formation process, from the existence of several reservoirs-predecessors to their final joining that resulted in an integral reservoir, and finally by intensive tectonic activity, which led to the formation of geographical barriers, sharp changes of environment conditions, etc.

Biological records ‘book’

According to contemporary calculations, 2,595 animal species and subspecies inhabit Baikal (Timoshkin, 2001). The number is practically twice as high as that for the closest competitor of Baikal, Lake Tanganyika. Furthermore, the list of Baikal fauna is incomplete and far from being completed. During the last 7—10 years only the Laboratory of Hydrobiology and Systematics of Aquatic Organisms (Limnological Institute of the Siberian Branch of the Russian Academy of Sciences) managed to open and describe on the average more than 20 species every year! There are plenty of reasons to believe that in the future the total number of Baikal animal species can reach 3,500 and plants, 1,500 species and subspecies.

Already now the specific diversity of Baikal fauna is comparable to that of hundreds and thousands of Eurasian lakes taken together. Considering the fact that about 60 % of Baikal animal species are endemic (i.e., are not met elsewhere), it is logical to conclude that Lake Baikal is the true center of biodiversity of the Eurasian continent, scientific importance of this fact cannot be overestimated.

Opening of new animal and plant species is as honorable as opening of new lands or galaxies. However, there are only a few places on Earth that offer similar taxonomic puzzles. This is not typical of freshwater faunal complexes, but Baikal is unique in this respect as well. The taxonomic ‘trophy’ of some zoologists from our institute amounts to as much as 100 to 150 discovered species for each!

The lake is a kind of the Guinness World Records Book, but these are the records held by nature. In addition to geological, paleoclimatic, and hydrological ‘achievements’, Baikal broke lots of biological records. Among the giants living in the lake there are the world largest representatives of free-living flatworms. Some species of bottom amphipods living at great depths are real giants, as, for example, Acanthogammarus. In their brood chambers, one can often find an unpleasant parasite, Pachyschesis. This is a unique case of parasitism of one amphipod species on another found in fresh waters.

Lately not only new species and genera, but even new communities of organisms, which are atypical of fresh waters, were found in Baikal! For example, ciliopsammon is the most interesting community of infusorians, specialized inhabitants of sandy soils. Other examples are the community of organisms living around underwater hydrovent (hydrothermal source) or cryophilic community living in the ice. The list of such biological records would make a volume of a small book.

It is absolutely impossible to encompass all data on the faunistic diversity of the unique lake ecosystem in a short article. We will consider only some most remarkable groups, which illustrate the whole spectrum of the classical problems concerned with the origin of unique diversity of Baikal fauna.

Sponges

Two families of sponges live in the lake: Spongillidae that include the species widely distributed almost worldwide and endemic inhabitants of open Baikal Lubomirskiidae.

From the species perspective, freshwater sponges belong to a few species, which is valid for Baikal as well, 14 species of Lubomirskiidae and 2 species of Spongillidae have been found here (Efremova et al., 2004). Notwithstanding the relative fewness of species, sponges are among the most widespread and unusual groups of Baikal organisms. They are an integral component of the biocenoses of stony soils at depths of up to 600 m. The flattened, spherical and especially treelike colonies of sponges shape unforgettable underwater landscapes in the near-shore zone of the western part of the lake. Green Lubomirskia baicalensis reaching 1 m in height looks like underwater bush.

Sponges are found practically everywhere in Baikal. Flattened colonies are abundant in the littoral or wash zone. The huge bright green spots of the sponges with the area of up to one square meter cover the surfaces of underwater rocks. Looking at the Baikal sponges, one could hardly believe that these are animals, though they have amazing tree-like shape and color spectrum: all shades of green, sometimes with a violet or pinkish shade, ivory and white coffee… There are sponges shaped as balls, spherical individuals that are freely rolling over the surface of sandy soil, in the Maloye Sea.

Sponges themselves are the habitat for many animal and plant species. Some species of amphipods gnaw out small ‘caves’ in the body of sponges; parasitic Brandtii ‘stick’ in clusters on the body of branchy Lubomirskii. Many species of flat worms, nematodes, and oligochaeta find ‘shelter’ in the bases of sponges. Large gastropods Megalovalvatae absorb the nutritious ‘cocktail’ of filtered fossils fastened by mucous discharges of the sponges from the surface of flattened sponges. After such a ‘cleaning’ sponges look like a well-groomed lawn.

Regardless of the fact that Baikal fauna has been studied for over 150 years, there are many gaps in our knowledge about Baikal sponges and many riddles have yet to be solved. So far their life cycles, reproduction terms, and even life expectancy have not been explored.

Flatworms

Endemic flocks of species of free-living flatworms (Turbellaria) are a very unusual phenomenon for freshwater ecosystems. Most large lakes of the planet, including the ancient ones, are populated with trivial, widespread species of Turbellaria.

The diversity of Baikal Turbellaria is unbelievable. All orders of free-living flatworms, except for typically marine orders, are available here: 8 orders and suborders, including 11 families and subfamilies, 41 genera, 164 species and subspecies. Most of them — about 73 % of genera and 99 % of species — are not found elsewhere on the planet! This suggests that thus far not more than 2/3 of the real number of Baikal Turbellaria species have been described. The numerous, diverse, and endemic Turbellaria fauna is indeed a unique biological phenomena for freshwater lakes.

The majority of Turbellaria species (more than 70 %) lives in littoral and sublittoral parts of the lake, where the worms are an indispensable and rather numerous component of many biocenoses. They have mastered practically all layers of the lake waters: representatives of different families of Turbellaria were found at the depths ranging from 380 to 1,620 meters! Deep-water species have slightly depigmented bodies and reduced eyes, some species have no eyes. Turbellaria is usually found in marine ecosystems, the availability of deep-water Turbellaria forms in Baikal is indeed a unique phenomenon for freshwater reservoirs.

Turbellaria, as one of the central and most interesting groups of Baikal invertebrates, can serve as a model for studying speciation and evolution processes, including the origins of the unique fauna of the lake (Timoshkin, 1994). For example, the presence of giant and dwarf forms, specialized deep-water forms and the groups that are supposed to be of sea origin, etc., in the Turbellaria fauna should enable the scientists to trace the most probable ‘scripts’, according to which Baikal invertebrates developed.

Oligochaeta

Oligochaeta are the invertebrates that belong to the most species-abundant groups of Baikal organisms. According to contemporary data, there are 204 species and subspecies of oligochaeta in the lake, 82 % of them are Baikal endemics (Semernoi, 2001; 2004).

Oligochaeta live on all soils, but prefer soft silts. They can be found even at maximum depths. Baikal oligochaeta have rather uncommon morphology: some of them are very large, others are very small, some have lively ‘tiger-skin’ color, others (Rhynchelmis) shimmer with rainbow colors… The length of some of them (for example, Rhynchelmis brachycephala) reaches 185 mm! Mature individuals of other species can be only 3—5 mm long.

The look of Baicalodrilus worms is astonishing. Their body is covered with numerous ‘wart’ excrescences and they have a back crest reminding that of a peacock or a miniature prehistoric dinosaur. In some species, excrescences are covered by crystalloids. Thus, only hardly appreciable setae and segmented body show that this creature is a relative of familiar earthworms.

As in other reservoirs, Baikal oligochaeta are favorite feed for many invertebrate and fish species.

Amphipods

Based on the number of species, amphipods Gammaridae are considered one of the largest species of multicellular Baikal organisms (Takhteev, 2000, Kamaltynov, 2001): they number 347—348 species. The lake hosts one fifth of the total number of the world gammarid species, all Baikal species being endemic.

Baikal gammarids are so variable and diverse that scientists are still discussing the number of families these species belong to. The problem of their origin is also the object of constant attention of scientists as it is difficult to believe that such a fantastic diversity of kinds could have originated from one ancestor.

Among amphipods there are predators, phytobenthos-eaters, corpse-eaters, cannibals, detritus-eaters, and many other groups with specialized ‘predilections for gourmet food’. The appearance of some of the most interesting Baikal gammarids is rather uncommon. Acanthogammarus has large pointed keels on both sides of the body. It is supposed that, in addition to protective purposes, in floating the keels have functions similar to those of wings. Some species of these genera are among the largest gammarids not only in Baikal, but also in other freshwater reservoirs. These huge crawfish are spread at great depths; the species living deeper have colorless body and smaller eyes. The catch of one deep-water draught yields several dozens of kilograms of the giant amphipods. Inexperienced biology students anticipate slaking of their hunger while boiling the crawfish in salty water. But what a disappointment! The body meat of the appetizing large crawfish weights only 2—3 grams. At least it tastes like crabmeat.

Macrohectopus (M. branickii) is a translucent large shellfish, a unique freshwater gammarid species that lives exclusively in the water depth. ‘The fair sex’ in Macrohectopus has almost 4 times longer body than the ‘strong sex’! It is also interesting that male individuals of this species are to a great degree vegetarians, while females are predators.

One more original amphipod — Spinacanthus parasiticus — is a symbiont of the branchy sponge Lubomirskia baicalensis. However, it can be called amphipod only with great reserve. First, it swims rarely and very clumsily; second, its body is always, even when it swims or moves over the sponge, in a strictly vertical position, not on one side.

Very unusual, though unattractive, is the appearance of the representatives of the genera of Ommatogammarus. These corpse-eaters with glossy ivory ‘block heads’ are not too fussy; they eat up the corpses of Baikal fish leaving bare skeletons behind.

Gammaridae is one of the dominating groups of the majority of the bottom communities; the species quite often dominates in the planktonic communities as well. They are also the basic food for the well-known Baikal fishes — omul and golomyanka — and even for seals. Thus, the importance of amphipods for the lake ecosystem can hardly be overestimated.

The origin of the Baikal fauna: a bunch of hypotheses

One of the most interesting and actively discussed problems is the age of Baikal fauna, its origin, and zoogeographical relations. In the early XX century three basic hypotheses of its origin were advanced on the basis of comparative-morphological (anatomic) analysis of various systems of organs. Only in the second half of the 20th century taxonomists started using a wider spectrum of research methods, including cytogenetic, biochemical, comparative embryological, etc.

The first hypothesis was elaborated by L. S. Berg (1910, 1922, etc.), who emphasized ancient and freshwater origin of Baikal organisms. G. Yu. Vereshchagin (1935, 1940), who advanced the second hypothesis, also considered the Baikal fauna as very ancient but supposed that it had marine ancestors. And, finally, V. C. Dorogostaisky (1923), and later D. N. Taliev (1955) and E. I. Lukin (1986) assumed that the lake fauna was basically young and actually there were no groups in its structure that could be called relic.

Vereshchagin’s hypothesis about the marine origin of most Baikal animals was disproved by further studies of Baikal fauna. The influence of marine ancestral forms on the Baikal animals could not be essential, as the lake had not been directly connected with any sea since the beginning.

The hypothesis about relatively recent marine origin proved to be true only with respect to one of the most known representatives of Baikal vertebrates, the seal (Kozhov, 1962; 1972; Lamakin, 1964, etc.). Taxonomists still have to revise the status and specific affiliation of Baikal omul, as the hypothesis about a relatively recent penetration of omul ancestors into Baikal from the Arctic Ocean has not been supported by molecular biology data (Sukhanova et al, 2000). Zoogeographical relations of some other Baikal organisms, whose close relatives live in the seas and salt waters, are still not clear. As yet one cannot completely deny some influence of marine ancestors on the formation of the modern Baikal fauna.

The abundance of endemic taxa, their very original appearance, unusual internal structure of many endemics, and at last the ancient age of the lake made the hypothesis on the ‘youth’ of Baikal fauna the least attractive for biologists. Nevertheless, endemic Baikal sponges are the first group of ‘classical’ relicts whose relic nature was disproved (Efremova & Gureeva, 1989; Efremova, 1994). It appeared that the most known group of Baikal animals, sponges Lubomirskiidae, actually were not relic, they originated from cosmopolitan families of freshwater sponges Spongillidae. This conclusion is based on the data on morphology and embryogeny, as well as on the ultrastructure of cells that are similar in endemic and cosmopolite sponges.

Molecular biologists come to the rescue

Surprising changes in the conventional notion of the age of different groups of Baikal fauna were made by molecular biology. During the last ten years many groups of Baikal animals and plants became the objects of molecular phylogenetic studies. Although there are certain differences in the age scales for the same species flocks estimated on the basis of genome comparison, one can make the following conclusion: the age (i.e., the time of existence since the divergence from the common ancestral forms) for the majority of considered endemic faunal groups, as a rule, is much less ancient than the age of the lake postulated by geologists.

First, several independent research teams confirmed the hypothesis on the young age of Baikal sculpins (Slobodyanyuk et al., 1994; Bowmaker et al, 1994, etc.). Even the most specialized among them — golomyanka (Comephoridae) — is relatively ‘young’ (not older than 1.5 million years). Then, the data on a relatively young age of most endemic genera of flatworms planaria (Kuznedelov, Timoshkin, Kumarev 1996) and mollusks Baicaliidae and Benedectiidae (Zubakov et al., 1997) were obtained. D. Yu. Sherbakov (Sherbakov, 1999) believes that the common ancestor of all modern Baicaliidae existed not earlier than 3 million years ago.

Thus, endemic Baicaliidae appeared to be at least several times younger than the lake. To reconcile the new estimates with the popular opinion on the relic character and antiquity of mollusks and to explain the succession of modern mollusk fauna to the well-known paleofauna of the Tankhoi sunk, the assumption was made on the period of the mass but not total extinction of Baikal gastropods (Zubakov, 1999).

The idea about the ‘nonrelic’ nature of Lubomirskiidae sponges was also confirmed using molecular-biological methods (Itskovich et al, 1998). It is amazing that the sites of the analyzed gene in Lubomirskiidae over about 630 nucleotide base pairs differ from the same gene site in Spongillidae by only one replacement! The number of replacements in some other groups of animals suggests intraspecific (occasionally — intergeneric) distinctions but not ‘interfamilial’ distinction.

Thus far only four faunal groups have been defined as ancient based on the results of molecular biological studies: gastropods from the genera Choanomphalus, almost all gammarids, chironomids of the genera Sergentia, and some of oligochaeta-Lumbriculidae. Of the two flocks of species of Baikal Lumbriculidae studied by molecular-biological methods, the first claimed to be ancient (15—18 million years) consists of several species only (Kaigorodova et al, 1997), whereas the second that unites the majority of modern endemic species of this family is young (‘only’ 2—3 million years).

Parallel history

One can easily notice that the age estimates of the start-up of the divergence of various endemic groups based on the molecular-biological data differ much from each other and vary within one to several tens of millions of years! At the same time many endemic families are apparently of relatively young or even very young age. This suggests that the deep and original morphological transformations underlying the separation of the families could take short time. On the contrary, the time of existence of some closer taxonomic groups (united by morphologists in genera) is estimated as millions, even tens of millions of years!

Attempts to superpose the above age estimates with available paleoreconstruction of Lake Baikal (Popova, et al., 1989) yielded more questions than answers. It’s a paradox, but sometimes it seems that the geological and biological history of lake Baikal developed independently of one another.

For example, if we admit that Baikal became ultradeep only about 0.80—0.15 million years ago, this will comply well with the posited beginning of the divergence for golomyanka fishes. Did the deepening of the lake impact Macrohectopus, the shellfish inhabiting water depths, if the age of family Macrohectopodidae is comparable with the age of Baikal (Kamaltynov, 1999)? We can admit that Macrohectopus originated from the same ancestor as the lake gammarus (Sherbakov et al., 1998). But why did it appear in Baikal and nowhere else (we know that for tens of millions of years Baikal was of the same depth as many other lakes)? The answer to this question, as well as to many others, is still unavailable.

Evolution through accidents?

Summing up, we would like to note that the natural difference in the age estimates suggests different time of appearance of ancestral forms in Baikal or at least different geological time at which one or another group reached its fullest flower or became endemic. This means that Baikal ecosystem developed dynamically throughout all its geological history. Some faunistic groups were replaced by others, the structure of the whole biocenosis varied essentially in parallel with the periods of explosive speciation in one or another group.

Undoubtedly, there are still many blank spots in our insight into speciation laws, and the age estimates based only on the ‘molecular clock’ hypothesis should not be considered as the only true. The problems of origin and age estimates cannot be solved similarly for all groups of Baikal endemics: each endemic group of the lake could have its own, sometimes, unique evolution history.

To explain the paradox between the ‘youth’ of many faunistic groups and the ancient age of the lake, most scientists use the traditional and well-known approach suggesting that all global geological cataclysms lead to total extinction of fauna and emptying of many ecological niches. Accidents are considered as one of the principal causes of ‘explosive’ speciation.

Geological catastrophes should have led to the change of thermal and gas regime of the lake, formation of poisonous (for example, hydrosulphuric) or anoxic (oxygen-free) zones on the bottom, etc. However, no authentic paleolimnological indications of such phenomena have been found till now and the fact of occurrence of the disasters within the scales of the lake has not been proved. Recently the evidence, which confirmed that small parts of the lake bottom probably had been subject to anoxic effect, was found, but the phenomenon was rather local and never covered the whole lake.

A formerly ‘unpopular’ hypothesis about mainly young age of the Baikal fauna has been recently confirmed; therefore it should be considered as a quite realistic scenario of the origin of many groups of the lake organisms. It appeared that Baikal faunal complex and the modern widespread paleoarctic fauna are very closely connected with one another and the gap between them is not as deep as it was assumed before.

However, despite all the achievements in zoology, taxonomy, and molecular biology, instead of a harmonious theory of the origin of Baikal fauna we still have only a set of hypotheses. The work should be continued to get more facts and evidence that should shed light on the natural history and origin of the unique fauna of such an extraordinary natural phenomenon as Lake Baikal. It is clear already that the role of Baikal will increase not only in the lacustrine science worldwide, but the discoveries made on the lake shores will favor the development or oblivion of many common paradigms accepted in biology, geology, and other natural sciences.

References

Annotated List of Fauna of Lake Baikal and Its Catchment Basin. Vol. 1, book 1. Lake Baikal. Timoshkin O. A. (Ed.). Novosibirsk, 2001.

Annotated List of Fauna of Lake Baikal and Its Catchment Basin. Vol. 1, book 2. Lake Baikal. Timoshkin O. A. (Ed.). Novosibirsk, 2001.

Atlas and Identifier of Lake Baikal Pelagobionts. Timoshkin O. A. (Ed.). Novosibirsk, 1995.

Kozhov M. M. An Outline of Baikal Studies. Irkutsk, 1972.

Kozhov M. M. Lake Baikal and Its Life. Dr. W. Junk Publ., Weisbach and van Oye (eds.) The Hague. 1963.