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



region, the number of molluscs increases markedly, and such typical North 

 Sea forms as Nucula nucleus, Mya truncata, Corbula gibba, Saxicava rugosa, 

 Teredo navalis, the species Syndesmya and Venus appear, while the Kattegat 

 is the habitat of various species like Leda, Yoldia, Area, Ostrea and Pecten. 

 Macoma baltica (Fig. 137) penetrates farther than any other form into the 

 Gulfs of Bothnia and Finland, surviving a salinity of 3-5 to 4% , and even 

 multiplying intensively in it. Next comes Mytilus edulis, with a salinity limit 

 of 4-5 to 5% , then Mya arenaria with a limit of 5% and Cardium edule reach- 

 ing a limit of 5-25 to 5-50% . 



It is characteristic that in the Gulf of Bothnia along the shores of Finland 

 all forms penetrate farther to the north than along the coast of Sweden ; this 

 is linked with the prevailing currents, which skirt the isohalines of the Gulf 

 of Bothnia to the northwest. 



Of the numerous Kattegat Opisthobranchia only five species penetrate into 

 the Baltic Sea proper : Retusa obtusa (as far as Gotland), Calvina exigua (as 

 far as the Stockholm Quarken), Embletonia pallida, Alderia modesta and 

 Limapontia capitata (the last three species as far as the southern shores of 

 Finland) (see Fig. 146). 



The number of Prosobranchia species in the Kattegat is more than 80, 

 in the Baltic Sea itself only three. Hydrobia baltica is the only species to reach 

 the Finnish coast, and along the southern shores of Sweden Hydrobia palu- 

 destrina {jenkimi) reaches Stockholm (see Fig. 146). 



Among the 1 1 species of marine Copepoda which penetrate into the Baltic 

 Sea proper, four forms common there should be noted : Acartia longiremis, 

 Centropages hamatus, Pseudocalanus elongatus and Temora longicornis. 



Of the three Cirripedia species found in the Baltic Sea, Balanus balanus, B. 

 cretanus and B. improvisus, only the last moves far into the Sea; it is still 

 found in considerable numbers off the shores of Finland, at a salinity of 

 5% (Fig. 146). 



Among the Amphipoda, of which there are 300 species in the North Sea 

 and 132 in the Kattegat, only 12 are found in the Baltic Sea, and only 9 marine 

 and brackish-water species in the waters of Finland. They are : Pontoporeia 

 femorata, P. affinis, P. sinuata (a very rare endemic species), Calliopius rathkei, 

 Gammarus locusta, G. duebeni, Corophium volutator, C. lacustrae and Pallasea 

 quadrispinosa. Pontoporeia affinis, both Gammarus and Corophium volutator 

 reach almost the innermost parts of the Gulfs of Bothnia and Finland (Fig. 

 146). 



The distribution in the Baltic Sea of the two Pontoporeia shows character- 

 istic differences. P. femorata reaches only the Aland Islands and partly enters 

 the Gulf of Finland. P. affinis is an Arctic brackish-water form. The density 

 of its population increases gradually as one moves north and east, as also 

 happens with Pallasea quadrispinosa and Limnocalanus grimaldi (Fig. 146). 

 P. affinis lives in many lakes of Northern Europe and Northern America as a 

 relict. Pallasea quadrispinosa is found in water with a salinity of up to 5 to 

 6% off the Swedish shores of the Central Baltic and the Gulf of Bothnia and 

 in the Gulf of Finland. 



Among the Isopoda, Mesidothea entomon and Iaera albifrons enter farther 



