WATER, SALTS AND MINERALS 



49 



occurring near or above high-tide mark to survive longer in the atmosphere 

 than more aquatic species. The terrestrial hermit crab Coenobita diogenes 

 and the land crab Gecarcinus lateralis live in air for 4 days or more whereas 

 the oceanic crabs Portunus sulcatus and P. spinimanus die in an hour. 

 Some determinations of blood concentrations of littoral and terrestrial 

 decapods made in Florida by Pearse appear in Table 2.5. 



The majority of terrestrial and semi-terrestial crabs are hypo-osmotic 

 to sea water, and are able to regulate in dilute and concentrated media. 

 Hyperosmotic regulation in these terrestrial crabs is an expression of their 

 ability to conserve water and discharge salts, both processes of adaptive 

 value in dry environments. When desiccated, Pachygrapsus is able to take 

 up water against a salinity gradient. The coco-nut crab Birgus latro shows 



TABLE 2.5 

 Blood Concentrations of Decapod Crustaceans from Different Habitats 



specific behavioural adaptations for life on land: it can drink water from 

 small puddles, moisten its respiratory membranes with external water, 

 and control the concentration of its blood by selecting water of the 

 appropriate salinity to drink (42, 57, 104). 



The shore isopod Ligia oceanica is also osmoregulatory to a notable 

 degree. The blood of this animal shows a value of A 2T5 in normal sea 

 water. The blood concentration is held fairly steady when the sea water 

 is varied in concentration from 100-50%. Outside this range the blood 

 concentration rises or falls, but osmoregulation still holds the blood hypo- 

 or hyperosmotic to the external medium, when this deviates from accus- 

 tomed values (101). 



Amphipods are very common inhabitants of brackish and estuarine 

 water. Various species of gammarids show well-marked temperature and 

 salinity preferenda (Fig. 2.15). The typically marine species such as 

 Marinogammarus marinus, and Gammarus locusta can withstand sea water 

 diluted down to 25%, under which conditions they maintain the blood 

 markedly hyperosmotic. The brackish-water species G. duebeni shows a 

 wide tolerance of concentrations from full strength to 2 % sea water, and 



