GENERAL CHARACTERISTICS AND GEOLOGICAL HISTORY 371 



some Bulgarian investigators, that in this area the aspect of the 'Caspian' 

 Sea fauna is even more a ' fresh water' one than it is in the Sea of Azov. 



The same fauna, or fauna very nearly the same from a taxonomic stand- 

 point, exists in the Caspian Sea at a much higher salinity (up to 12 or 13% ). 

 Some species, which in the conditions of the Caspian Sea must be considered 

 comparatively stenohaline and avoiding fresh water, enter the rivers in the 

 Azov-Black Sea basin (Table 150). Among such forms the following may be 

 mentioned : Pandorites podoceroides, Pontogammarus maeoticus, Caspia 

 gmelini, Clesissiola variabilis (Dnieper inlet), Dreissensia rostriformis (Bug 

 inlet) and others. M. Bacesko (1948) discovered the polychaete Manayimkis 

 caspia, which lives in saline water in the Caspian Sea, in the Danube. Lateo- 

 labrax has a fresh-water habitat in the Bug inlet and a marine one in the 

 Caspian Sea. Birstein pointed out that the Azov Monodacna colorata and 

 Dreissensia polymorpha perish very rapidly in the Azov and Caspian waters 

 at a salinity of 5% (by chlorine). 



The following relicts, among others, live in the Don ; two species of coelen- 

 terates, two bivalves and one gastropod mollusc, two polychaetes, one species 

 of leech and 38 species of higher crustaceans. Only three species of relict 

 forms living in the Sea of Azov are absent from the Don delta. 



Among relicts, apart from Malacostraca, only one polychaete was en- 

 countered in the Volga and Ural rivers. In addition, all the relict forms live 

 in the open parts of the Sea. 



The most natural explanation of this remarkable difference in the distribu- 

 tion of the autochthonous fauna in two seas situated side by side seems to 

 lie in the difference of their historical past and their salinity, and finally in the 

 influence on the autochthonous fauna of a stronger rival — the Mediterranean 

 fauna. A stronger fauna displaces the weaker one from all regions where it 

 can live itself. This proposition can only have a most general character. Some 

 of the forms of Caspian fauna in other bodies of water are very powerful 

 competitors of the local fauna, as are, for example, the Caspian immigrants 

 into the Baltic Sea, and still more, into fresh water. According to Mordukhai- 

 Boltovskoy (1960) some Caspian forms attain greater numbers and biomass in 

 the Black and Azov Seas than in the Caspian Sea. Thus, for example, in the 

 Azov-Black Sea basin Dreissensia polymorpha yields a biomass of up to 4-7 



Table 150. Distribution of autochthonous relict forms in the Caspian Sea, Vo Iga and Ural 



