THE BALTIC SEA 341 



As for the glacial marine relicts inhabiting saline-brackish waters which 

 could not have populated the fresh-water ice lakes, Ekman, sharing the 

 point of view of Miinthe and Sauramo, thinks that they arrived in the Yoldian 

 basin from the west. 



The climatic conditions of the North Sea and the adjacent parts of the 

 Atlantic at the time were so severe that the Arctic fauna may have migrated 

 far to the south and lived in the North Sea. However, this fauna may have 

 come from the northeast, if we assume that the Yoldian Sea was connected 

 with the White Sea, or if this connection existed at later periods. 



Sagerstrale (1957) likewise accepts this route of the penetration into the 

 Baltic Sea of a part of the brackish-water Arctic relicts ; he divides them into 

 several groups according to the time and route of their penetration into the 

 Baltic Sea. 



(7) Limnocalamis macrurus (according to Sagerstrale the Baltic form L. 

 grimaldii evolved from it), Pontoporeia affinis, Pallasea quadrispinosa and 

 Mysis relicta were the first immigrants from the fresh waters in the north to 

 the Baltic Sea. Sagerstrale thinks that the isopod Pallasea came from the 

 fresh waters of Siberia. 



(2) The second group of relicts, Mesidothea entomon, Gammar acanthus 

 lacustris, Cottus quadricornis and Phoca hispida, penetrated into the Baltic Sea 

 during the period of ice-recession, when the Gulf of Finland was freed of ice. 

 They may have migrated from the northeast from an ice lake in the area of 

 the White Sea, perhaps during the Littorina period when the water was not 

 yet saline. 



(5) During the Littorina stage all these relicts were pushed into the least 

 saline areas and the penetration of Atlantic fauna and flora from the south- 

 west began ; for example : Littorina J it tor ea, Pontoporeia femorata, Mysis 

 mixta, Halicryptus spinulosus, and other remains of the cold-water fauna of the 

 Ice Age. In the case of some forms — Pallasea quadrispinosa is given as an 

 example — S. Sagerstrale (1957) accepts the view of P. Pirozhnikov (1937) and 

 E. Gurjanova (1946, 1951) regarding the west Siberian (Kara Sea) centre of 

 the evolution of a number of forms, and of their migration south down to the 

 Caspian Sea west of the Ural mountains via the Ob basin ; this theory is based 

 on the fact that the Kara Sea is now inhabited by a community of brackish- 

 water forms nearest to the Caspian immigrants (Fig. 169). For the rest, 

 Sagerstrale is inclined to consider P. quadrispinosa as genetically related to the 

 Lake Baikal P. kessleri. The fresh-water bodies of water of the Ice Lake 

 period may have served as further routes of migration (Fig. 170). Following 

 the opinion of Soviet authors (N. Lomakin, 1952), Sagerstrale is inclined to 

 connect the migration of the Caspian Pontoporeia (P. affinis microphthalma), 

 Gammaracanthus (G. loricatus caspius) and Mesidothea entomon with the 

 fate of the Siberian ice lakes. 



In whatever way these forms penetrated into the basin of the Baltic Sea, as 

 a result of a subsequent change in the coastal contour and the rise 

 of temperature in the adjacent areas of the Atlantic, a discontinuous habitat 

 was created, the conditions of the Baltic were altered, and forms became 

 partly extinct, partly relicts. 



