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



In the northern Sivash the mollusc Cardium edule is present in various 

 combinations as the dominant species, with a number of others. In the summer 

 it is associated with Chironomus, Syndesmya ovata and Gammarus locusta, 

 in the autumn with Hydrobia ventrosa, Chironomus, Syndesmya and Pecti- 

 naria, in the winter with Chironomus, Hydrobia and Syndesmya, in the spring 

 with Syndesmya and Hydrobia. Among the other forms Nephthys hombergi, 

 Mytilaster, Nereis diversicolor, N. zonata and Chironomus may be noted. 



In the northern Sivash the mean biomass is 200 to 300 g/m 2 {Table 223). 



Table 223 



In the summer other forms are present only in small numbers. In autumn 

 the numbers of Hydrobia may attain 17,230 specimens per 1 m 2 with a bio- 

 mass of 44-54 g/m 2 . The numbers of Pectinaria (up to 13 g/m 2 ) and of 

 Nereis diversicolor (up to 5-9 g/m 2 ) are considerably increased. Lamelli- 

 branchiata composes from 66-7 to 93-9 per cent of the total biomass, Hydro- 

 bia in autumn up to 13 per cent, Vermes up to 11-5 per cent, Chironomidae 

 by the end of the winter up to 18 per cent. 



In the outermost part of the bay the Syndesmya biocoenosis is preponder- 

 ant in benthos almost in the same combination of species, but with a biomass 

 of up to 400 to 500 g/m 2 . 



The site occupied by the Syndesmya biocoenosis (1st northern Sivash) gives 

 shelter to a fairly abundant ichthyofauna. According to N. Tarasov's data 

 (1927) 53 species offish were recorded which feed there, but rarely penetrate 

 into the second part of the northern Sivash. Vorobieff suggests that 'all the 

 production of this biocoenosis is completely consumed by fish'. 



In the autumn Hydrobia becomes the dominant form in the area formerly 

 occupied by the Cardium biocoenosis, and partly in the first northern Sivash 

 inhabited by the Syndesmya biocoenosis. The distribution of Loripes, Myti- 

 laster, Gammarus and Vermes (Nephthys and Nereis) communities is limited 

 in time and space. 



More than two-thirds of the Sivash area is occupied by the Chironomus 

 salinarius biocoenosis. This biocoenosis inhabits some parts of the northern 

 Sivash and the whole of the central and southern Sivash. Chironomus is 

 found in the central Sivash in various combinations with the same Hydrobia, 

 Cardium, Gammarus, Ostracoda and Artemia; in the northern Sivash, with 



