GENERAL CHARACTERISTICS OF THE EASTERN SEAS 729 



only endemic species, but even endemic genera (Lucicutia, Heterorhabdus, 

 Parenchaeta, Bathypontia, Bathycalanus, Spinocalanus, Pachyptilus, Hete- 

 roptilus) with a majority of endemic species. Some of these forms penetrate 

 into the Bering and Okhotsk Seas through the deep straits. 'Abyssal species', 

 writes Brodsky, ' are not widely distributed, certain species being endemic to 

 certain areas of the World Ocean'. 



Developing his idea further K. Brodsky notes (1948) that a 'series of species 

 of Calanoida are wrongly said to be widely distributed'; this happened be- 

 cause different species were known under the same name. Thus Metridia 

 pacifica was classified with M. lucens, M. okhotensis with M. longa, Calanus 

 pacificus with C.fimnarchicus, etc. When these forms were distinguished from 

 one another their habitats naturally became more limited. As a result of the 

 revision of the taxonomic composition of the Far Eastern fauna of the species, 

 Calanoida was found to be 60 per cent, while its cosmopolitan species formed 

 only 1-8 per cent. 



A similar vertical distribution of zooplankton in the column of water was 

 traced for other pelagic organisms by M. Vinogradov (1955) from the data 

 obtained from a series of vertical catches of plankton according to horizons, 

 during trawling down to 8,500 m in the region of the Kuril-Kamchatka 

 trench. In the 500 to 1,000 m layer the predominant zooplankton species were 

 Calanus cristatus, С tonsus, Eucalanus bungii and Sagitta elegans. Hymeno- 

 dora frontalis appears at a depth of 200 to 500 m, attaining its greatest numbers 

 within the 500 to 1 ,000 m horizon. Below 1 ,000 m it is replaced by H. gla- 

 cialis, and by Eukronia fowler i among the Chaetognatha. Among the mysids at 

 a depth of 500 to 2,000 m Eucopia grimaldi is predominant, while at 4,000 m 

 E. australis and Gnathophausia gigas assume this role. Among the Euphau- 

 siaceae Euphasia pacifica lives at depths down to 500 m, and Bentheuphausia 

 amblyops between 3,000 and 4,000 m. Similar pictures are observed with many 

 other species. In the trench itself, below 6,000 m, the usual abyssal species 

 disappear, and plankton comprises mainly the species Copepoda and Amphi- 

 poda. 



Yet not all the plankton groups have this type of vertical distribution ; thus. 

 for example, among the Chaetognatha (Yu. Chinodonova, 1955) only one 

 group of abyssal forms (in the broad sense) can be distinguished {Sagitta 

 macrocephala, S. planctonis, Eukronia fowleri and Heterekronia mirabilis). 



Qualitative changes with depth of the bottom-living fauna can be deter- 

 mined also by various other biotic and abiotic factors. Such alterations should 

 first be linked with the tropical factor (M. Sokolova, 1956, 1958, 1959). 

 Macro- and micro-zonal distribution of bottom-living fauna is readily ex- 

 plained when this method is applied, and when the properties of the soil 

 (mechanical and chemical), the rates of the movement of bottom-water 

 masses, their content of suspended substances and the general composition 

 of the fauna are taken into account. The detritus-eating group is markedly 

 predominant among the benthos of the abyssal. Sokolova distinguishes among 

 them those which consume the upper layer of the soil indiscriminately, those 

 which discriminate roughly the surface layer of the sea-bed, and those which 

 make a delicate choice of detritus on the surface layer of the sea-bed. 



