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



Black Sea fauna is the absence of deeper-water fauna or a fauna connected in 

 its development with great depths (below 125 to 150 m). 



The ' Atlantization' of the Mediterranean fauna in the Black Sea 

 The characteristic differences between the fauna of the Black and Azov Seas 

 and that of the Mediterranean had already been observed by the first investi- 

 gators of the fauna of our southern seas. K. Kessler (1860) pointed out that 

 fish in the Black Sea are often smaller than in the Mediterranean, 'which is the 

 result, probably, of lower temperature and less salinity'. This was also noted 

 by H. Ratke as early as 1837. V. Sovinsky too dwelt on this phenomena (1902). 

 S. Zernov (1913) pointed to the fact that the Mediterranean crab Carcinus 

 maenas is considerably smaller in size than those in the Black Sea and off the 

 shores of Great Britain. Zernov expressed the opinion that 'once in the 

 Mediterranean the crab became smaller, and when passing into the colder 

 water of the Black Sea it grew again in size'. 



A. Sadovsky (1934) approached this problem on a wider front. He estab- 

 lished for 14 species of Black Sea molluscs (including sea mussel, oysters, 

 Patella, Syndesmya and others) a closer relationship in the shell structure 

 (size, shape, thickness, sculpturing, colouring) with Atlantic species than with 

 Mediterranean. Sadovsky considers that once the Atlantic forms got into the 

 Mediterranean they underwent definite changes as a result of higher salinity 

 and temperature. In the Black Sea, under the influence of lower salinity 

 (from 37 to 18% ) and temperature (the minimum Mediterranean temperature 

 is 13°; that of the northern part of the Black Sea descends to zero) there 

 took place a 'reshaping' of the original Mediterranean aspect into the 

 Black Sea one, which developed autochthonously in the Black Sea, since in 

 the hydrological conditions described we have, as it were, a return from 

 Mediterranean conditions to those of the North Atlantic. This author thinks 

 that the rule noted by him for molluscs must also be applicable to other 

 groups of Black Sea fauna. The species of molluscs which have undergone 

 'Atlantization' form 11-4 per cent of all Black Sea malacofauna. Sadovsky 

 points to the fact that in the 'warmer part of the Sea, in the Batum region, 

 one observes a greater similarity between some of the molluscs and the Medi- 

 terranean ones than one sees in the northern part of the Sea'. Finally, Sadov- 

 sky observes another interesting phenomenon in the case of Patella and 

 Mytilus : when young they resemble the Mediterranean forms more closely, 

 in maturity this resemblance is lost. 



The affinity between the Black Sea fauna and that of the northern parts of 

 the Atlantic Ocean lies first in the selection of genera and species, and 

 secondly in the above-mentioned morphological resemblance between the 

 Black Sea and the Atlantic Ocean forms. As regards the former feature of 

 resemblance, Sovinsky says that in the Black Sea a selection was made of the 

 northern forms which had remained there since the Ice Age, and which 

 had died out or were poorly represented in the Mediterranean. Thus the Black 

 Sea fauna is a selection of cold-water relict species. To what extent can the 

 morphological peculiarities mentions d above also be explained by their 

 relict character, i.e. did these forms get into the Black Sea as 'northern' forms 



