I38 A BIOLOGY OF CRUSTACEA 



Table 4 gives the rate of urine production in several different 

 Crustacea. The fresh-water crayfish also seem to have adapted them- 

 selves so that they do not have to produce a great flow of urine, 

 The most interesting feature of the table is the great production of 

 urine by the fresh-water prawn Palaemonetes antennarius, which, 

 in relation to its size, produces ten times as much urine as a cray- 

 fish. It seems that P. antennarius has comparativelv recently entered 

 fresh water and has not yet adjusted its rate of urine production. 

 The table also shows that the rate of urine production is about that 

 which might be predicted from its brackish water relative P. varians 

 if it were to live in fresh water. 



Fresh-water Crustacea make some concessions to their environ- 

 ment in that they do not maintain the concentration of their blood 

 as high as that of their marine relatives. A reduction in the con- 

 centration of the blood is a convenient method of reducing the 

 osmotic stress, for the gradient between inside and outside will not 

 be so great and so will be easier to maintain. The concentration of 

 salts in the blood of the European crayfish is just under half that 

 of sea water, and it maintains this concentration in river water 

 with less than one-hundredth the concentration of the sea. 



There is evidence from the structure of the crayfish antennal 

 gland that this organ can reabsorb salts from the urine. The fresh- 

 water crayfish has an extra loop in the gland, which is not present 

 in its marine relatives. Fresh-water amphipods also have larger 

 excretory organs than the marine members of the same order. But 

 this is not a general rule; the fresh-water crabs have antennal glands 

 which are similar in size and structure to those of their marine 

 relatives. That the extra loop in the crayfish antennal gland func- 

 tions in reabsorbing salts has been shown by measuring the con- 

 centration of chlorides at the beginning and end of the loop. At the 

 end of the loop, nearest the opening to the outside, the concentra- 

 tion is less than half the concentration at the beginning. 



Large fresh-water Crustacea, such as crabs and crayfish, deal with 

 the problem of osmotic stress in three ways. They are only slightly 

 permeable, they take in salts through their gills, and they reabsorb 

 salts from their urine. The smaller Crustacea are not so easy to 

 study, and much less is known about their methods of regulating 

 the concentration of their blood. Many of them are very permeable 

 to water. Experiments with Daphnia magna show that there 

 is 80 per cent exchange between the blood and surrounding water 

 in about two minutes. Nevertheless Daphnia magna can live for 



