568 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



The taxonomic composition of the Caspian Lamellibranchiata has been 

 more thoroughly studied than that of the Gastropoda. Apart from Mytilaster 

 lineatus, a recent immigrant from the Black Sea, and Syndesmya ovata, trans- 

 ferred from the Sea of Azov, all the Caspian bivalves are endemic forms of the 

 Pontic-Caspian basin. They are represented by three tribes of the Hetero- 

 donta suborder, one species of the Corbiculidae family (C. fluminalis), 

 five species of the genus Dreissena (D. polymorpha, D. rostriformis, D. cas- 

 pica, D. grimmi, D. andrussovi), one species of Cardium (C. edule), two 

 species of Monodacna (M. caspia and M. edentula) and seven species of the 

 genus Adacna {A. trigonoides, A. crassa, A. pyramidata, A. longipes, A. bar- 

 bot-de marnyi, A. baeri and A. latens). Except for Cardium edule, which had 

 penetrated into the Caspian Sea through Manych in the Khvalyn period, 

 and Dreissena polymorpha and Corbicula fluminalis (an ancient fresh-water 

 immigrant), which have migrated far beyond the limits of the Caspian Sea, 

 the endemic nature of the bivalves is most pronounced; in Derzhavin's 

 opinion they are all autochthonous forms of the Pliocene Seas. Sixteen species 

 are endemic forms of the Caspian, while Didacna is an endemic genus of it 

 {Table 237) (A. Derzhavin, 1951). 



Table 237 



Number of endemic forms among them 



Total amount of Caspian molluscs 



Pontic-Aralo- Caspian 



Caspian 

 Genera Species Genera Species Genera Species 



Number 16 57 53 4 50 



Percentage 100 100 62-5 93-0 25-0 87-7 



The heterogeneous nature of the Caspian fauna is well illustrated in the 

 Turbellaria group. V. Beklemishev established (1915) the presence of 29 

 species of Turbellaria in the Caspian Sea (Triclada — 6 species, Acoela — 

 11, Alloeocoela — 5, Rhabdocoela — 7). Twenty-seven species of Turbellaria 

 are endemics ; in fact there are no less than two endemic genera. In Bekle- 

 mishev's opinion 18 species of Turbellaria are undoubtedly marine forms 

 (Acoela, part of Rhabdocoela and the majority of Alloeocoela) ; they originated 

 in the Tertiary period when the Caspian basin was still connected with the 

 ocean. The 7 species of Rhabdocoela are ancient (Tertiary) immigrants from 

 fresh water into the Caspian Sea. The other species have only recently come 

 from fresh waters. 



The so-called negative features are more sharply pronounced in the popu- 

 lation of the Caspian Sea than in that of the open sea; many typically marine 

 groups are either completely absent from the Caspian or represented by very 

 few species. Strictly speaking only fish, crustaceans and, to a smaller extent, 

 the molluscs are varied here. The number of species of these three groups 

 constitutes about 60 per cent of all the species of the free-living animals of 

 the Caspian. 



