22 



even here in the 750-1,000 m layer additional species do not appear. As 

 for the ratio of bathy- and abyssopelagic species in the tropical regions, 

 the data available is inadequate for any definite inferences. 



Geographic Distribution 



At present, 81 species (not considering subspecies) of hyperiidean Physo- 

 somata are known from the world ocean. They belong to 19 genera. The 

 family Scinidae has the maximum number of species (39 species). 



The Pacific Ocean leads in faunal diversity. The fauna of the Atlantic 

 Ocean is depauperate, which might possibly be due to the fact that fewer 

 investigations have been conducted, mainly in the North Atlantic Ocean; 

 the number of deepwater catches in the tropical and southern regions is 

 comparatively small. Finally, in faunal composition, the Southern Ocean 

 (south of 50° S) lags considerably behind not only the Pacific or Indian 

 Ocean, but even the Atlantic. 



It is known that the area of distribution of the majority of deepwater 

 bathyal animals, both in the pelagic and benthic parts of the oceans, is 

 quite extensive and often has a panoceanic character (Brodsky, 1955; 

 Vinogradova, 1958; Birshtein and Vinogradov, 1955). Hence it is quite 

 natural that the degree of commonality of fauna in the Pacific, Indian, and 

 Atlantic oceans be quite high. The cold and faunistically impoverished 

 Southern Ocean has few endemic forms and its fauna is entirely formed 

 by species from other oceans (Table 4). In the Pacific and Atlantic 

 oceans, 72% of the genera and 63% of the species are common. In the 

 , Indian Ocean, 89% of the genera and 63% of the species are common 

 to the Pacific Ocean, and 81% of the genera and 64% of the species 

 common to the Atlantic Ocean (Table 3). If we do not take into account 

 species inhabiting the northern temperate regions of the Pacific and 

 Atlantic oceans, which have no parallel in the Indian Ocean, the degree of 

 commonality of fauna rises still further and a more intimate relationship 

 is seen between the Indian and Pacific oceans rather than the Indian and 

 Atlantic oceans. Thus, in the Physosomata fauna of the tropical regions 

 of the Pacific and Indian oceans, 94% of the genera and 72% of the 

 species are common; the figures for the tropical regions of the Indian 

 and Atlantic oceans are 81% and 69% respecti.vely. 



The high degree of commonality of fauna is not only due to pan- 

 oceanic or circumtropical species common to all three oceans, whose 

 number is not that high (72% genera and 48% species), but to a consider- 

 able extent due to different species, some of which inhabit the Indian and 

 Pacific oceans, while others occur in the Indian and Atlantic oceans. This 

 is true not only of species, but of genera too. Thus, the genus Megalance- 

 ola inhabits only the Atlantic and Indian oceans, while Prolanceola, Chu- 

 neola, and Microphasmoides inhabit only the Indian and Pacific oceans. 



