Hydrobiologia (2006) 568(S):207–211 G. Coulter, O. Timoshkin, L. Timoshkina & K. Martens (eds), Species and Speciation in Ancient Lakes DOI 10.1007/s10750-006-0313-y
Ó Springer 2006
Endemic gastropod distribution in Baikal T. Ya. Sitnikova Limnological Institute RAS, Ulan-Batorskaya str., 3, 664033 Irkutsk, Russia (E-mail:
[email protected])
Key words: Baikal, gastropod species, distribution, depth, geographic regions, province
Abstract A peak in overall gastropod diversity (80 of the 117 endemics species) was observed at a depth 5–20 m. About 30 species live deeper than the photic zone which is about 100 m in Baikal, but their reproductive areas are located at a depth 15–50 m. Only 10 are true deep-dwelling (>100v) species. Twelve gastropod species occur at different depth in all three basins of the lake. Species distribution varies considerably; some are linearly fragmented, many species have patchy distribution, and several have been found in many points in the lake. On the basis of statistical analysis of endemic gastropod occurrences in different regions, it is suggested that Baikal should be divided into two provinces: Northern Baikal (Northern basin, Maloe More Strait, Ushkany Islands, Chivyrkuy Bay) and Southern Baikal (Southern and Middle basins). It appears these Baikal provinces bear some similarities to bivariate distributional provinces of the Pacific Ocean.
Introduction Horizontal and vertical distribution of the Baikalian gastropods has been studied for a century (Kozhov, 1936, 1963; Starostin, 1926; Starobogatov, 1970). Results have shown that depths distribution is uneven and most species diversity is found in a relatively shallow zone down to 15–20 m depth (Kozhov, 1936). Gastropods do not occupy the surf zone except in spring when only a small number of species occur there for a short period of calm water (Kaplina, 1974). Kozhov (1936) pointed out that endemic gastropods occur in a littoral lagoons (‘‘sor’’) only rarely and, in general, inhabit the lake proper. All endemic gastropod species without exception can be found at depths between 20 and 30 m, and this intermediate zone is densely populated. Most of the species live at depths less than 20 m, and some deepwater species are abundant in this zone as well (Kozhov, 1936). From his analysis, Kozhov proposed two provinces of gastropod distribution: Northern Baikal and Southern Baikal, and assigned several divisions in both. In this
scheme, Northern Baikal included Maloe More Strait and Barguzin-Chivyrkuy Bays. Southern province consisted of Turka, Goloustnoe and Anga division and Selenga shallows. Considering Baikal as a distinct biogeographical designation, Starobogatov (1970) subdivided it into seven provinces according to horizontal and vertical distribution of gastropods and amphipods. Four of these provinces were shallow, from 0 to 100 m deep (Southern Baikal, East Baikal, West Baikal including Maloe More and Northern Baikal). Two provinces (Northern-Middle Baikal and Southern Baikal) were regarded as ‘‘supra-abyssal’’ (adopting oceanographic terminology referring to depth between 100 and 250 m). Lastly, a province named ‘‘pseudo-abyssal’’ embraced the remainder of Baikal below 250 m depth. Moreover, Starobogatov (1970) indicated that the malacofauna of the shallow Western and Eastern Baikal provinces does not differ. Karabanov (1990) suggested subdividing the lake into three basin provinces, 13 districts and about 60 regions (landscapes).
208 In the last few decades several new species have been described, and the distributions of many species better defined. On the basis of these data it is possible to undertake a new analysis of gastropod distribution in the lake.
Material and methods Gastropods of the families Baicaliidae, Benedictiidae, Valvatidae and Planorbidae were collected by SCUBA diving and by dredge and grab in 1993–2001, and stored in Limnological Institute SD RAS. Published data of Kozhov (1936), Starostin (1926), Bazikalova (1971), Beckman & Starobogatov (1975), Slugina (1983), Sitnikova & Shimaraev (2001) are included in this review along with our own unpublished data. Representatives of the family Acroloxidae were not included because data on their distribution in Baikal are incomplete. Ten regions (Fig. 1) of the Lake were considered according to their differences in physical– geographical conditions (see review, Lake Baikal..., 1998). Each region has several or at least one endemic species or subspecies. For comparing the gastropod faunas of these regions we used statistical processing (UPGA) by the PAST software and Excel for Windows.
Results Vertical distribution At present 148 gastropod species are described in Lake Baikal, 78% of them endemic. Only one non-endemic species, Lymnaea (Radix) auricularia intercisa Lindholm, 1909 [or L. (R.) intercisa sensu Izzatulaev et al., 1983] occurs in open Baikal, populating depths to 20–30 m (Sitnikova & Ro¨pstorf, unpublished). Another 30 non-endemic (Palaearctic and Siberian) species inhabit the lagoon-sor zone, where they live from 0.1 to 5 m, and sometimes down to 10 m depth. Analysis of gastropod occurrence along depth gradients shows more than 80 endemic species living between 5 and 20 m (Fig. 2). With increasing depth, species diversity decreases. Thirty-nine
Figure 1. The two Baikal provinces: 1 – South-Baikal, 2 – North-Baikal, 3. – distribution overlap zones between provinces, 4 – overlap zones between Baikal and Palaearctic distributions.
species were found at 100 m, while only a few occur deeper than 500 m. One specimen of the species Cincinna (Pseudomegalovalvata) laetmophila (Beckman & Starobogatov, 1975) was described from 1380 m depth, but this needs validated by further research. Only 10 species can truly be termed deep-dwelling, i.e. living deeper than the photic zone (100 m). About 30 gastropods may be called eurybathic, being found 15–100 m or some of them ranging even deeper.
Figure 2. Species number and depth.
209 Horizontal distribution Horizontal distribution to 100 m depth only is considered here. Twelve littoral and sublittoral species have linearly fragmented distribution around the three basins of the lake. Some occupying rocky bottom, for example, Megalovalvata baicalensis (Gerstfeldt, 1859) and Teratobaikalia ciliata (W. Dybowski, 1875), others sand, such as Parabaikalia oviformis (W. Dybowski, 1875). A few species prefer to live opposite river mouths on soft substrates, for instance, a group of Benedictia fragilis (W. Dybowski, 1875). Many species have a patchy distribution, and they usually populate one or two basins of the lake, for example, Liobaicalia stiedae (W. Dybowski, 1875) inhabiting soft substrates in Southern Baikal. Among 22 species and subspecies usually called south-Baikalian, some occur in both Southern and Middle Baikal, and some of them can be found in Malye Olkhonskie Gate. Similarly, 36 north-Baikalian species and subspecies live not only there but are also encountered in Maloe More and Middle Baikal also. Where two closely related species (or subspecies) inhabit similar biotopes in different basins of the lake their distributions usually overlap. For instance, southern Maackia herderiana (Lindholm, 1909) and northern M. variesculpta (Lindholm, 1909) occur together from Anga Bay to Malye Olkhonskie Gate along the western shoreline and along the east littoral of Olkhon Island. There are species inhabiting different biotopes in two basins of the lake, for example, Maackia costata (Lindholm, 1909) living in South-Western Baikal on rocky sediments but on soft substrate in MiddleEastern Baikal. Ten species were found at just a few points in the lake, and their exact distribution is uncertain. However, a few species are indeed restricted in distribution, for example, Teratobaikalia macrostoma (Lindholm, 1909) and Choanomphalus patulaeformis (Lindholm, 1909), which populate hard sediments in the littoral of Maloe More, Ushkany Islands, east liitoral of Olkhon Island, Bolshoy and Maly Kyltygey Islands in Chivyrkuy Bay and along the shore of Svyatoy Nos Peninsula; additionally they occur in small number along the eastern and western shorelines of Middle Baikal. About four species are limited to Academic Ridge (100–300 m depth), and are unknown elsewhere.
A dendrogram (UPGA, Jaccard coefficient) of 8 regions, comprising 45 species and subspecies that live in stony or rock substrate, shows two main groups (Fig. 3a), the first composed of species inhabiting Southern and Middle basins (along both shorelines), the east littoral of Olkhon Island, Selenga Region and Barguzin Bay; while the second comprises species living in Northern basin,
(a)
(b)
Figure 3. Dendrogram of species occurrence in regions (according to Jaccard coefficient). (a). 7 regions of 45 littoral species inhabiting hard substrate. (b). 9 regions of 103 species living on hard and soft substrate excluding deepwater species. W-E – south-western region (from Kultuk to Aya Bay), E-S – south-eastern region (from Slyudyanka to Selenga region), Sel – Selenga Region, E-M – middle-east region (from Proval to entrance of Barguzin Bay), E-O – east littoral of Olkhon Island, SN-Cv – Svyatoy Nos Peninsula and Chivyrkyy Bay, Bar – Barguzin Bay, Ush – Ushkany Islands, W-E-N – northern eastwest region (from Chivyrkuy to Zama), MM – Maloe More Strait.
210 Maloe More, Chivyrkuy Bay and Ushkany Island. Our analysis of 103 species from 9 regions (stony, sandy and silt substrates) gave a similar result (Fig. 3b). (Note: both analyses exclude true deepwater representatives and second exclude the east littoral of Olkhon Island because at present distributions are not known for sand-silt species).
Discussion The bathymetric distribution of Baikalian gastropods is similar to that observed in other ancient lakes (see review in Michel, 1994) and in the oceans (Valentine & Jablonski, 1988). However, in Lake Tanganyika Michel (1994) estimated that the zone of 2–5 m depth carries the greatest species diversity. In marine ecosystems, e.g. the NorthEastern Pacific, the average distribution depth of 14 families (202 species) of shell-bearing gastropods is 40 m; 46% of these occur deeper than 50 m; 25% deeper than 100 m; and only 8.4% reach 250 m depth (Valentine & Jablonski, 1988). Baikal is an exception among ancient lakes, because some of its gastropods live in great depths. The peculiarity of mosaic gastropod distribution in Baikal deep-water zones was considered by Sitnikova & Shimaraev (2001). Michel (1994) pointed out that depth is not the main factor influencing vertical gastropod distribution in ancient lakes, and that species diversity is correlated with resources (peak diversity is observed usually in the most productive regions of the photic zone of the littoral and sublittoral). In Baikal the 5–20 v depth zone contains the largest number of different biotopes – about 20 (Kozhov, 1936, 1963), and the highest species abundance occurs there. It is likely the number of biotopes in this zone in Baikal is considerably more, taking into account, for example, that hard substrates (stones, pebbles, rock and boulders) may have different orientations of space. Moreover, stones may be situated on sandy or rocky sediments, disposed in single or more layers, overgrown by sponges or macrophytes and having different size of ‘‘caves’’. The most important factors affecting horizontal distribution of gastropods in Baikal are the great length of its shoreline, the different geological histories of the basins, and the mosaic-type
differentiation of biotopes in the littoral and sublittoral. Moreover, species inhabiting this shallow water zone have a linearly fragmented area along the littoral and sublittoral (i.e. photic zone) zones. Our statistical results confirm the opinion of Kozhov (1936) that Baikal may be subdivided into two provinces, Northern and Southern, with characteristic gastropod compositions. But whereas Kozhov (1936) thought Barguzin Bay must be included in the Northern Province, this region is regarded here as within Southern province. Distributional boundaries between the provinces are indistinct, and overlap zones extend over several kilometres. A zone of this sort between two provinces passes along the western shoreline from Anga Bay to Malye Olkhonskie Gate and then along the east shore of Olkhon Island to Ezhimey. Along the Eastern Baikal shore the zone goes from Cape Sukhinsky through Barguzin Bay to Svyatoi Nos peninsula. Area boundaries of some species, both northBaikalian and south-Baikalian, do not coincide with the province boundaries. (Similarities with gastropod distribution in provinces have been observed in the Pacific Ocean, where neighbouring provinces differ by not more than 50% of the species composition, and the area boundaries of some species do not coincide with province boundaries (Valentine & Jablonski, 1988)). Baikal provinces are not strictly linear. They have some of the peculiarities of bivariate provinces in the ocean; where faunas on islands and shallow-water banks do not differ from faunas occupying similar biotopes in adjacent continental shallows. For example, Murinskaya Banka in Southern province, islands in Maloe More Strait, Ushkany Islands and islands in Chivyrkuy Bay of Northern province. It is possible that physical–geographical subdivision into districts might be applied to gastropod distribution in Baikal, as suggested by Karabanov (in Karabanov et al., 1990), but further research would be required to test this.
Acknowledgements The author is very thankful to the divers V. Votyakov, I.Yu. Parfeevets, Dr. A.B. Kupchinsky, and Dr. P. Ro¨pstorf for helping to collect the
211 samples, to Dr. G.Coulter and E.M. Timoshkina for correcting English and Dr P. Ro¨pstorf for critical remarks. This work was supported by RFRBR, Projects 01-04-49339, 01-04-97214, 9804-63064, 99-04-63058, 00-04-63099 and 01-0463098.
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