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



and Astarte elliptica, polychaetes Onuphis conchylega, Thelepus cincinnatus 

 and Nephthys ciliata, and the brittle stars Ophiocantha bidentata are reduced 

 in numbers. They are gradually replaced by the thermophilic forms of echino- 

 derms, molluscs and worms. At the same time the biocoenosis biomass in- 

 creases, reaching 10 kg/m 2 in Danish waters, in Gulmarfjord (S. Sweden) up 

 to 7 kg/m 2 and off the Faroe Islands up to 16-5 kg/m 2 (up to 300 specimens/ 

 m 2 ). 



The biocoenosis Mactra elliptica has similar ranges, and changes in the same 

 manner with its advance to the east (according to the data of V. Zatzepin and 

 Z. Filatova, 1946). This biocoenosis does not extend, as did the former one, 

 eastwards to Sviatoi Nos ; it is distributed only in the western and along part of 

 the central Murman coast, but it is also found on large-grain sand and fine 

 gravel. 



Whereas the previous biocoenosis consisted in its basic mass of epifauna 

 seston-feeding forms, * the Mactra elliptica biocoenosis is mainly represented 

 by the infauna bottom-feeding forms. 



Besides Mactra elliptica, the bivalves Astarte borealis, Cyprina islandica 

 and the polychaetes Onuphis conchylega and Thelepus cincinnatus and others 

 play an essential role in this biocoenosis. 



The quoted biomass of this biocoenosis is considerably smaller than that 

 of the Modiola modiolus-Pecten islandicus community, both on coasts and 

 in the north Atlantic. The average biomass off the Murman coast is, 

 according to V. Zatzepin (1946), only 50 g/m 2 and the largest hardly 

 reaches 100 g/m 2 . 



On the shores of Northern Norway the biomass of the Mactra elliptica 

 biocoenosis sometimes reaches 200 g/m 2 and in the waters of Iceland it 

 reaches 270 g/m 2 . 



Zatzepin confirms R. Sparck's (1936) opinion that the M. elliptica bio- 

 coenosis of the Faroe and Iceland waters (and according to Zatzepin's data, 

 those of the Murman coast as well) should be considered as colder-water 

 north-boreal modifications of the south-boreal groups of the Venus sand bio- 

 coenosis. 



Among the biocoenoses peculiar to Murman coastal waters, it is possible to 

 distinguish other north-boreal ones with ranges similar to the two previously 

 mentioned. Such are the biocoenoses Cyprina islandica, Pseudopotamilla reni- 

 formis, Brisaster fragilis, Waldheimia cranium and others. 



At great depths (150 to 350 m), on slightly silty sand floors containing rocks, 

 the Brachiopoda Waldheimia cranium community is greatly developed; it 

 stretches east almost to the Rybachiy Peninsula. Waldheimia, a typical 

 planktophage, forms mass accumulations of some hundreds of grammes per 

 m 2 in sectors with strong currents. In this community Waldheimia comprises, 

 on the average, more than 50 per cent of the whole population. Moreover, 

 there is an abundance of Porifera {Geodia baretti, Craniella cranium, Thakellia 

 and others), polychaetes Placostegus tridentatus and Eunice norvegica, 

 molluscs Astarte sulcata, Anomia squamula, sea-urchin Brisaster fragilis, 

 asterid Cribrella sanguinolenta, crab Ну as coarctatus and others. In the west 



* Zatzepin's terms. 



