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



about 579-73 g/m 2 . The autumn increment of biomass {Table 209), obtained 

 in spite of this loss, is the result of intensive increase of the molluscs remaining 

 in the population. The actual production of M. Meatus can be determined as 

 900 g/m 2 , while the average P/B ratio is 3-22. This high ratio for the Sea of 

 Azov is explained by its high temperature and the abundance of food. 



The Syndesmya ovata biocoenosis — In the deepest part of the Sea, beyond 

 the Cardium edule biocoenosis, there lay in 1934-35 the Syndesmya ovata 

 biocoenosis, occupying an area of about 14,500 km 2 ; this latter is somewhat 

 reduced in the autumn, since it is replaced by its contiguous C. edule bio- 

 coenosis. In the deepest part of the Sea (12 to 13 m), over an area of 4,500 

 km 2 , the number of Syndesmya is small, and the gastropod mollusc Hydrobia 

 ventrosa is predominant. In a wide zone surrounding this deepest part (10 to 

 11 m), over an area of 10,000 km 2 , Syndesmya ovata is greatly preponderant, 

 while at depths less than 9 m Syndesmya is replaced by Cardium. 



S. ovata is one of the most numerous molluscs in the Sea of Azov. In the 

 Black Sea it is mostly found in fairly shallow low-salinity sectors, especially 

 under the roots of Zostera and the Chareal sea-weeds in lagoons and inlets. 

 This species is widely distributed in the Mediterranean Sea and in the Atlantic 

 Ocean off the coast of Europe. Specimens living on sand or shell gravel are 

 larger in size (up to 25 mm) and have a thicker shell, while on mud soils 

 they are smaller (up to 20 mm) and have a thin transparent shell. In the Sea 

 of Azov they live in largest numbers on silt or silty sand. S. ovata feeds on 

 detritus and dwells in the upper layer of the sea-bottom. It has extensible 

 siphon-tubes which help it to endure the unfavourable gas conditions of the 

 near-bottom layer. In general this species is hardier than C. edule and goes 

 to greater depths in the Sea of Azov than other molluscs, excepting only 

 Hydrobia ventrosa, and it is adapted to the zone of 'blackened shell gravel' 

 with an admixture of mud, in which the proportion of fines is 40 to 50 per cent. 

 In deep and less well-aerated sectors of the bottom Syndesmya displaces 

 Cardium; both molluscs are found in almost equal numbers at depths of 

 9 to 10 m ; in higher layers Syndesmya is replaced by Cardium. S. ovata is 

 found at all depths from 1 to 13 m in the Sea of Azov, but it reaches a maxi- 

 mum at 10 to 1 1 m. However, in shallower places (4 to 6 m) S. ovata produces 

 a second maximum on silty sand or shell gravel and mud, since it does not 

 find there its powerful rivals Cardium and Mytilaster, which displace it at 

 depths of 6 to 9 m. 



This eurytopic capacity of S. ovata is also shown in its response to salinity. 

 In the Sea of Azov it survives salinity fluctuations of 5-5 to 7-0% in the Gulf 

 of Taganrog and up to 55% in the Sivash. Its optimum, however, is reached at 

 9 to 12% . It can live in the presence of hydrogen sulphide and ammonia and 

 can even exist for some time (5 to 8 days) under anaerobic conditions. 



S. ovata is one of the favourite foods of almost all the bathypelagic fish 

 of the Sea of Azov, especially sturgeon and golden shiner, and it has a high 

 food-value, partly due to its small size and thin shell. 



S. ovata has a very high fecundity : the number of its eggs reaches some 

 hundreds of thousands. It breeds from the end of April to the end of Septem- 

 ber. 5 1 . ovata reaches its sexual maturity in the third year of its life, rarely in 



