66 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



littoral fauna by specific conditions of the littoral microclimate. Some 

 authors divide the Arctic littoral region into two provinces— the high Arctic 

 one (Kara-Siberia) with its uninhabited littoral zone, and the Arctic one with 

 its traces of littoral fauna and flora (Matochkin Shar, Spitsbergen). But it is 

 impossible to accept either of these divisions. The separation of an unin- 

 habited littorial zone into a zoogeographical province is not justified since 

 animal life is absent from it. Again there are no grounds for placing 'traces 

 of littoral fauna' in a separate province, since these contain no original 

 features and are actually 'traces' of a boreal littoral fauna penetrating from 

 the south. 



G. Madsen (1936) has approached this problem somewhat differently. He 

 divides the littoral of the northern part of the Atlantic into sub-Arctic (we 

 call it boreal) and Arctic. 



The sub-Arctic littoral ends when the periwinkle, sea mussel, Balanus are 

 absent; the Arctic littoral is characterized by such groups as Oligochaeta, 

 Hydracarina, Turbellaria, Amphipoda and Harpacticidae. 



This view too, however, cannot be accepted. All the groups noted by Madsen 

 live also on the littoral of the boreal region, and to separate the Arctic littoral 

 into a group it is necessary to establish the specific features of its faunal 

 species and the adaptability of its main forms to only one given zoogeo- 

 graphical category. 



The high Arctic shallow-water sub-region in its turn is not homogeneous. 

 It is known that the shallowest parts of the epicontinental Arctic seas, especi- 

 ally the Siberian seas, shelter a rich relict brackish fauna, both of plankton 

 and benthos, which is rich both qualitatively and, especially, quantitatively. 

 The northern parts of the epicontinental bodies of water of the Arctic have a 

 typically marine fauna. 



On these grounds G. Gorbunov suggested (1941) the division of the high 

 Arctic sub-region into two parts: 'the high Arctic or continental water dis- 

 charge and the high Arctic of the open sea'. However, it is better to designate 

 them differently and to divide the high Arctic shallow-water sub-region into 

 sea-water and brackish-water provinces, thus marking the most characteristic 

 difference in the fauna of both parts. The brackish-water province could 

 probably be further divided into several zones according to their degree of 

 salinity and the fauna corresponding to them. Evidently the outer circle of the 

 brackish-water province, adjacent to the sea province of the shallow high 

 Arctic sub-region, is the zone so clearly defined by G. Gorbunov (1941) as 

 that of the distribution of the bivalve, Portlandia arctica (Fig. 1 7), comparing 

 it with that of the distribution of another high Arctic mollusc, Propeamussium 

 (Pecten) groenlandicum major. 



Unlike P. arctica this mollusc cannot endure a lowering of salinity and lives 

 outside the zones influenced by river waters. 



The two molluscs exclude each other, as it were, and are very rarely found 

 in large quantities in the same place. 



Although P. arctica can live under conditions of full sea-salinity, the zones 

 of its mass development are connected with regions exposed to a greater or 

 lesser extent to river discharge. 



