THE BALTIC SEA 311 



Many fresh-water forms decrease in size when they penetrate into brackish 

 water as, for example, Theodoxus fluviatilis or Bithynia tentaculata. 



The change in the size of the body with the passage from one medium to 

 another is illustrated in Fig. 147. 



A. Remane (1935) has observed that alongside the decrease in size there is 

 a reduction of the calcareous skeleton as one moves into less saline areas. 



Brackish-water forms are also reduced in size as they move into fresh 

 water, but they do not become smaller when they move into more saline 

 waters ; examples are Gasterosteus aculeatus, Pleuronectes flesus, Hydrobia 

 ulvae, etc. 



Preponderance of North Atlantic littoral species. One of the most remarkable 

 features of Baltic Sea fauna is the huge preponderance of typical littoral 

 forms belonging to the North Atlantic. Almost all the main forms of the 

 littoral of the North Sea, Scandinavia, the Murman Peninsula and the White 

 Sea are encountered here : among the polychaetes : Fabricia sabella, Arenicola 

 marina, Pygospio elegans, Nereis diversicolor, Nephthys coeca; among the 

 Gephyrea: Priapulus caudatus and Halicryptus spinulosus; among the mol- 

 luscs : Macoma baltica Mya arenaria, Cardium edule, Mytilus edulis and some 

 species of Hydrobia and Limapontia capitata ; among the Crustacea : Gam- 

 marus locusta, G. duebeni, Jaera albifrons, Balanus improvisus; among the 

 echinoderms : Asterias rubens ; and even the common littoral fishes : Pholis 

 gunellus and Zoarces viviparus. This phenomenon, wholly exceptional in its 

 scale, of almost all the littoral fauna migrating into the sublittoral, deserves 

 the closest attention of biologists. 



Presumably the colonization of the sublittoral in the Baltic Sea by the 

 biocoenosis Macoma baltica could have taken place only in circumstances 

 under which this horizon was poorly colonized by other organisms. The 

 phenomenon of competition or, so to speak, the biological resistance offered 

 to the colonization of the sublittoral by the fauna already existing there, was 

 either very weak or non-existent. 



Probably during the Littorina Period the littoral biocoenosis of Macoma — 

 highly eurybiotic as regards salinity, temperature and oxygen — penetrated 

 without difficulty into the Baltic Sea. Meeting no serious competitors, it 

 populated densely the upper levels of the sublittoral. Eurytopic to a high 

 degree, these littoral forms penetrated farther into the Baltic Sea to waters 

 which are less saline. The Baltic Sea is tideless and their allied biotope is 

 absent there but, owing to their euryhalinity and the absence of competition, 

 they took almost complete possession of the upper level of the sublittoral. 

 The Arctic relict cold-water community is predominant at the lower hori- 

 zon but it too moved to much lower levels : it is related to the zone of the 

 shore off Greenland, while in the Baltic Sea it is concentrated in the deep- 

 water zone. 



Fresh-water forms. As one moves farther into the Sea the marine forms be- 

 come less numerous at the same time as the fresh-water forms come more and 

 more into evidence ; in the least saline parts of the Sea they form a considerable, 



