THE SEA OF AZOV 501 



in the most westerly part of the Gulf of Taganrog ; it is very widely distri- 

 buted in the open parts of the Sea of Azov, and in 1934 occupied about one- 

 third of its bottom. 



A bottom-area of 10,000 km 2 of the Sea of Azov is occupied by this bio- 

 coenosis in the spring, and 12,000 km 2 in the autumn. The widening of its 

 habitat is due to its pushing out other biocoenoses {Nereis succinea, Syn- 

 desmya, Mytilaster). 



Cardium does not form such massive populations in the Black Sea as in 

 the Sea of Azov, where it has found exceptionally favourable conditions for its 

 mass development. Cardium edule is a typical filter-feeder since it lives on 

 plankton and detritus suspended in water; it competes with Mytilaster, 

 Balanus and Corbulomya in its feeding, forming with them a powerful filter. 

 Huge plankton development and the abundance of detritus in the Sea of Azov 

 create most favourable conditions for the existence of C. edule. This biocoeno- 

 sis reaches its highest development on silty sand bottoms. This species is also 

 widely distributed in the Atlantic Ocean, reaching the western part of the 

 Murman peninsula in the north. The Baltic Sea also is thickly populated by 

 it. In the northern part of the Atlantic C. edule is adapted mainly to the tidal 

 zone and is a typical littoral organism. Thus it is a very widely distributed 

 eurytopic species, with a great capacity for adapting itself to different condi- 

 tions of life : temperature, salinity, soils and depths. This mollusc is devoured 

 in large numbers by fish (in the Sea of Azov by bullhead, Acipenseridae, flat 

 fish, golden shiner, roach, Rutilus rutilus heckeli and others); for many 

 thousands of years it has also been used as food by man. 



As a result of C. edule 's capacity for adapting itself to different conditions 

 of existence, numerous varieties have been evolved from it ; C. edule var. 

 maeotica lives in the Sea of Azov, while in the Utlyuksk inlet and the Sivash 

 С edule var. picta is also found. 



In the Sea of Azov C. edule is found on various sea-bottoms, but it prefers 

 soft beds. A single biomass of this mollusc varies from a few grammes to 2 kg 

 and more per 1 m 2 . C. edule does not require a great amount of oxygen but it 

 cannot survive a considerable lowering of oxygen content. In the Sea of Azov 

 its greatest numbers are adapted to a depth of 6 to 10 m. At a greater depth 

 it is replaced by Syndesmya ovata. Since Cardium is fairly tolerant to consider- 

 able fluctuations of salinity it can compete successfully with all the forms of 

 benthos of the Sea of Azov ; Mytilaster alone pushes it out on the harder sea- 

 bed of the Zhelezinskaya Bank, while Corbulomya does so on sand, or silty 

 sand bottom at a depth of less than 4 m. C. edule begins to multiply during 

 the second summer of its life, but its breeding reaches its greatest intensity 

 only in its third or fourth summer. С edule may perhaps breed three times a 

 year, laying some tens of thousands of eggs. 



A comparison of the rate of growth of C. edule off the English and German 

 coasts shows that in the Sea of Azov it is much slower and that C. edule does 

 not reach as advanced an age. As regards the rate of growth this can appa- 

 rently be explained by lowered salinity and by the unfavourable aeration 

 conditions of the Sea of Azov, and partly also by the higher temperature, 

 causing earlier sexual maturity ; the shorter life-span of the Azov C. edule 



