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



Besides Phoca groenlandica and Ph. vitulina other seals — Phoca hispida, 

 Ph. foetida, Halichaerus gryphes — lived in the Littorina Sea. In keeping with 

 the higher salinity during the Littorina stage Cardium edule and Mytilus 

 edulis were larger than they are now in the same places. 



The last phases in the formation of the present-day fauna 

 Approximately 2,000 years B.C. the straits again became shallow ; the sea lost 

 much of its salinity, and entered its present phase. Part of the fauna dis- 

 appeared {Scrobicularia piperata, Rissoa species, both littorines, Phoca 

 groenlandica). The sea was populated by fresh-water species, first of all 

 Limnaea ovata baltica, after which the phase is named the Limnaean Sea 

 (Loven, 1864 and Munthe, 1931). 



Still later, during the second half of the Iron Age, and possibly in historical 

 times, the mollusc Mya arenaria (Myan Sea) and the fresh-water Limnaea 

 palustris, L. stagnalis, and later Dreissena polymorpha, migrated to the Baltic 

 Sea and multiplied abundantly in it. Each of these phases lasted for about 

 4,000 years, differing from the Baltic Sea of today not in their hydrology, but 

 in their fauna. The present phase of the Baltic Sea might quite justifiably 

 be called the 'Macoma Sea' because of the huge predominance in it of the 

 mollusc Macoma baltica. 



Zoogeographical classification of the Baltic Sea 



Owing to the heterogeneity of its fauna components a biogeographical 

 classification of the Baltic Sea presents considerable difficulties. From the 

 Littorina stage the Atlantic fauna vigorously populated this body of water, 

 and as regards this fauna the Baltic Sea should be related to the boreal region. 

 The deep parts of the Baltic Sea, however, and its shallow northern parts are 

 populated by cold-water Arctic relicts of varied genesis : partly relicts of the 

 cold Yoldian Sea, partly members of the original brackish- water community, 

 which in the Baltic Sea found only a secondary centre of settlement, and which 

 probably arrived as early as the time of the Ice Lake, possibly from the far 

 northeast. Both have marked Arctic characteristics and cannot be related to 

 the boreal region. Thus the Baltic Sea has a double zoogeographical aspect : 

 the more shallow, the southern and the southwestern parts of the Sea are 

 populated mainly by boreal fauna, the deeper places, and the northern and 

 eastern parts, by fauna of an Arctic aspect. 



Zonation 



We have had to point out several times that it is impossible to create a single 

 system of division of marine and brackish-water fauna according to the 

 salinity of the water, and that the zonation of each low-salinity body of water 

 must have its own special features. 



The first schemes for the classification of waters according to their salinity 

 were worked out for the Baltic Sea. The problems of brackish waters were 

 also first studied in the Baltic Sea. The scheme of the German hydrobiologist 

 H. Redeke (1922), worked out for the Zuyderzee, was used as the basis of 

 these classifications. 



