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



Copepoda Acartia tonsa, Eurytemora hirundo, Limnocalanus grimal- 



dii, Nitocra lacustris, N. spinipes, Ectinosoma curticorne, 

 Mesochra rapiens, M. liljeborgi, Laophonte mohammedi, 

 Schizopera clandestina, Cletocamptus confluens, Horsiella 

 brevicornis, Idunella muelleri 



Ostracoda Cyprideis littoralis, Loxoconcha gauthieri, Leptocytere 



castanea, Cypridopsis aculeata, Heterocypris salina, Can- 

 dona angulata 



lsopoda Tanaidacea, Mesidothea entomon, Sphaeroma rudicaudum 



Mollusca Hydrobia Jenkins i, Conger ia cochleata. 



A list of 1 5 plant and 87 animal forms is obtained from both investigators 

 taken together. 



What prevents this fauna from leaving the limits of the brackish-water zone 

 and from becoming euryhaline forms in the broadest sense of the word? After 

 all they are all descendants of either marine or fresh-water forms. This is a 

 complex phenomenon, which cannot be explained by a single cause. First 

 one must point out that the brackish-water fauna of the Baltic Sea consists 

 basically of three groups — crustaceans, fish and Rotifera. Hence not all the 

 animal groups constitute equal parts in the population of the brackish-water 

 zones. 



The origin of the local brackish- water fauna can most probably be ex- 

 plained by the centuries-long fluctuations of salinity suffered by a zone of 

 transitional salinity. The aquatoria, which contain the most typical brackish- 

 water community, are known to have passed through continuous changes of 

 salinity (named by L. Zenkevitch (1933) 'salinity pulsations') during previous 

 geological periods (the Quaternary and to some extent the Tertiary too), and 

 at the present day a typical instability of saline conditions is characteristic of 

 them. Salinity fluctuations in one direction or the other must inevitably have 

 attracted certain forms from both the marine and fresh-water fauna; and 

 after that, in the order of species formation, the salinity fluctuations must have 

 strengthened in a hereditary way the adaptation of an organism to varying 

 salinity. In this process the biocoenotic factor no doubt played a significant 

 role. 



Remane accepts the possibility of specific action of brackish water on 

 organisms. However this is only a surmise ; there are no precise data about 

 it. For the rest, if the explanation given above is accepted, there is no need of 

 any further explanation. It is of interest to note that forms of marine origin 

 are preponderant in the brackish-water fauna, comprising about 60 per cent. 



Other zonal classifications according to salinity have been used for parti- 

 cular areas of the Baltic Sea. Thus A. Wilier (1925), in his magnificent survey 

 of Frishhaff, used the generally accepted terminology of the classification of 

 bodies of water according to salinity (eury-poly-meso-oligo-steno-halinity), 

 but he attached to it a purely local meaning, as if Frishhaff had been a marine 

 body of water of full value as regards salinity. 



He distinguishes inside Frishhaff, for example, stenohaline brackish- water 

 organisms, typical of a 'polyhaline' zone, and 'euryhaline brackish- water 



