1 34 A BIOLOGY OF CRUSTACEA 



Balanus improvisus appears to have been transported accidentally 

 by vessels from the Sea of Azov through the Volga-Don Canal. 

 This latter species has increased rapidly in what has been described 

 by a Russian biologist as a ' biological explosion '. Balanus eburneus 

 is the most recent species found in the Caspian, but no explanation 

 of its occurrence has yet been offered. 



The Aral Sea has a very poor selection of Crustacea, with no 

 crabs, only one amphipod (Pontogammarus aralensis) and under 

 thirty of the smaller Crustacea. The numbers of each species are 

 also low compared with other areas. The ostracod Cyprideis littoralis 

 is found in hundreds of thousands per square metre in the Sea of 

 Azov, but barely a thousand per square metre are found in the 

 Aral Sea. 



The Baltic differs from the other inland seas in that it is further 

 North and, on the average, has less salt dissolved in it. The com- 

 munication between the Atlantic and the Baltic is by means of the 

 Skagerrak and Kattegat, then through the narrow channels be- 

 tween the Danish Islands. There is a considerable difference between 

 the salinities of the waters on the two sides of the Danish Islands. In 

 the middle of the Kattegat the salinity is about two-thirds that of 

 the North Sea, while at the southern tip of Sweden there is only 

 half this concentration of salts. The salinity decreases as one passes 

 up the gulfs of Bothnia and Finland. Many Crustacea of the 

 Kattegat do not enter the Baltic; 64 species of decapods are known 

 from the Kattegat, only thirteen from south of the Danish Islands, 

 and but two from the gulfs of Bothnia and Finland. Different 

 species of the same genus penetrate the Baltic to different extents; 

 of the barnacles, Balanus crenatus get as far as the South of Den- 

 mark, while B. improvisus gets well into the gulfs of Finland and 

 Bothnia. 



The reduced salinity of the Baltic is not the only factor which 

 limits the spread of Crustacea. Rhithropanopeus harrisi tridentatus, 

 a small crab, was introduced into Europe before the end of the 

 eighteenth century and lived mainly in the brackish waters of 

 Holland. From there it was transported by Dutch boats to the Kiel 

 Canal and has since entered the Baltic north-west of Kiel, but it 

 does not seem to be spreading any further. Salinity is not the 

 limiting factor, because this species can live in fresh water, it seems 

 that the low temperature of the Baltic in winter is too severe to 

 allow any further spread. 



Apart from the marine animals which have penetrated from the 



