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



to autumn, for the Cardium biocoenosis is 185 g/m 2 . Hence to a first apoprxi- 

 mation, the actual production of Cardium in this biocoenosis is 1,146 g/m 2 . 

 The P/B coefficient for the original spring biomass will be 1,146 :279 = 4: 1. 



Balanus sometimes settles on mollusc shells in large numbers and take away 

 tieir food and oxygen. On the other hand, Mytilaster young also settle at 

 times on the Balanus cases and even on top of them, tightening up the cases 

 with their bysus threads. By taking away its food and oxygen and in a purely 

 mechanical manner Mytilaster young, when settling in masses, may destroy 

 B. improvisus. The destruction of one of the components of this close sym- 

 biosis may cause the destruction of another, especially when a mass of B. 

 improvisus settles on С edule. The mollusc dies and stops the aeration of the 

 water, and the Balanus settled there are deprived of their food. 



In fish feeding-grounds masses of С edule and M. lineatus are devoured and 

 B. improvisus acquires a dominant position. Moreover, the last named grows 

 more vigorously than its rivals, but it multiplies less intensively. It produces 

 hundreds of eggs, while C. edule produces tens of thousands and Mytilaster 

 hundreds of thousands. Thus the changes in the groupings Balanus, Mytilaster 

 and Cardium, and their transition from one into another, are caused primarily 

 by their struggle for the site, food and oxygen, and by their being eaten by 

 fish. Large areas of sea-bed at depths of 6 to 8 m with hard soils, in cases when 

 Cardium, Mytilaster and Syndesmya are devoured in masses by fish, are 

 rapidly populated by the intensively developing Balanus, which has a very 

 long period of puberty. This might be the reason why the Balanus improvisus 

 biocoenosis is found in patches, mainly along the routes along which fish 

 travel. Moreover, the considerable washing out of the bed soil during a vio- 

 lent swell affects Cardium and Mytilaster very strongly, while a low oxygen 

 content kills off Balanus. That is why a biocoenosis with a marked pre- 

 dominance of Balanus is more often found in the autumn than in the spring, 

 Its total area in the spring is 607 km 2 , and in the autumn 2,200 km 2 . Apart 

 from on rocks and cliffs, which are rare in the Sea of Azov, Balanus develops 

 best on shell gravel, either pure or with an admixture of sand or mud, at a 

 depth of 4 to 6 m. 



As with other bottom groupings of the Sea of Azov, so with the biocoeno- 

 sis in which Balanus is predominant one can readily establish its plasticity 

 and the most varied combinations with other mass benthos species, especially 

 Nereis succinea, Cardium edule, Mytilaster lineatus, Hydrobia ventrosa, 

 Syndesmya ovata and Brachynotus lucasi. 



The P/B ratio of Balanus improvisus varies within the limits of 1 to 4-76 

 in different biocoenoses, depending on the density of the population of other 

 species present and primarily, of course, on that of Balanus itself (intra- 

 specific and inter-specific competition). Another variant of the Cardium bio- 

 coenosis, scattered in separate patches like the previous one, is the variant 

 with a marked predominance of Mytilaster lineatus. The main accumulations 

 of M. lineatus were adapted to the Zhelezinskaya Bank, off the craggy 

 southern shores of the Sea of Azov and also to the coast of the Arabat 

 Strelka. On the Zhelezinskaya Bank it had pushed out almost all the other bio- 

 coenoses. The total area occupied by this grouping — 1,470 km 2 — is almost 



