324 THE DEEP SEA 



Table I. Approximate Number of Benthic Animal Species at \'arious Latitudes 



and in Various Depths 

 (180,000 accepted as total number of marine species) 



From 40°X 



North of 40°N to 40°S South of 40°S 



Dwelling Realm In All 



No. % No. % No. % 



Shelf zone 180,000 15,000 8.3 150,000 83.3 ? 8.3? 



Below 2000 m 1,080 439 40.7 591 54.7 274 25.4 



(V'inogradova, 1959) 

 Below 4000 m 189 61 31 98 51 oi 16 



(Vinogradova, 1959) 



belonging to the fauna of the continental shelf increases approxi- 

 mately ten times. In contrast, the number of species at depths 

 exceeding 2000 m in both zones is approximately the same. This 

 suggests that the migration of animals to the great depths in high 

 and midlatitudes is less complicated than in the tropics. 



Recently the biogeographical homogeneity of the abyss has 

 become quite obvious and the great depths have been called the 

 "realm of monotony." However, now it is possible to propose a 

 different viewpoint. X'inogradova (1959), who has studied this 

 problem over a period of years, subdivides the abyss into three 

 realms, six subrealms, and eight zoogeographical pro\inces. These 

 subdivisions may be even further subdivided in the very near 

 future. This likelihood is particularly pronounced in the distinct 

 bipolar distribution of the abyssal fauna (Fig. 1). It is also note- 

 worthy that a circumtropical distribution of the abyssal fauna is 

 equally pronounced. Indeed, there are some relic forms in the 

 abyssal fauna of these areas. Two species of Neopilina as well as 

 both species of Spinula (Dall, 1908) are limited to depths of about 

 3000 m off the west coast of Central America. 



What limits representatives of the abyssal fauna (with but few 

 exceptions) to areas with similar environmental conditions, such 

 as temperature, salinity, and the oxygen content of the water? 

 Possibly, to some extent, this may be ascribed to the bottom 

 topography and to pressure, but it is unlikely that these are the 

 only factors concerned. Perhaps the chemistry of the bottom 



