Recent near-shore ostracod fauna of the Caspian Sea

Толық мәтін

1. Introduction

The Caspian Sea is the largest enclosed water body in the world. As a Paratethyan remnant it is assumed that many of the modern ostracod species in the Caspian Sea originated from marginal marine environments during the late Neogene (Boomer et al., 2005). The Caspian ostracod fauna of marine origin is constituted by representatives of Leptocytheridae, Hemicytheridae, Loxoconchidae and Xestoleberididae, however, none of them can be assigned to original genera of these families since the endemic taxa have been formed there on the level of genera (with the only exception – C. torosa) (Schornikov, 2011b) due to significant paleogeographical events, geographical isolation and unique brackish-water habitat (Mandelshtam et al., 1962; Svitoch, 1991; Rychagov, 1997; Yanina, 2012, etc.). Despite the high-amplitude changes in the sea-level of the Caspian Sea during the Quaternary, its deep-water areas retained a certain volume of water even during regressive episodes and must have acted as refugia (Boomer, 2012).

However, during a highstands, an exchange of fauna occurred – for example, as a result of the connection between the Caspian and the Black Seas through the Manych Strait (Jones and Simmons, 1996; Rögl, 1999; Popov et al., 2006). Some researchers are of the opinion that only coastal shallow-water populations migrated to Azov-Black Sea basin through the Manych Strait. Specimens of deep-water populations could not migrate through a shallow straight (e.g., Schornikov, 2017) that existed at different times in the Quaternary history of the Caspian Sea (Svitoch et al., 2011; Semikolennykh, 2022). Others find a greater similarity between the shallow-water assemblages of the Caspian Sea and the fauna of the Aral Sea; between the deep-water assemblages of the Caspian Sea and the Black Sea (e.g., Boomer, 2012). In total, 26 species of Caspian origin were identified in late Pleistocene and Holocene sediments of the Azov-Black seas, 17 species occurred alive in the river deltas, bays and estuaries (Schornikov, 2011a). The same work notes that three Caspian species invaded in the Aral Sea. They inhabited that area in the 60s of the 20th century, but now only C. torosa remains in the Aral Sea.

We encountered some difficulties when attempting to arrive at a homogeneous picture of the ostracod distribution in the Caspian Sea. One of the greatest problems was the retention of open nomenclature for many of the taxa. About 350 species of ostracods are known from the Pliocene and Post-Pliocene deposits of the Caspian basin area. However, most of them are difficult to determine, since it frequently remains uncertain which taxon the authors of publications meant under a certain name (Schornikov, 2011b). The abundance of taxonomic problems is discussed in detail in the Schornikov’s work (2017), the following factors were noted as their main reasons of the incorrect identifications: many species were described repeatedly by different researchers and published independently; different forms were described as different species. It is extremely difficult to revise the majority of ostracod species of the Caspian Basin, since the reports and collections, which were the basis for the initial descriptions have been lost or appeared inaccessible. New sampling of sediment from the bottom of the Caspian Sea without equipping large interdisciplinary expeditions, the closedness and fragmentation of available drilling materials for various reasons (like implementation of commercial projects, the search for oil and gas, the conflict of interests of different countries in the Caspian region) still hinder work in this direction, especially for small research groups. Therefore, the Gofman’s collection is of great importance. It is the result of the Institute of Geology and Development of Fuels work, devoted to the study of the ecology of ostracods and foraminifera in brackish and freshwater basins. Later, E.A. Gofman was forced to study the stratigraphy of the Jurassic deposits of Mangyshlak, leaving work on the Caspian ostracods unfinished and providing a report (Gofman, 1966) focused on identifying the most favorable conditions for the particular species, as well as determining the habitats of various species of ostracods. As a result, the work with the unique collection material was not completed; some samples needed additional elaboration. In particular, the issues of freshwater ostracods distribution and shallow-water zone along the eastern shore of the Caspian Sea were not covered (Gofman, 1966).

Nowadays, according to the latest estimates the Caspian Sea ostracod fauna is now known to comprise more than 70 species, however only for 16 the soft parts were described (Schornikov, 2011b). Saidova (2014) found 61 species in modern sediments of the Caspian Sea. According to earlier works by Naidina (1968), 23 species of ostracods were found alive in the Caspian Sea. According to Gofman (1966) more than 80 species of ostracods live in the Caspian Sea, 57 of them were identified and 39 were described there in detail. It was noted that to compile the report, data was used from 300 sampling sites from the entire Caspian Sea area (Gofman, 1966). Although for this study we used specimens from the collection that were identified as living by Gofman, we were not convinced that the studied shells were alive at the time of collection. No soft parts (appendages and other internal organs) preservation was established. Unfortunately, due to the lack of illustrations, it is sometimes difficult to determine which taxa were actually described by Gofman (1966). In addition, certain names have been changed in some cases since the time of publication. For example, Schornikov (1964) considered Graviacypris as a junior synonym of Candona and, since Candona elongata Schweyer would have been a junior homonym of Candona elongata Herrick, 1879, he changed its name to Candona schweyeri (Boomer et al., 2010; Spadi et al., 2019).

This work is a continuation of the distribution and ecology researches of the modern ostracod species in the Caspian Sea initiated by Gofman (1966). Here an attempt was made to expand our knowledge of the distribution and ecology of some modern ostracod species in the Caspian Sea, however, due to the difficulties and limitations mentioned above, the focus of the work was only on the species that are most widely represented in the studied samples, and those species that was able to be determined unambiguously. Thus, the presented generalization is based on the results of the author’s study of the Gofman’s collection and analysis of a number of publications devoted to ostracods of the Caspian basin. It should be noted that the current work mainly meets the paleogeographic interests of the Caspian region studying, since the information provided can be used to understand the environmental changes corresponding to the time of sediments accumulation in which the described ostracod species could be found.

2. Materials and methods

The present study is based on the E.A. Gofman’s collection of recent ostracods, completed during the spring season in 1964, which is now being stored at the Laboratory of Pleistocene Paleogeography, Faculty of Geography, Lomonosov Moscow State University. There are no analogues to this collection, containing ostracod shells of the entire Caspian Sea area.

According to Gofman (1966), a total of 900 samples were initially taken from 300 sampling sites (three samples from each site), but the author received significantly reduced collection. Over the past decades, some of the samples were partially or completely lost, therefore, the quantitative analysis in this study was replaced by a qualitative one. As large areas of the Caspian Sea are shallow, especially in the northern part of the sea nearby Russian shore, we studied the near-shore zone. It includes sampling sites, where water depth is less than 50 m. A total of 45 near-shore sampling sites were studied, where the largest number of ostracod shells have been preserved (Fig. 1, 2).

Fig.1. Location of the studied sampling sites.

Fig.2. Water depth at the sampling sites.

Ostracods from the bottom sediments of the Caspian Sea (upper ~0–5 cm) were collected during the summer seasons of 1961–1963 with a bottom sampler. For the collection each dry sediment sample weight was 100 g by Gofman. These samples were washed with tap water through a 63 μm sieve, the remaining >63 μm -size fraction was air-dried. Using a binocular microscope, ostracod shells were hand-picked from the dried sediment using a fine-tipped brush and transferred to a slide (Krantz-Cells or microcells). These shell samples (valves or carapaces) preserved in the collection were studied in the current work.

Here we present summarized data on our results of studying 45 near-shore sampling sites (Fig. 1) combined with published data, which was the first one to investigate the distribution of recent ostracods in the Caspian Sea and to discuss the influence of such factors as bottom sediments type, water temperature and salinity (Gofman, 1966; Schornikov, 1973; Yassini, 1986; Chekhovskaya et al., 2014). For each sampling site water depth, mean annual bottom water temperature and salinity at the time of sampling are available. This data is presented in Fig. 2, 3, 4.

Fig.3. Mean annual bottom water salinity at the sampling sites.

Fig.4. Mean annual bottom water temperature at the sampling sites.

3. Results and Discussion

Following the geographical features (Fig. 2, 3, 4), the samples are combined into three groups – Northern Caspian (Fig. 1, points 1-24, 28, 31, 41, 46-47), Western Caspian (Fig. 1, points 25, 32-40, 42-44), Eastern and Southern Caspian (Fig. 1, points 26-27, 29-30, 45). Since it is impossible to give an exact quantitative assessment, to indicate the frequency the following indexes were used: a – “abundant” for numerous shells of a certain species, c – for “common”, r – for “rare”, s – for “single”. Three coastal ostracod assemblages have been identified (Table 1).

Table 1. The identified near-shore ostracod assemblages

The C. torosa assemblage – was identified in the waters of the Northern Caspian Sea. The influence of Volga is significant near its delta, which can be seen at sampling sites 7, 10, 13. Shells of D. stevensoni is numerous here – this species is mostly coincided with the stream beds and the river mouths, and inhabits depths up to 8 m with salinity below 7‰ (Fig. 3). D. stevensoni is an euryhaline and eurythermal species (Gandolfi et al., 2001), which probably explains its wide occurrence at sampling sites, where seasonal changes in water temperature can reach 24 ˚C (Gofman, 1966). Shells of freshwater I. bradyi, L. inopinata, Cypridopsis sp. and Candoninae spp. were also found at the indicated sampling sites. According to Bronshtein (1947), who described the habitats for the freshwater and slightly brackish species representatives, I. bradyi prefers puddles and river oxbows from which it is being transported to rivers. L. inopinata inhabits freshwater and brackish-water basins. The occurrence of D. stevensoni decreases away from Volga delta area, and C. torosa dominates in the assemblage. This trend is slightly less pronounced near the mouth of the Ural River (sampling sites 1, 9, 14). Similar patterns were noted earlier (Naidina, 1968).

The highest abundance of C. torosa was noted in the Northern Caspian Sea. It is a highly euryhaline species, occurring from fresh to hyperhaline water in Europe, western Asia, and North Africa. According to Gofman (1966), Yassini (1986), Boomer et al. (2005), Chekhovskaya et al. (2014), Berdnikova et al. (2023) etc. in the Caspian Sea this species was recorded at numerous sites at water depth from 0.2 m to 250 m (usually shallower than 50–60 m) across the whole sea area. However, maximum abundance corresponds to 3–5 m depths (sometimes reaching almost 90% of the total assemblage), and decreases with depth. Population peaks were also obtained in the transitional zone from marine to freshwater conditions – owing to its strong hypoosmotic regulation capacity, it is abundant in such environments, where marine species cannot survive due to low salinity, whereas freshwater species cannot survive due to high salinity. In the Aral Sea it is dominant species of shell crustaceans (Schornikov, 1973). Studies by Aladin (1993) showed that C. torosa from the White and Barents Seas has marine origin, while its form from the Baltic, Black, Azov, Caspian and Aral seas has freshwater origin.

The high abundance of C. torosa in the studied samples in areas of high temperature variability (up to 24 ˚C), where average annual salinity values differ by 10‰ or more (Fig. 3), may be associated with the large amount of nutrients, since the mixing zone of brackish and fresh waters, rich in zooplankton and phytoplankton, is the most productive. According to Schornikov (1973) C. torosa became euryhaline under favorable conditions during colonization of the non-marine basins, which differs greatly from marine conditions. However, as a result, the competitive advantage was lost in marine basins, where the marine fauna is more or less represented in full volume. C. torosa becomes abundant in the northern near-shore part of the Caspian Sea with its unstable hydrological regime. Presumably the maximum abundance reaches because of wide salinity range and better adaptability in such conditions. Although C. torosa tolerates changes in salinity, it disappears off the eastern coast, where a higher mean annual salinity is observed (Fig. 3).

Almost all species of this assemblage are tolerant to wide changes in salinity – they are able to live both in marine conditions and at low salinity – for example, in estuaries with a salinity up to 5‰ in association with freshwater species. For example, A. longa and A. cymbula were also found in estuaries in the Black Sea at depths up to 5 m (Zenina et al., 2017). Shallow-water areas of the Caspian Sea are characterized by significant seasonal fluctuations of temperature as well. The species adapt well to changes in temperature, which allows them to live in such conditions. Frequent findings of C. gracilis, E. baquana and L. gibboides only at sampling sites remoted from the large river deltas (sites 12, 15, 22, etc.) allow to conclude that although the species generally tolerate seasonal changes in water temperature, they prefer salinity of 10–13‰. Representatives of the genus Loxoconcha, which do not descend to depth, are also common. Generally, the assemblage shows the extent of the Volga delta influence in Northern Caspian area as well as the importance of seasonal variability of the water parameters.

In the T. amnicola donetziensis assemblage, found in the Western Caspian Sea, C. torosa is also widely represented (Table 1). The species T. amnicola donetziensis is widespread in the Caspian Sea at depths of less than 90–100 m, where it forms assemblages at depths of less than 30 m and salinity ranges from 4 to 13.5‰ (Tkach et al., 2024). It was also found in the Black Sea and the Sea of Azov at salinity less than 5‰ and in brackish and freshwater lakes of Caspian Lowland and Manych Strait (Schornikov, 1973; Zenina et al., 2017). This species is common in the littoral zone and tolerates temperature and salinity seasonal changes. In Northern and Central Caspian Sea both adult specimens and instars were found. In addition, almost all studied samples contained C. bogatschovi, Camptocypria sp., A. striatocostata, A. caspia, A.? quinquetuberculata and E. virgata. The species E. virgata is often found in samples from the Gofman’s collection (Table 1). In the studied sites this species inhabits littoral zone with depths usually less than 30 m and with significant seasonal variation in temperature and salinity. Living species were found in the Northern Caspian Sea by Maria Zenina in 2013 in water of 25.4–27 ˚C temperature, 10.02–12.01‰ salinity, pH 7.30–8.33. The species typically inhabits sandy sediments (Tkach et al., 2024). It was also found in Azov-Black Sea in rivers estuaries, limans and lakes with salinity less than 5‰ (Zenina et al., 2017).

Less common in the assemblage of Western Caspian Sea (Table 1) are E. bosqueti, E. baquana (although finds of E. baquana are frequent at sampling sites 32, 37 and 39), and H.? azerbaidjanica. Single specimens of A. longa and C. schweyeri (sampling sites 32, 33, 34), C. gracilis (sampling sites 36, 38, 42) and L. gibboides (sampling sites 32, 33) were also found in the Western Caspian Sea. L. gibboides is common for water depth less than 90–100 m. This species tolerates temperature changes from 4.5 to 15 ˚C, prefers salinity 10.5–13.5‰, withstanding dynamic hydrological conditions and salinity decrease down to 7‰ (Gofman, 1966; Yassini, 1986).

The T. amnicola donetziensis assemblage identified in the Eastern and Southern Caspian Sea, is different from the community described for the western area (Table 1). Thus, here the species C. torosa was not found and recent near-shore assemblage is represented mainly by the species T. amnicola donetziensis and stenohaline species like P.? naphtatscholana, L. gibboides, Camptocypria sp. и C. bogatschovi. The former is generally widespread in the Central and Southern Caspian Sea, especially its eastern area (Yassini, 1986). The species C. bogatschovi inhabits waters with a salinity 12.5–13.25‰; it prefers shelf environments with depths of about 60–200 m (rare findings were noted at depths of less than 30 m and 200–315 m) (Gofman, 1966; Boomer et al., 2005; Chekhovskaya et al., 2014). L. gibboides – common for the Eastern Caspian – was found in the Northern Caspian only at depths deeper than 15 m (Fig. 2), probably due to the fact that this species prefers greater depths and/or more saline environment. Findings of various Leptocythere sp. are also common along with rare finds of A. caspia, E. bosqueti, E. virgata and Xestoleberis sp. Single shells of E. baquana and A.? quinquetuberculata were found in sample from the site 45 (Fig. 1). Their widespread presence was previously noted in the Caspian Sea, as well as in the Dniester estuary and the Don delta (Naidina, 1968), however, according to the current research, these species are more often found in brackish water environments.

Along the western coast of the Central Caspian species with a thick shell, practically not sculptured, were noted. Such a variability may be related to the higher energy environment (wave action), intense bottom water currents, as well as frequent severe storms in this region. There are no freshwater species in the assemblage mainly due to the absence of large rivers in the area. Although, the smaller number of studied sample sites, compared to the Northern and Western Caspian Sea, should be noted.

The results presented allow to conclude that the recent ostracod fauna of the Caspian Sea changes with distance from the coast with increasing depths in accordance with changes in the bottom water temperature and salinity. In the shallow Northern Caspian Sea, partially frozen in winter and warmed up to 24 ˚C in summer (Gofman, 1966), samples were taken from depths of up to 20 m (Fig. 2), the mean annual salinity does not exceed 10‰ (Fig. 3) and changes significantly as the influence of fresh water from river runoff weakens. The ostracod assemblage here is represented by species that adapt well to changes in temperature and salinity. Despite the predominance of the species T. amnicola donetziensis in the assemblages of the Western, Eastern and Southern Caspian Sea, their composition differs along with water parameters at the sampling sites. The east and south part of the Caspian Sea are areas of the highest salinity (13–14‰) (Fig. 3), although mean annual bottom water temperature here is lower than in the western zone – about 11–13 ˚C (Fig. 4) (up to 5 ˚C in winter and 19 ˚C in summer in the Middle Caspian, up to 11 ˚C in winter and 25 ˚C in summer at sampling site 45) (Gofman, 1966). Here we found more thermophilic and stenohaline species. Moreover, due to the greater depth of sampling (25–50 m, Fig. 2), single shells of deep-sea Caspian species were noted here (e.g., B. dorsoarcuata), which inhabits depths above 50 m (Gofman, 1966; Yassini, 1986; Boomer et al., 2005; Tkach et al., 2024)).

The assemblage described for the western part of the Caspian Sea, although slightly different in some components to the north and to the south of the Absheron Peninsula, contains both representatives of the North Caspian community (especially in shallow-water areas) and species most often presented in samples from the eastern part of the sea. Such pattern could be explained by larger depth range of sampling in the Western Caspian (from 0–5 to 35 m), and by different distances of sampling sites from the large river mouths. The analysis of the recent near-shore ostracods of the Caspian Sea allows to conclude that salinity, along with water depth, are the leading factors determining the composition of ostracod assemblages.

4. Conclusions

Distribution of the ostracods in the Caspian Sea shows a remarkable depth control. Overall, the near-shore ostracod assemblage of the Caspian Sea indicates shallow water conditions. The low salinity of the greater part of the near-shore Caspian Sea has forced development of the brackish-fresh water forms. The corresponding near-shore assemblages (e.g. North Caspian) is dominated by C. torosa and contains abundant euryhaline species tolerant to reduced salinities, high temperature changes and unstable hydrological regime. At the same time near-shore assemblages of Central and Southern Caspian areas are represented mostly by stenohaline ostracod species, reflecting the unique brackish-water environment of the Caspian Sea.

5. Acknowledgments

This research was funded by Government assignment «Paleogeographical reconstructions of natural geosystems and forecast of their future changes» №121051100135-0. The author is grateful to Maria Zenina for her help and counseling.

Conflict of interests

The authors declare no conflicts of interest.

Авторлар туралы

A. Tkach

Хат алмасуға жауапты Автор.
Email: alinaberdnikowa@yandex.ru
ORCID iD: 0000-0002-9391-894X

Laboratory of Pleistocene Paleogeography, Faculty of Geography

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2. Fig.1. Location of the studied sampling sites.

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3. Fig.2. Water depth at the sampling sites.

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4. Fig.3. Mean annual bottom water salinity at the sampling sites.

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5. Fig.4. Mean annual bottom water temperature at the sampling sites.

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