WATER, SALTS AND MINERALS 45 



show well-marked zonation from permanently submerged to splash levels 

 and species inhabiting the upper shore must be subject to considerable 

 osmotic stress. In a South African investigation it was found that littoral 

 snails could be graded in sequence according to their tolerance of desicca- 

 tion and changes of salinity; this sequence accords with their vertical 

 distribution and with gradation of like conditions in their environment 

 (17, 22, 51). 



Osmoregulatory Powers in Crustacea. The majority of Crustacea are 

 wholly marine, but various groups have representatives which have pene- 

 trated into brackish or even fresh waters, or have invaded the land and 

 become terrestrial. As a class the Crustacea are characterized by a hard 

 exoskeleton which may be strengthened by lime deposits. Among other 

 functions the exoskeleton reduces permeability. 



The strictly oceanic Crustacea found in the lower littoral zone or in 

 sublittoral habitats are stenohaline and have body fluids isosmotic with 

 sea water, e.g. Maia, Portimus and others. The blood concentrations of 

 these animals follow closely that of sea water over the range A 2-5 to 1-5 

 or somewhat lower (Fig. 2.13). The spider crab Maia is unable to survive 

 for more than a few hours in sea water diluted below one-fifth. In 80 % 

 sea water it quickly swells, but in less than a day its weight returns to 

 normal as salts and water are lost, and its body fluids regain osmotic 

 equilibrium with the environmental medium (Fig. 2.14). 



A slight tendency to maintain the blood hypertonic to the external 

 medium at concentrations below A 1-5 is shown by the edible crab Cancer 

 pagurus. Cancer swells to a smaller extent in dilute media than Maia, its 

 rate of swelling is less and volume regulation is also slower (Fig. 2.14). 

 Cancer is less permeable to water and salts than Maia, and manifests a 

 slight amount of osmoregulatory ability in hypotonic media. 



By the use of iodide, which can be detected easily, estimations have 

 been obtained of the permeability of the integument of various stenohaline 

 and euryhaline crabs. In solutions containing this ion it has been found 

 that the stenohaline brachyurans Hyas araneus and Portunus depurator 

 are much more permeable than the relatively euryhaline Cancer pagurus 

 and Carcinus maenas. Penetration takes place mostly through the gills but 

 also proceeds at a slow rate through other external surfaces. 



The common shore crab Carcinus maenas shows well-developed powers 

 of osmoregulation in hypo-osmotic media and can survive dilutions down 

 to A 06. In normal sea water the blood is isosmotic with the external 

 medium, but in increasing dilutions it develops hypertonicity until at an 

 external concentration of zl 0-6 the internal concentration is maintained at 

 twice that value; lower external concentrations are lethal (Fig. 2.13). 



Carcinus shows little swelling in dilute sea water when compared with 

 other brachyurans previously considered (Fig. 2.14). Permeability to water 

 and salts is relatively low in Carcinus, but nevertheless some transfer still 

 occurs across the gills in hypo-osmotic media, water being absorbed and 

 salts lost. The water continually flowing in is eliminated by the kidneys, 



