THE SEA OF OKHOTSK 805 



' The predominant forms in the zone of detritus-collecting organisms are 

 either one or another species of bivalves (species of the genera Macoma, 

 Yoldia, Leda and Nucula replacing each other) or some mass species of Ophiura 

 (for example O. sarsi or O. leptoctenia), or detritus-collecting Holothuria or 

 Polychaeta. 



'This change in the composition of species of the dominant ecological 

 group of animals within each zone is the result of an alteration in the mani- 

 festation of the factors in the environoment to which the ecological animal 

 group is adapted and of the inclusion of some new factors in it. 



' A certain consequent replacement of one ecological zone by another in 

 proportion to the distance from the coast and the increase of depth is also 

 observed. Hence there arises the possibility of the occurrence of a certain 

 vertical zonation in the distribution of the ecological groups of bottom-living 

 animals. Rock soils in the coastal areas are commonly predominantly occupied 

 by a fouling fauna. . . . With increase in depth the zone of the predominance 

 of fouling fauna is replaced by that of the predominance of mobile sestono- 

 phages. The last is adjacent to the wide zone with a predominance of detritus- 

 collecting forms. . . . Mollusca are replaced by Ophiura, and finally, in the 

 lower horizons of the zone (mainly in the bathyal) where the finest detritus 

 fraction is deposited and the aeration of bottom-water layers becomes less 

 satisfactory, the Polychaeta, as the most eurybiotic forms, acquire a dominant 

 role in the biocoenoses. . . . The group of bottom feeders or sessile sestono- 

 phages of soft soils becomes intensely developed in the central deep-water 

 part of the Sea, on diatomaceous oozes rich in plant food.' Thus similar 

 ecological groupings, but having different compositions of species are found 

 in various parts of the Sea at different depths, on more or less common soils 

 and at currents of similar strength. 



Quantitative distribution of sessile sestonophages (Fig. 400) and bottom 

 feeders (Fig. 401) could be used for the comparison of the nature of the distri- 

 bution of Savilov's ecological groups of benthos. 



The distribution of Savilov's ecological groups and the total biomass and 

 its connection with bottom topography and currents is most graphically 

 shown in the longitudinal cross section of the Sea of Okhotsk (Fig. 402). This 

 picture is wholly comparable with the ecological profile due to Sokolova, con- 

 sidered above (page 730). 



Mollusca occupy the first place (about 30 per cent) in the total benthos 

 biomass of the Sea of Okhotsk ; Echinodermata come second (about 25 per 

 cent) and the Polychaeta third (about 12 per cent) (Fig. 403). 



Ushakov (1953) gives the simplest diagram of the distribution of bottom 

 biocoenoses in the Sea of Okhotsk on a chart of the Sea and on a latitudinal 

 cross section through the central part of the Sea (Figs. 404 and 405). 



Colonies of luxuriant pink hydroid corals of the family Stylasteridae (St. 

 norvegicus f. pacifica, St. solidus, St. eximius, St. scabiosa, and Errinopora 

 staliferd) develop sporadically on rocky bottoms in the uppermost horizon 

 of the bathyal, especially in the area of Iona Island, at the entrance to Shelek- 

 hov Bay, at the northern end of Sakhalin and in the area of the Kuril Straits, 

 at depths of 100 to 200 m and somewhat deeper. 



