394 THE DEEP SEA 



particular species may sun'ive but not breed. A thorough study of 

 the Hfe history of selected animals on the slopes and neighboring 

 abyssal regions within a restricted area is necessary to e\'aluate 

 the significance of this. 



Also for the true abyssal animals, meaning all species which live 

 their entire life in the abyssal zone, much more collecting is needed 

 before we can ascertain their distribution, etc. Elpidia glacialis, for 

 instance, considered by Ekman (1953) to be restricted to the North 

 Polar Basin, has now been found in all oceans (Zenkevich et aL, 

 1955; Hansen, 1956). Several more examples could be given. This 

 does not exclude the possibility of certain species or higher taxons 

 having a more restricted distribution, governed by the supply of 

 food, the composition of the sediments, etc. 



Origin 



Without doubt the abyssal fauna stems from shallower depths. 

 Even in the case of the recent Alonoplacophora (Lemche, 1957; 

 Clarke and Menzies, 1959), which hitherto have been found only in 

 abyssal depths, it is still possible that other representatives of this 

 order will be found on the slopes. A comparison of the abyssal 

 fauna and the slope fauna indicates that the latter harbors a much 

 more varied fauna than the abyssal zone. At the end of the last 

 century, Perrier (1899) was aware that many ancient animal groups 

 like the siliceous sponges and the eryoneid crustaceans were relicts 

 of former shelf faunas. The find of the coelacanth fish Latimeria 

 is a more recent example of this. The present abyssal group is on the 

 whole of a fairly recent date. The hundreds of millions of years 

 before the Pleistocene may very well ha\'e developed an abyssal 

 fauna. However, the present fauna has only few species in common 

 with this earlier one. The Pleistocene glaciation with the two polar 

 ice caps must have produced a disastrous decrease in temperature 

 in the abyssal zone, perhaps from about 10° to about 2° C, as in- 

 dicated by Emiliani and Edwards (1953) so that only eurythermic 

 animals could survive. A similar situation may have prevailed in 

 the trenches and affected the composition of the hadal fauna; here 



