328 



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



depth of 14 to 17 m, the second at 9 to 10 m. To the west, in the Twerminn 

 area, there are at certain stations communities identical with these. At other 

 stations a number of new forms are found ; and if, off Pellinge, we find the 

 Macoma-Pontoporeia-Mesidothea biocoenosis, here at deeper places (25 to 

 37 m) on the same gyttja, we find Macoma-Pontoporeia-Halicryptus (Fig. 

 159). At lesser depths (10 to 25 m) there is an extreme paucity of forms; 



86-и 



81-0 



111-0 



Fig. 159. Composition of bottom communities off the shores of 

 Finland (Twerminn) (Sagerstrale, 1932). Mean biomass, g/m 2 , is 

 shown above the circles. 1 Macoma baltica ; 2 Pontoporeia affinis ; 

 3 Halicryptus spinulosus; 4 Mytilus edulis; 5 Chironomidae ; 6 

 Cardium edule; 1 Corophium volutator; 8 Mesidothea entomon; 

 9 Others. 



Macoma baltica is predominant with a small admixture of Pontoporeia affinis, 

 Halicryptus spinulosus and Mytilus edulis (Fig. 159). Moreover at times the 

 whole population consists solely of Macoma baltica (Fig. 159, III). Examples 

 of such a degree of uniformity of benthic marine communities are found again 

 only in the Sea of Azov. 



At depths of less than 10 m the variety of the fauna increases and several 

 forms are added : Corophium volutator, Cardium edule, Mesidothea entomon, 

 Nereis diversicolor ; Chironomidae appear in large numbers, while Halicryptus 

 spinulosus disappears (Fig. 159). 



However, as had been shown by Sagerstrale, Chironomidae in the Baltic 

 Sea are adapted only to the shallows and disappear with increasing depth 

 {Table 135). 



