GENERAL CHARACTERISTICS OF FAR EASTERN SEAS 725 



The most complete and, graphic data on this phenomenon are provided 

 by N. Vinogradova (1958), who has examined the vertical distribution of 

 1,144 species of deep-water animals (below 2,000 m). The first horizon with 

 a pronounced change of bathyal and partly sub-abyssal species lies at a depth 

 of 2,500 to 3,500 m, the second at 4,000 to 5,000 m (Figs. 351a, в and 352a, 

 b). A large number of new species and even new groups appear in both hori- 

 zons, while those inhabiting higher ones disappear. 



Z. Shchedrina (1958) notes also that 'the most luxuriant and varied Fora- 

 minifera fauna . . . was recorded on two zones or horizons ; at depths of about 

 3,000 m and at 4,850 to 5,570 m. Between these two zones of maximum Fora- 

 minifera variety two more transitional zones, characterized by scarcer Fora- 

 minifera fauna, can be distinguished.' With some groups a third horizon of a 

 more marked change can be distinguished at 6,000 to 7,000 m, the threshold 

 of the ultra-abyssal zone. This worker does not explain the cause of such 

 vertical distribution : 'the explanation of this phenomenon should be sought 

 both in the ecology of the animals inhabiting the deepest water, and in the 

 historical causes which promoted their existence'. 



It is most interesting that similar kinds of principles in the alterations in the 

 qualitative composition according to depth have been observed also for the 

 pelagic fauna of the Gammaridae (Ya. Birstein and L. Vinogradov, 1955) in 

 the Kuril-Kamchatka trench (Table 297 and Fig. 353). 



Table 297 



Zones Sub-zones Gammaridae forms 



Deep-water, Upper, л Cvphocaris challenged, Cyclocaris guilelmi, 



500-6,000 m 500-2,000 m Korogam egalops, Paracalanus alberti, Paran- 



Lower, [ dania boecki, Eusirella multicalceola, Rhach- 



2,000-6,000 m ) otropis natator 



Lower, Cyphocaris richardi, Astyra zenkevitchi, A. 



2,000-6,000 m bogorovi, Halice aculeata, H. shoemakeri, 



Cleonardo macrocephala 



Ultra-abyssal, Tetronychia gigas, Hyperiopsis latiearpa, Anda- 



6,000 m niexis subabyssi, Halice quarta, Vitjaziana 



gurjanovae, Protohyperiopsos arquata 



These workers note also that the number of new forms not found in the 

 higher horizons increases with depth — there are two (15-4 per cent) such 

 forms in the upper deep-water zone ; seven (52 per cent) in the lower, and six 

 (100 per cent) in the ultra-abyssal. These last are considered by them as ende- 

 mic to the Kuril-Kamchatka trench. Moreover, Birstein and Vinogradov 

 arrange their data according to their zoogeography. Among the deep-water 

 Gammaridae they distinguish four main groups : (1) Organisms with a pan- 

 oceanic type of distribution (6 species) ; (2) Atlantic-Pacific (amphi-boreal) 

 forms (7 species); (5) Arctic forms (1 species) and (4) North Pacific forms 

 (2 species). 



It is to be noted that the endemic nature of the fauna increases with depth — 



