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



and at times the predominant, part of the population. They penetrate into 

 Baltic Sea waters with a salinity of 4 to 6% , while some forms are found 

 even at a salinity of 7% . Among the fresh-water plants which penetrate the 

 saline Baltic waters we can point out the water moss : Fontinalis dolecorlica, 

 Phragmites communis, several species of Scirpus, Potamogeton, Myriophyl- 

 lum, Ranunculus, Chara, Enteromorpha, Cladophora and Ulotrix. 



The larvae of insects (chironomid, dragonflies, mayfly, etc.) form a highly 

 characteristic part of the population of the considerably diluted waters of the 

 Sea. 



In the least saline parts of the Sea the following fresh-water molluscs are 

 strongly represented: Neritina (Neritella) fluviatilis, Bythinia tentaculata, 

 Physa fontinalis, Paludinacontecta, Limnaeastagnalisvar. livonica, L. ovatavar. 

 baltica, L. peregra, L. palustris var. litoralis, Planorbis vortex, Anodonta and 

 Unio. Among the fresh-water crustaceans, Asellus aquaticus is common in the 

 off-shore waters (up to 6T3% ). In the plankton, even in the open sea, such 

 forms of Rotifera as Anuraea cochlearis, Notholca longispina and Asplanchna 

 priodonta are common. 



Among the fresh- water fish, Coregonus lavaretus, С albula, Abramis brama, 

 Esox lucius, Lota lota, Perca fluviatilis and Thymallus thymallus are widely 

 distributed and are of commercial importance. 



A certain number of plant and animal forms — emigrants from fresh waters 

 and now living in the less saline parts of the Baltic Sea — are either very rare 

 or completely absent from the adjacent fresh-water lakes. Among the plants 

 one may name : Najas marina, Zannichella repens, Z. pedunculata, Potamo- 

 geton panormitonus, Myriophyllum spicatum and Utricularia neglecta; and 

 among the animals : the Porifera Ephydatia fluviatilis, the mollusc Theodoxus 

 fluviatilis, together with some species of water bugs and water beetles. 



Penetration into the Baltic Sea of new species from the Atlantic. The Baltic 

 is a young sea, but it may be assumed that the relationships of the components 

 of its fauna are fairly stable, and that the population of it by marine forms, and 

 the distribution of different inhabitants throughout the Sea, are in the main 

 a complete process. Some forms, however, are still penetrating it, either actively 

 or passively, and migrating from west to east. 



Among new, contemporary immigrants the following groups may be dis- 

 tinguished: (/) immigrants from distant seas; (2) new immigrants from the 

 North Sea ; (5) forms migrating from the western parts of the Sea to the central 

 and eastern parts. 



To the first group belongs the diatom algae Biddulphia sinensis, the gastro- 

 pod mollusc Potamopygus jenkinisi, the copepod Acartia tonsa and the bryo- 

 zoan Alcyonidium palyonum, two crabs — Rhitzopanopeus harrisi spp. triden- 

 tata (Birstein, 1952) and Eriocheir sinensis, perhaps the most interesting 

 representative of this group, is also called the Chinese hairy-legged crab ; it has 

 rapidly populated the shores of the North and Baltic Seas, as if it had found its 

 second home there. Some earlier immigrants should be included in this group, 

 such as Mya arenaria, found off the shores of Europe since the sixteenth and 

 seventeenth centuries (I. Hessle, 1946), and some Caspian elements which 



