THE BALTIC SEA 



333 



out during 1928-31, have shown that Corophium volutator lives for only one 

 year. Over this period the fluctuations in the number of specimens of this cray- 

 fish per 1 m 2 , all collected in the same place, are given in Table 137. 



Table 137 



1928 



May Jul Sep Nov 

 244 184 5,429 4,210 



1929 



May Jun Jul 

 3,151 1,712 105 



Nov 

 1,774 



1930 



Apr May Jun Oct Nov 

 338 188 4 81 124 



1931 



Apr Oct 

 56 1,992 



Nov 

 1,834 



The same type of fluctuations were observed by Sagerstrale in the case of 

 another amphipod, Pontoporei aaffinis. These fluctuations are of special 

 interest since both crayfish are important items in the diet of fish. 



A very approximate estimate, probably with considerable errors, can be 

 made for the benthos biomass of the whole Baltic Sea and its separate regions 

 for the summer season {Table 138). 



VI. ORIGIN OF THE FAUNA 

 The main components 



Four main components can be distinguished in the Baltic Sea fauna : (7) marine 

 cold-water relicts of the post-glacial period ; (2) true brackish-water fauna, 

 consisting mainly of Arctic brackish- water relicts of the Ice Age ; (3) marine 

 fauna, representing a greatly impoverished Atlantic fauna ; and (4) fresh-water 

 fauna (its most euryhaline representatives). The first group, and to some extent 

 the second, form groups of relicts of cold-water European boreal and Arctic 

 fauna. 



The marine cold-water relicts of the Ice Age 



According to Ekman's determination, a form may be considered a relict for 

 a given area if its habitat is cut off from its main habitat and if it or its original 



