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



reaches 600 g/m 2 , forming more than 75 per cent of the total benthos. The 

 Pechora region and the shallow off Novaya Zemlya must obtain large amounts 

 of detritus from the abundant sea-weed growths off the southern coast of 

 Novaya Zemlya, and from the outflow of the waters of the Pechora, the 

 Novaya Zemlya rivers and from the Gorlo of the White Sea. 



The following thirteen species are preponderant among the biomass of bi- 

 valves of the open parts of the Barents Sea: Astarte crenata, A. borealis, A. 

 montagui, A. efliptica, Cardium ciliatum, C. groenlandicum, Macoma calcarea, 

 Area glacialis, Leda pernula, Yoldia hyperborea, Nucula tenuis, Portlandia 

 arctica and P. intermedia. 



The qualitative distribution of the echinoderms is almost the reverse of that 

 of the molluscs. Only in the southwest of the Sea, off the shores of Novaya 

 Zemlya, the echinoderm biomass is about 50 g/m 2 . The Pechora region is 

 characterized by its extreme poverty in echinoderms (less than one per cent 

 of the total biomass). The small number of echinoderms in the Kanin region 

 — west from Kolguev Island to the Sviatoi Nos meridian — is particularly 

 striking. The main mass of echinoderms is adapted to the deep western and 

 central parts of the Sea (30 to 50 per cent of the total biomass), to the slopes 

 of the Bear Island shallow and farther north and northeast. The reason for the 

 shortage of echinoderm representatives in the Kanin and Pechora regions is 

 not clear ; it can hardly be explained only by some decrease of salinity (33-0 to 

 34-5% in the bottom layer) and the shallowness of the region. However, the 

 mass development of bivalves in this region is very characteristic. It is well 

 known that echinoderms are natural enemies of bivalves, since they devour 

 their young fry. As has been shown by Petersen (1913), this antagonism may 

 have a decisive influence on the distribution of bivalves. The following eleven 

 species are the main quantitative forms of echinoderms in the Barents Sea: 

 Ctenodiscus crispatus, Strongylocentrotus droebachiensis, Brisaster fragilis, Mol- 

 padia sp., Ophiura robusta, O. sarsi, Ophiopholis aculeata, Ophiopleura borealis, 

 Ophiocantha bidentata, Ophioeten sericeum and Stegophiura nodosa. 



The picture of quantitative distribution of polychaetes is different from that 

 of the molluscs and echinoderms. The greatest gathering of polychaetes is 

 adapted mainly to the halistatic regions and the softer floors connected with 

 them. The deeper western part of the Sea, so rich in echinoderms, is particu- 

 larly poor in polychaetes. Its main polychaete forms are : Spiochaetopterus 

 typicus, Maldane sarsi, Pectinaria hyperborea, Onuphis conchylega, Thelepus 

 cincinnatus, Myriochele oculata, Owenia assimilis and Scoloplos armiger. 



Of all the remaining fauna the large sipunculids Phascolosoma margari- 

 taceum should be distinguished ; in the central parts of the Sea and on the 

 slopes of the southern island of Novaya Zemlya it forms dense colonies (15 

 to 65 g/m 2 ) and frequently forms more than 50 per cent of the total benthos 

 biomass. 



The distribution in depth of the three main above-mentioned groups of 

 Barents Sea benthos shows substantial differences (Fig. 54). The bivalves are 

 considerably reduced with depth, the echinoderms, on the contrary, increase in 

 numbers, while the polychaetes remain essentially unchanged. The same rela- 

 tionship in the vertical distribution of the three main groups of benthos has 



