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



Khvalyn age of the 'Caspian' fauna in the Azov-Black Sea basin. 'The 

 comparative richness of the Caspian fauna in the northern approaches to the 

 Black Sea', he writes, 'is apparent not only in its high biomass, but also in 

 the considerably greater number of its species (as compared with the Sea of 

 Azov basin — L.Z.). Whereas we used to reckon 49 species of (Caspian — 

 L.Z.) invertebrates for the river Don, and only 23 for the River Kuban, for 

 the rivers Dnieper and Bug we have no less than 69 (Markovsky), and 64 for 

 the Danube. ' 



A very curious phenomenon comes to light in a comparison of the distribu- 

 tion of the autochthonous relict community in the Azov and Caspian Seas 

 (V. Beklemishev, 1922; later developed by J. Birstein, 1946, F. Mordukhai- 

 Boltovskoy, 1953 and Yu. Markovksy, 1953—56). In the Sea of Azov basin 

 the relicts are in the main concentrated in the area of the river mouths in 

 fresh water. In the Caspian Sea, however, most of them live in the saline 

 waters of the Sea itself. 



Mordukhai-Boltovskoy points out, for example, that the mollusc Caspia 

 gmelini, which he found in the delta of the Don, in the Caspian lives only in 

 the Sea itself. The Don delta is in fact the main place where the relicts in the 

 Sea of Azov accumulate ; some of them are found there in large numbers 

 (Mesomysis kowalewskyi up to a few g/m 2 , Hypania and Hypaniola at times 

 up to 10 g/m 2 , and large quantities of Monodacna and Dreissensia). Only 

 six or seven species of these relicts are found in the Sea of Azov itself, while 

 in the Gulf of Taganrog their number rises to 25 forms in the less saline parts 

 (counting not only the peracarids, but also the molluscs, polychaetes and 

 coelenterates) Table 149). 



Table 149. Distribution of autochthonous relict forms in the Sea of Azov and the River Don 



The fact that the main mass of the relicts is adapted to the Don delta is 

 particularly interesting, since the pre-delta zone of the Gulf of Taganrog is 

 abundantly populated by fresh-water forms, which are often accumulated 

 there in very considerable numbers. Thus the Caspian relicts in the basin of 

 the Sea of Azov cannot endure a rise of salinity as well as the fresh-water 

 forms ; in other words they are more ' fresh water ' than the fresh- water organ- 

 isms themselves. 



It has been shown, by the research done in the lower reaches of the rivers 

 and in the inlets of the northwestern part of the Black Sea by Markovsky and 



