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



the number of specimens is no less than 85 to 90 per cent, whereas the total sum 

 of the biomass increases two to three times. The winter frosts are also destruc- 

 tive of the settled balanus ; their mortality during the winter may be 95 to 98 

 per cent. 



Somewhat below the layer of highest balanus development in the second 

 and third zones of the middle horizon, 1-3 m above zero depth, lies the 

 Mytilus biocoenosis — Mytilus edulis. Mytilus attains its highest development 

 in places where there is little swell, on cliffs and rocks and on rising ground 

 on the silty sand beaches. On cliffs and rocks the amount of Mytilus reaches 

 7 to 10 thousand specimens with a biomass of 10 to 15 kg/m 2 , and sometimes 

 up to 13 thousand specimens with a biomass of 19 to 25 kg/m 2 . Among the 

 fuci the quantity of Mytilus is smaller (2-5 to 3 kg/m 2 ) and they themselves 

 are smaller in size. 



The Mytilus biocoenosis is characterized also by the presence of a large 

 amount of Nucella lapillus, Acmaea testudinalis, molluscs, hydroids and 

 bryozoans and, in the lowest levels in autumn, of asterids Asterias rubens and 

 gastropods Buccinum undatum {Table 45). 



Table 45 



On silty sand of the lower littoral zone dense Mytilus colonies are common 

 (the so-called Mytilus banks) ; they form a kind of defensive border to the 

 littoral. The total amount of Mytilus on such banks is somewhat smaller 

 than on the cliffs, but it also can reach 19 to 21-5 kg/m 2 and may be more 

 than 10,000 specimens {Table 46). Thus here a considerable part of the 

 population is represented by the Macoma community. 



The main gatherings of Mytilus, forming powerful biofilters, are situated 

 within the lower level of the littoral and in the upper (1 to 3 m) level of the 

 sublittoral. 



