DISTRIBUTION I 35 



Atlantic there are several genuine brackish water Crustacea living 

 in the Baltic. These are also found living in brackish water in 

 other parts of Europe, and are frequent in the middle reaches of 

 estuaries. The amphipods are the best represented, with such species 

 as Bathyporeia pilosa, Gammarus zaddachi, G. duebeni and Melita 

 palmata. 



Migration into the Baltic from fresh water has also occurred. 

 This has been enabled by the great stability of the salinity over 

 wide areas. Many fresh-water Crustacea can become adapted to a 

 certain amount of salt in the water provided it does not fluctuate 

 very much. The common fresh-water isopod, Asellus aquaticus, 

 seems to have made the greatest progress in this direction, and may 

 be taken in dredges from the sea bottom at the entrance to the 

 Gulf of Finland. 



The final, and in many ways the most interesting, component of 

 the Baltic fauna is made up of glacial relicts. Most of the glacial 

 relict Crustacea are also found in the Arctic Sea, and seem to have 

 entered the Baltic from an ice lake which was formed near the 

 White Sea and which must have communicated with the Gulf of 

 Finland after the last glaciation. Some of these relicts, such as the 

 isopod Mesidotea ento?non, and the amphipod Pontoporeia affinis, 

 have since become isolated in certain fresh-water lakes around the 

 Baltic. This isolation has been brought about by the gradual 

 elevation of the land (which is still proceeding in some parts of the 

 Gulf of Bothnia at a rate of three feet in a century) cutting off basins 

 which gradually freshened due to the inflow from rivers. 



SALINITY AND THE DISTRIBUTION OF CRUSTACEA 



The importance of the salinity of the medium as a factor in the 

 distribution of Crustacea has already been mentioned, but it is 

 worthwhile examining the effects of dissolved salts in more detail. 

 As mentioned earlier (p. 39), the amount of salt in the blood of a 

 marine crustacean is similar to that in sea water. When the amount 

 of salt in the surrounding water is reduced, as in estuaries, the 

 animal will be subjected to osmotic stress — water will tend to flow 

 into the blood, and the animal will swell, unless it can prevent the 

 entry of water, or has an efficient means of removing it. If for 

 instance the crab Maia sqainado is placed in 80 per cent sea water 

 its weight increases rapidly. About three hours later the weight is 

 reduced to its normal level, and it is found that the concentration 



