DISTRIBUTION 1 39 



about ten days without food in pure distilled water, and can live 

 a normal life in very dilute solutions. There must be a very 

 efficient mechanism for retaining salts and for taking them into 

 the blood from dilute media. 



Most fresh-water Crustacea have become so completely adapted 

 to their medium that they cannot tolerate sea water. Daphnia 

 pitlex dies in less than twenty hours in water containing one part 

 in six of sea water, and dies correspondingly quicker in higher con- 

 centrations. A notable exception to the general rule is the fresh- 

 water race of the large isopod Mesidotea entomon from lakes in 

 Sweden. This species also lives in the Baltic and Arctic seas. The 

 specimens from the Baltic cannot be acclimatised to live in fresh 

 water, but the specimens from the fresh-water lakes can be acclima- 

 tised to live in full-strength sea water. This is taken as a sign that 

 Mesidotea is a recent immigrant into fresh water, and in fact the 

 fresh-water lakes in which it lives came into being after the Ice 

 Age (see p. 135). 



So far we have been dealing with the problems which arise when 

 a crustacean lives in water which is more dilute than sea water, but 

 there are some inland waters which contain more salt than the sea. 

 Few Crustacea can live in such places, and by far the most successful 

 of these is the brine shrimp, Artemia salina. This anostracan can 

 survive in water so saturated with salts that crystals form around 

 the edge of the pool. It can also survive in water that is only one- 

 tenth the strength of sea water. However, in nature it is not usually 

 found in water less than twice the strength of sea water. Clearly 

 such an animal must have an efficient means of regulating the com- 

 position of its blood. Recent studies by Croghan (1958) have shown 

 that Artemia can keep the concentration of salt in its blood well 

 below that of the strong brines in which it swims. When in water 

 which is eight times as strong as sea water Artemia keeps its 

 blood only slightly more concentrated than sea water. The gills 

 play an important part in this process by excreting salt, while the 

 gut is also important in actively taking in water to compensate for 

 that which tends to be lost by osmosis to the strong outside medium. 

 Salt is also taken in through the gut, and a balance is established 

 between the intake through the gut and the output through the 

 gills. 



It is clear from the last paragraph that Artemia could live in the 

 sea, or even in more dilute water. Its absence from such situations 

 is to be explained by its vulnerability to attack by predators; it is 



