508 



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

 Table 212 



Age group 



Under one year 

 One-year-olds 

 Two-year-olds 

 Three-year-olds 



Total 



No. of specimens 



3,756 



683 



371 



7 



3,817 



Weight, g 



68-36 



78-55 



124-66 



4-71 



276-27 



The central, deepest (11 to 13 m) part of the Sea, over an area of 4,500 

 km 2 , is inhabited by the variant Hydrobia-Nephthys-Syndesmya of this bio- 

 coenosis. It lives on grey liquid clay-mud with a small admixture of blackened 

 shell gravel, which smells of hydrogen sulphide. This biocoenosis consists of 

 about 16 species only; most of them, moreover, are temporary inhabitants, 

 while the permanent components of the biocoenosis are Hydrobia ventrosa, 

 Nephthys hombergi, Nereis succinea and Syndesmya ovata, which are the most 

 eury-oxybiotic species. Among the other species only Corbulomya maeotica 

 and Cardium edule are found more or less frequently. A remarkable feature of 

 this group, which is determined by the phenomenon of suffocation frequently 

 occurring there, is the marked uniformity of the age of the mollusc popu- 

 lations, which have settled after the suffocation and survive until the following 

 one. Considerable numbers of Cardium edule migrate, as a result of a shortage 

 of oxygen, into the neighbouring shallower sectors of the sea-bottom. The 

 density of the population fluctuates greatly from zero up to 38,400 speci- 

 mens per 1 m 2 , and its biomass up to 120 g/m 2 . The predominant biomass is, 

 however, 10 to 15 g/m 2 . The relationship between the components and the 

 fluctuations from spring to autumn are given in Table 213. 



Among all the species found in this grouping a gain in biomass is observed 

 only with Syndesmya ; moreover its PjB ratio is only 0-99 here. For all the 



