Province of the Tropical Circumequatorial 

 Region. 



The similar zoogeographical composition 

 of the copepods and the cladocerans of the area 

 studied indicates the presence in the area of 

 at least two different zoogeographical regions. 

 Actually, the northern boundary of the north- 

 western part of the Kuroshio is at the same 

 tinne the zoogeographical boundary of two re- 

 gions: the North Pacific Temperate and the 

 Tropical Circumequatorial's Indo-Pacific 

 Subregion. 



Distribution of species . Let us look at the 

 distribution of individual species, mostly of 

 copepods. In view of the large number of 

 species, we will separate them into groups and 

 show the distribution only of sonne representa- 

 tives of each group. The species fall into three 

 natural groups according to their distribution 

 and their relation to the water temperatures in 

 the area investigated: 



1) Relatively cold-water species (Calanus 

 cristatus , C. tonsus, and Eucalanus bungii; 



2) Relatively warm-water species 

 (Calanus pacificus); 



3) Subtropical and tropical species 

 Nannocalanus minor, Labidocera japonica, 

 Corycaeus spp., Sapphirina spp., Copilia spp., 

 Penilia avirostris, Evadne tergetina, and 

 salps). 



The distributions of the three species 

 named for the first group are very similar; we 

 shall therefore limit ourselves to the charac- 

 ter of the distribution of the species which is 

 present in the greatest mass in the area 

 studied, Calanus tonsus. This species is 

 broadly distributed in all three of the Far 

 Eastern seas (the Sea of Japan, the Okhotsk, 

 and the Bering), and extends as far as the 

 southern part of the Chukotsk /Chukchi/ Sea. 

 In the area investigated it occupies a narrow 

 expanse in the form of a band along the Kurile 

 chain. The southern boundary of this band runs 

 at a distance of 240 nniles off the southern 

 Kuriles and more than 400 miles from the 

 northern islands of the chain, corresponding 

 approximately to the 12°-13°C. isotherms of 

 the surface waters in July-August. 



The distribution of the two remaining 

 species (Calanus cristatus and Eucalanus 

 bungii) is similar to that of Calanus tonsus, 

 but it has some specific peculiarities. In par- 

 ticular Calanus cristatus, which is distributed 



like Calanus tonsus in an elongated band 

 parallel to the Kurile chain, forms accumula- 

 tions in places of upwelling of cold waters to the 

 surface layers. 



Among the species of the second group, the 

 most typical distribution is that of Calanus 

 pacificus, which, like the two species pointed 

 out in the first group, is distributed in a band 

 along the Kurile chain, with, however, this 

 difference, that this band (or zone) occupied by 

 Calanus pacificus is displaced somewhat to the 

 south-southeast. The band-like or zonal dis- 

 tribution of this species is especially noticeable 

 in September. 



The third group is heterogeneous. Both 

 subtropical and tropical species enter into it, 

 but they naturally differ in the degree of their 

 stenobiontism. As an example let us take a 

 species less stenobiontic than the rest, the 

 cladoceran Penilia avirostris. The northern 

 boundary of the distribution of this crustacean 

 coincides with the northern boundary of the dis- 

 tribution in general of all of t h e subtropical 

 fauna in the area investigated. As an example 

 of a more stenobiontic species we can take the 

 copepod Nannocalanus minor, the northern 

 boundary of which is in good agreement with 

 the northern boundary of the tropical fauna in 

 the area. 



The region of distribution of Penilia 

 avirostris in the waters of the Pacific Ocean off 

 the Kuriles occupies a nnore southern position 

 than the regions of distribution of the species of 

 the first and second groups, that is, the mod- 

 erately cold-water group and the moderately 

 warm-water group. The northern boundary of 

 Penilia avirostris, where the species is repre- 

 sented by only a small nunnber of specimens 

 (from 1 to 4 per m.-^), does not come any closer 

 to the Kurile Islands than 300 miles in the south- 

 ern part and 400 miles in the middle of the 

 chain. The greatest quantity of individuals of 

 this species is distributed considerably farther 

 south than the region of distribution of the mod- 

 erately cold-water and moderately warm-water 

 species and lies at a distance of 400-500 miles 

 to the southeast from the chain, which position 

 corresponds to the 17°C. surface isotherm in 

 July. In September the aggregations of this 

 crustacean are displaced somewhat closer to 

 the chain and are even distributed slightly north 

 of the 40° parallel. 



The distribution of Nannocalanus minor 

 resembles that of Penilia avirostris with the 

 difference that in 1953 the greatest quantity of 

 individuals of this species was encountered in 



18 



