WATER, SALTS AND MINERALS 



47 



which produce a greater volume of urine under these conditions (cf. 

 Fig. 7.3, p. 299). 



In all concentrations of the external medium the urine tends to be 

 isotonic with the blood with some variation in either direction. The kid- 

 neys, therefore, play no part in salt conservation and the hypertonic level 

 of the blood is maintained by active salt absorption across the gill mem- 

 branes. This involves osmotic work, necessitating a higher rate of metabol- 

 ism. Increased respiratory activity has been detected in prawns (Palae- 

 monetes) and crabs {Carcinus, Ocypode) that have been transferred to 

 anisosmotic media. It has been estimated that the osmotic work performed 

 at the body surface and excretory organs of brackish and freshwater 



24 



Time (hours) 



Fig. 2.14. Changes in Weight of Brachyuran Crabs 

 when Transferred to Dilute Sea Water 

 Time, in hours, after placing in experimental medium. Cancer pagurus and Portunus 

 depurator in 67% sea water (from Hukuda, 1932). Maia verrucosa and Carcinus maenas 

 in 80% sea water (from Schwabe, 1933.) 



animals amounts to about 1% of total metabolic energy (32, 57, 77, 19a, 

 103, 106tf, 119, 142). 



Many decapods are able to regulate the concentration of their blood in 

 hyperosmotic as well as hypo-osmotic media, and some prawns and 

 grapsoid crabs are actually hypo-osmotic to normal sea water under 

 natural conditions (Fig. 2.13). The Australian rock crab Leptograpsus 

 variegatus, which lives on the shore, displays definite hypotonicity to 

 normal sea water (blood A 1-97, in sea water A 2T4) and similar condi- 

 tions are found in Heloecius cordiformis, a crab inhabiting mangrove 

 swamps where it is uncovered at low tide. The American shore crabs Uca 

 crenulata and Pachygrapsus crassipes have blood concentration curves that 

 reveal a high degree of osmotic regulation, the blood becoming hypotonic 

 when the external medium exceeds 31-2% , and remaining hypertonic 

 when the external medium falls below this value (57, 109). 



Some of these animals are normal inhabitants of brackish and estuarine 

 waters where their osmoregulatory ability is of adaptive value. Blue crabs 



