708 



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



vertical migrations of plankton. Diurnal vertical migrations of many plankton 

 species of the surface zone are either absent or only feebly developed, being 

 determined either by the season of the year or by the age of the organism, 

 the latter being much more important. On the contrary, species inhabiting 



much greater depths (for instance 

 Metridia pacifica, M. ochotensis, Para- 

 themiato japonica) descend many 

 hundreds of metres and rise again. 

 There are reasons for thinking that 

 throughout the 5 km deep Ocean waters 

 plankton follows a steplike system of 

 vertical migrations. 



Apart from the diurnal vertical 

 migrations, ranging between 300 and 

 5,000 m, numerous crustaceans have 

 seasonal migrations extending for 

 Calanus tonsus and C. cristatus to 2 or 

 3 km (K. Brodsky, 1956). 



K. Brodsky (1956) in his attempt to 

 divide the pelagic zone into districts 

 correctly takes the quantitative signifi- 

 cance of certain plankton species 

 (number of specimens per m 3 ) as the 

 basis of his work. He uses only one 

 dominant plankton group, the 

 Calanoida, for the zonation of the Far 

 Eastern Seas. Certain individual species 

 are characterized by several quantita- 

 tive indices — the frequency of their 

 occurrence, the number of specimens 



10 



100 



1000 mg/m 3 



Fig. 342. Vertical distribution of zoo- 

 plankton biomass in different layers 

 at the deep-water stations in the 

 northwestern Pacific. / Tropical 

 waters; 2 Boreal waters (Bogorov 

 and Vinogradov). 



per m 3 , the percentage of the num- 



ber of specimens to the total number 

 of Calanoida. The main forms of 

 Calanoida are: Pseudocalanus elongatus, Calanus tonsus, C. cristatus, 

 Eucalanus bungii, Metridia pacifica, Scolecithricella minor, Pareuchaeta 

 japonica and Microcalanus pygmaeus. Brodsky's proposal to include the 

 northwestern part of the Seas of Japan and of Okhotsk, and of the Bering 

 Sea, and the southeastern part of the Chukotsk Sea is based on the 

 distribution of Calanoida in the boreal regions. Moreover, he distinguishes 

 the northern Japanese, northern Okhotsk and northern Bering provinces, all 

 three with Calanus finmarchicus as a predominant form; this is widely distri- 

 buted in the boreal and Arctic waters of the Atlantic and Arctic Oceans. 

 Brodsky calls the fauna of these three provinces pan-Arctic: 'similar to the 

 Arctic, but not identical with it, i.e. analogous but not homologous'. 



The vertical distribution of Calanoida in the northwestern part of the 

 Pacific Ocean is as follows : poor variety of species in the surface waters ; a 

 still smaller number of species in the cold intermediate layer; the greatest 



