Oligochaete fauna of Lake Baikal

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<ul><li><p>Hydrobiologia 218: 21-34, 1994. T.B. Reynoldson &amp; K.A. Coates (eds), Aquatic Oligochaete Biology V 0 1994 Kluwer Academic Publishers. Printed in Belgium. </p><p>27 </p><p>Oligochaete fauna of Lake Baikal </p><p>Ljudmila N. Snimschikova &amp; Tamara W. Akinshina Limnological Institute, Irkutsk ScientiJic Centre, Siberian Division of the USSR Academy of Sciences, Ulhan-Batorskaya Street, 3,664033 Irkutsk, Russia; 2 Scient$c Research Institute of Biology of Irkutsk State University, Lenin Street, 3, P.O. Box 24, 664003 Irkutsk, Russia </p><p>Key words: Oligochaeta, distribution, endemism, radiation of species </p><p>Abstract </p><p>Oligochaetes are one of the most abundant groups of invertebrates in Lake Baikal. They compose up to 70-90% of the biomass and numbers of zoobenthos and are distributed from the water edge to the maximum depth. There are 207 identified species and subspecies belonging to 44 genera and seven families, of which 160 species and 13 genera are endemic. Many are relict species. The main peculiarities of oligochaete fauna of Lake Baikal are: immiscibility of its ecologically different complexes (Palaearctic and Baikalian), its antiquity and heterogeneity, and intense radiation of the species. </p><p>Introduction </p><p>Baikal is one of the most thoroughly and com- prehensively studied lakes in the world but all the scientific achievements on Baikal are only a stage to a more profound knowledge of the Lake which is a remarkable natural phenomenon (Kozhov, 1963). The more thorough is the knowledge, the more riddles and secrets are opened up to the investigators by the amazing nature of Baikal. During the last decade interest in this Siberian inland sea grew not only in Russia but in the whole world. </p><p>The oligochaetes of Lake Baikal are one of the most abundant and complicated groups of inver- tebrates and play an active role in the ecology of the lake. However, up to 1987 the Baikal oli- gochaete fauna had not been studied as a whole, and no list of common species existed. Previ- ously, either individual families (Lumbriculidae, Naididae) or the fauna of separate regions of the lake (Selenga Region, north end, Maloye More) </p><p>were investigated to determine their oligochaete composition. Information accumulated during re- cent decades prompted Timm &amp; Finogenova (1987) to compile a list of aquatic oligochaetes of the USSR which included those of Baikal. A major work has also appeared on the origin and evolution of the Baikalian oligochaetes (Semer- noy, 1987). </p><p>We undertook the task of compiling a system- atic list and table of distribution of the oligocha- etes in Baikal which will be published in the new edition of the book Lake Baikal and its life (Kozhov, 1963). The information from this list is the basis for the present review. The limited size of our article here does not enable us to cite it completely (see Appendix 1). </p><p>Materials and methods </p><p>All the available literature sources on the subject were used. Insufficient data on oligochaete dis- </p></li><li><p>28 </p><p>tribution in Baikal are to some extent completed by the authors unpublished observations, ob- tained from the benthic collections made by the Limnological Institute and the Institute of the Bi- ology of the Irkutsk University. These collections were made from 1972- 1989 in South Baikal in the vicinity of Solzan and Bolshie Koty; in Middle Baikal in the straits of Maloye More and Olkhon Gate and in the Mukhor Gulf or Selenga Shoals: in North Baikal in the vicinity of Sosnovskaya Bank (Fig. 1). </p><p>Samples were collected with a Petersen bottom grab (0.10 m2 or 0.025 m2), with a dredge and a trawl. The animals were preserved in formalde- hyde or ethanol. </p><p>Results </p><p>Oligochaetes compose up to 70-902 of the bio- mass and numbers of individuals of bottom com- munities of the lake and are distributed from the waters edge to the maximum depth (1620 m). Near the shore in open Baikal, not less than 20- 30% of the total benthic biomass is oligochaetes. In silty-sands at a depth of lo-20 m about a half of the total biomass is oligochaetes. On silty- sands at a depth over 50 m, ohgochaetes form up to 90% of the biomass and occasionally the benthic population consists almost completely of oligochaetes. On Selenga Shallows, the most pro- ductive region of the lake (enriched by the organic substances brought down the River Selenga), the maximum benthic biomass is 360 g m2, the bio- mass of oligochaetes is 233 g m2 and their abun- dance is 14,460 individuals m2. In North Baikal, in the areas influenced by the rivers Verhknyaya Angara and Kichera, the benthic biomass is 50 g m2, the biomass of oligochaetes 28 g m2 and their abundance 20,870 individuals m2. In South Baikal in the vicinity of Solzan the average benthic biomass is 3 1 g m2, with the oligochaete biomass 17 g m2. The biomass of the benthos decreases to 5 g m2 with increasing depth, and the proportion of oligochaetes increases to 90% (Bekman, 1983; Snimschikova, 1987; Akinshina &amp; Lezinskaya, 1980). </p><p>The species diversity of the oligochaetes of Baikal is astonishing - up to 30 species per sample of I 1 m2. The species diversity is especially high in sandy-stony littoral (depth of 5-20 m), where 75 species and subspecies have been observed, and on silty sediment at depths of 50 m where 63 species and subspecies were identified, </p><p>At present 207 species and subspecies of oli- gochaete have been counted in Baikal from 44 genera and seven families, out of which 160 spe- cies and 13 genera are endemic. This does not complete the diversity of the Baikalian oligocha- etes. Study of the fauna is continuing and it is certain that new discoveries will be made. </p><p>The Lumbriculidae is represented in Baikal by 59 species and subspecies (this is one third of the whole worlds lumbriculid fauna) of which 5 1 are endemic. Lumbriculidae dominate the biomass in the abyssal. Among them are the largest Baikal species: Rhynchelmis brachycephala Michaelsen, 190 1, up to 185 mm long and Teleuscolex baicalensis (Grube, 1873) 75 mm, but dwarf spe- cies also occur, including Lamprodrilus achaetus Isossimov, 1962 and Lamprodrilus pygmaeus Michaelsen, 1902. Several truly abyssal species with specific morphological adaptations have been found: Lamprodrilus infatus Michaelsen, 1905, Lamprodrilus bythius Michaelsen, 1905. </p><p>Tubificidae are more diverse in Baikal than Lumbriculidae. To date 92 species and subspe- cies have been identified, 72 of which are en- demic. They are most abundant in shallow areas (Snimschikova, 1987). Many small-sized species were discovered among tubificids: Baikalodrilus exilis Snimschikova, (2.0-2.3 mm long); Baikalo- drilus kozovi (HrabC) (1.2 mm); Lymphachaeta pinnigera Snimschikova, (1.8-2.3 mm). Many tu- bificid species do not exceed lo-12 mm in size. Giant forms are Baikalodrilus crassus Snim- schikova, (32-35 mm) and Baikalodrilus discolor Snimschikova, (28-30 mm). Materials were ob- tained proving the eurybathic nature of tubificids. Some specimens from abyssal depths were de- scribed as Rhyacodriloides abyssalis, ce- kanovskaya, and others (cekanovskaya, 1975; Snimschikova, 1987). </p><p>Of Naididae 47 species and subspecies are </p></li><li><p>29 </p><p>Ushkaniy </p><p>Irkutsk Reservoir </p><p>Kaloye More Strait / </p><p>/rr Olkhon Island </p><p>say </p><p>-.- \ .p i ~{.:,.;yJaarguzin Bay </p><p>Fig. I. Schematic map of Lake Baikal. </p></li><li><p>30 </p><p>known in Baikal including 28 endemics. They mainly inhabit the coastal zone and only a few are found at depths (N&amp;s koshovi Sokolskaja, was found at a depth of 93 m). Of special interest is the discovery of Nais abissalis Semernoy, at a depth of 700 m (Semernoy, 1984). </p><p>Enchytraeidae is represented by 3 species (2 endemics), Propappidae by 2 species (1 endemic), Haplotaxidae by 2 species and Aeolosomatidae by 2 endemic species. The latter family has tra- ditionally been included in Oligochaeta, although Brinkhurst &amp; Jamieson (1971) withdrew them from this subclass and included it in Archianne- lida, while Timm (1987) placed them into sub- class Aphanoneura of the class Clitellata. De- scription of a representative of Branchiobdellidae in Baikal (Snimschikova, 1987) was erroneous, as it turned out to be a new endemic leech (Fi- nogenova &amp; Snimschikova, 1991). </p><p>There are few abundant species of oligochaetes that are found distributed over the whole lake. In the coastal-sor (Sot- is the local name for half- isolated shallow bays of Lake Baikal) zone and in the open Baikal the palaearctic species Propappus volki Michaelsen, 1915, as well as the endemic species Rhyacodrilus korotnej? (Michaelsen, 1905), Rhyacodrilus multispinus mul- tispinus (Michaelsen, 1905), Isochaetides arenarius (Michaelsen, 1926), Isochaetides baicalensis (Michaelsen, 1902) L. pygmaeus pygmaeus and Teleuscolex korotnefi Michaelsen, 190 1 are abun- dant. However, these species do not occur at depths over 260-300 m. Lycodrilides schizo- chaetus Michaelsen, 1905 and Styloscolex asym- metricus Isossimov, 1962 are often found in small numbers or individually in the sors, bays and open waters of Baikal. The above endemic species ex- cept L. pygmaeus pygmaeus have dispersed from Baikal and are found in the Angara River, which discharges from the Lake, while R. korotneji and L. schizochaetus are in the Yenisei River. Lam- prodrilus pygmaeus pygmaeus was introduced into the lakes of Karelia (North Europe) where it has formed populations by asexual reproduction (Popchenko, 1972). </p><p>In the open gulfs (Mukhor, Proval) and in the lake proper Tasserkidrilus acapillatus (Fino- </p><p>genova, 1972) is numerous. Its distribution is lim- ited to the Caspian Sea, Lake Peipsi and Lake Baikal. As well, the endemic L. achaetus achaetus, Lamprodrilus wagneri Michaelsen, 190 1, and Stylodrilus opisthoannulatus (Isossimov, 1962) are abundant in these habitats. Lycodrilus dybowskii Grube, 1873, is frequent but not numerous. These species do not appear to enter the sors although T. acapillatus and L. wagneri are known from Angara and S. opisthoannulatus is known from Yenisei. The above-mentioned species are found to depths of 500-860 m. </p><p>It is difficult to speak confidently about com- mon species of oligochaetes distributed over the whole Lake. It would be more correct to describe the common species of a certain region of the lake or even of a biocoenosis, for example, of the Se- lenga region or of the stony littoral. The lake re- gions differ considerably from each other in their hydrological and hydrochemical conditions, bot- tom sediment, underwater landscape etc. The biocoenoses of river mouth shoals which have predominantly soft sediments on relatively gentle slopes cannot be compared to the biocoenoses of the stony steep drops. Such areas differ consid- erably in their species compositions. Typical spe- cies of soft sediments in the Selenga region are 1. baicalensis, different species of Baikalodrilus, L. dybowskii, and L. wagneri. At the North Baikal where the biotopes are more diverse and the change of biocoenoses with increasing depth is faster, P. volki, T. acapillatus, Rhyacodrilus isossi- movi Cekanovskaya, 1975, Svetlovia maculata ce- kanovskaya, 1975, L. achaetus and species of Stylodrilus predominate. Still more different is the region of Bolshie Koty (South Baikal) with largely a sandy-stony substratum, where on the periphy- ton between stones Naididae (Nais baicalensis Sokolskaja, 1962, Nais tygrina Isossimov, 1962, Uncinais minor Sokoloskaja, 1962), small-sized Enchytraeidae of the genus Mesenchytraeus, P. volki, L. achaetus, L. pygmaeus, species of the genera Stylodrilus and Isochaetides, and L. shizochaetus are very abundant. </p><p>Differences in the ecological conditions de- crease as the deep water zone (250 m and more) is approached and the species composition also </p></li><li><p>31 </p><p>becomes more uniform. Palaearctic species dis- appear and subsequently the endemic species that are abundant at middle depths gradually disap- pear. Typical inhabitants of the abyssal zone are R. brachycephala, L. wagneri longus, T. baicalen- sis, R. abyssalis and B. discolor with subspecies. </p><p>Discussion </p><p>Together with the unusual species richness and high degree of endemism, the Baikalian fauna is defined by relative immiscibility of the two eco- logically and historically different complexes of which it consists. The coastal-sor zone is inhab- ited by a palaearctic species complex widely dis- tributed in Siberia while the open lake is inhab- ited mainly by the endemics composing the Baikalian complex (Wereschagin, 1935; Levani- dova, 1948; Kozhov, 1962). </p><p>Among the oligochaetes, in contrast to mol- lusts, amphipods, and other benthic groups, there is not a sharply defined immiscibility barrier be- tween the Palaearctic and Baikalian species. In sors, the Baikalian complex is represented only by its eurybiontic component but composes 63 % of the total species number (68 species and sub- species are accounted for in the Baikal sors and bays). On the other hand, the palaearctic species Rhyucodrilus coccineus (Vejodovsky), Rhyacodri- lus sibiricus Semernoy, T. acapillatus, P. volki, and Haplotaxis gordioides (Hartmann) have colonized the open Lake to depths up to 200-270 m. Nev- ertheless, the fact of immiscibility of the Baikalian and the Palaearctic complexes also occurs in this group. Over 10 palaearctic and cosmopolitan species inhabit only sors and do not venture into the open Lake. This is confirmed by distribution of the papillate tubificids. The palaearctic Spirosperma ferox Eisen distributed in the North- Baikalian Sor could be transported by undercur- rents of the rivers, Verkhnyaya Angara and Kichera into the North Baikal shallows, whereas they are not found in either the river mouths or beyond the sand spit that separates the sor from Baikal proper. Beyond this spit in the open Lake, on sandy shoals at depths of 2-20 m, and even </p><p>on silted sand at depths of 20-50 m, the papillate tubificids of the endemic genus Baikalodrilus are distributed in masses. No one species from this genus was found in the North-Baikalian Sor. </p><p>An exception is the invasion of the cosmopoli- tan Tubifex tubif&amp; (Mtiller) into open Lake Baikal. Akinshina &amp; Lezinskaya (1978) have also recorded it in South Baikal, in the vicinity of Sol- zan in sediments polluted by sewage waters at depths of 5- 130 m where its frequency increased with an increase in the degree of pollution. The future behaviour of T. tubifex in Baikal can only be conjectured. At best, if the public pressure for protection of Baikal achieves closure of the Baikal pulp mill, the discharge of sewage will terminate. The disturbed biocoenoses will be rehabilitated and T. tubzfex may disappear. However, if the pollution continues for a long period, conditions will be favourable for wider distribution of T. tu- bifex, and a decision whether this species should be left unchanged or subjected to biological ir- radication will be required. </p><p>Immigration of freshwater species into Baikal has taken place throughout the history of the lake, i.e. 28 x lo6 years. The fauna1 elements of the preceding geological epochs were not only main-...</p></li></ul>

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