THE BALTIC SEA 



313 



have penetrated from the south through river systems : Cordylophora caspia, 

 Dreissena polymorpha and Corophium curvispinum. I. Nikolaev (1951) points 

 out that brackish-water forms are the most significant in this group. Evidently 

 they were chiefly transported across the oceans by ships which remained for a 

 long time in harbours, where the salinity of the water is usually low and vari- 

 ous brackish-water forms are numbered among the inhabitants. 



The Chinese crab was first discovered in the lower waters of the Elbe and 

 Weser. It has been suggested that it was brought from China around 1912 by 

 ships, possibly in their water tanks or in the growths which covered the sides 

 of the ship. During the last twenty-five years the crab has migrated along the 

 southern shores of the North Sea, the straits and the shores of the Baltic Sea, 



го intend of 1924 

 VZHfrom1925 to the 



mm 1930-1932 



E3 W33-1935 



Fig. 148. Distribution of the Chinese crab Eriocheir 



sinensis in the Baltic basin (Peters and Panning, 1933). 



Penetration up the rivers is shown by О an d #■ 



and up the river systems. Its migration in the last fifteen years is shown on the 

 chart (Fig. 148). The fact that in new places the crab appears first of all near 

 large ports is evidence of its being brought by ships. Now it has settled over 

 an area of no less than 1,000,000 km 2 . This crab is a small, very active animal 

 (the largest are 7 cm long) which in unfavourable conditions is capable of 

 coming out on land and traversing it for quite considerable distances. In 

 come areas, especially in Germany, the Chinese crab has multiplied greatly 

 and become a very serious pest. It damages fishing nets, but the greatest harm 

 it does is through the destruction of the shore by its innumerable burrows. A 

 persistent campaign is waged against it. In some places as many as 50,000 

 crabs are caught in a day. In the Elbe alone the catch (1935) was more than 

 500 tons a year, i.e. no less than ten million specimens. The crab cannot 

 breed in fresh water ; it comes down to the estuaries for this purpose. 



I. Nikolaev has assembled the data on the second and third groups of forms 



