GENERAL CHARACTERISTICS OF THE NORTHERN SEAS 53 



Zoobenthos and the history of its formation 



The fauna of the Arctic basin, including the Greenland and Norwegian Seas, 

 can be divided into the following main groups (according to E. F. Gurjanova, 

 1939, with some alterations): 



I. The Arctic autochthonous forms 



1. Species endemic in the Arctic region 



(a) Eurybiotic circumpolar species 



(b) High Arctic epicontinental species 



(c) Forms of the depths of the Arctic basin 



2. Brackish-water relicts 



3. Arctic boreal species (partly) 



II. Immigrants from the North Atlantic 



1. Post-glacial and contemporary immigrants (part of the Arctic 

 boreal species) 



(a) Littoral boreal species 



(b) North Atlantic forms of the continental shelf 



2. Relicts of the Littorina period 



III. Immigrants from the Northern Pacific 



1. Post-glacial and modern immigrants 



2. Pliocene relicts 



L. Berg (1934), as has been mentioned before, thinks that in the Pliocene 

 Period the Arctic basin was widely connected with the Atlantic and Pacific 

 Oceans, and that its climate was considerably warmer. At that time a large 

 exchange of fauna between the two oceans must have taken place via the 

 Arctic basin, and the fauna of these three water bodies was very similar. As 

 early as the Pliocene Period, before the closing up of the Bering Strait, the 

 fauna inhabiting the Arctic basin began to be pushed southwards into the 

 Atlantic and Pacific Oceans under the influence of the continuous cooling of 

 this basin. In the opinion of L. Berg the main stock of amphyboreal forms 

 were evolved at that time. 



E. Gurjanova (1938, 1939) gives a somewhat different explanation for this 

 phase of the history of the Arctic fauna. In her opinion the endemic character 

 of the Arctic fauna is so clearly reflected not only in its species but also in its 

 genera (Acanthostepheia, Onisimus, Pseudalibrotus, Mesidothea) that the 

 formation of the main autochthonous stock of the Arctic basin should be 

 ascribed to a period earlier than the Pliocene. The warming up during the 

 Pliocene Period gave the Pacific fauna as a whole the opportunity to pene- 

 trate into the Arctic, but its autochthonous stock had already been formed. 

 Later during the Ice Age the Pacific fauna, which had penetrated into the 

 Arctic basin, ' was almost completely destroyed and replaced by a new high 

 Arctic fauna, which had developed mostly from the ancient autochthonous 

 fauna of the Arctic '. 



A. M. Djakonov (1945) also thinks that the Pliocene fauna of the Arctic 

 basin perished in the Ice Age, except for the species which moved into 

 the depths and there survived the period of unfavourable climate. The 



