48 THE BIOLOGY OF MARINE ANIMALS 



(Callinectes sapidus) from different environments show osmotic acclima- 

 tion : those from low salinities (CI about 6% ) survive in extreme dilution 

 (CI < 2-5% ) longer than crabs from high salinities (CI about 25% ). 

 Other examples of hyperosmotic regulating decapods are brackish-water 

 crabs such as Sesarma erythrodactyla, and certain penaeid and palaemonid 

 prawns such as Metapenaeus monoceros and Palaemon serratus. Some 

 prawns even penetrate into fresh water, but the female returns to salt 

 water to breed (98, 102, 108). 



The final stage in euryhalinity, the penetration of fresh water, is depen- 

 dent on the ability of the animal to regulate its blood concentration in 

 highly dilute media, below 8% . The blood of many euryhaline species 

 becomes swamped at these low concentrations but in the Chinese crab 

 Eriocheir sinensis the blood is maintained at a steady level under such 

 conditions. This crab is an Asiatic species which was first noticed in 

 Europe in 1912, and has since extended its range widely. Individuals grow 

 to maturity in fresh water, but return to the sea to breed. The factors 

 permitting Eriocheir to live in fresh water are low permeability to water 

 and salts, and the ability of the gill membranes to absorb salt against an 

 osmotic gradient (77). 



During the moulting process of crabs there is a striking increase in size 

 and weight which is correlated with osmotic changes in the blood. Prior 

 to the moult water is lost, the osmotic pressure of the blood rises and the 

 animal shrinks within its shell. Shortly after moulting sea water is 

 absorbed osmotically and the osmotic pressure falls. In Carcinus maenas, 

 an animal weighing about 50 g absorbs about 35 g during the moult. As a 

 result the animal increases considerably in size and fills out its new exo- 

 skeleton (77, 110). 



In the inter-tidal zone crabs occurring at higher levels may be exposed 

 to the atmosphere for several hours during tidal ebb and remain active. 

 Carcinus maenas has been shown experimentally to live up to 8 days in 

 air, during which time its blood concentration increased from about 610 

 mM to 815 mM. Jones (57) has determined the effect of air exposure on a 

 series of crabs (Hemigrapsus, Pachygrapsus, Uca) during ebb tide and has 

 found that the osmotic pressure of the blood increases, on the average, 

 A 0T8 during this period. A semi-terrestrial crab Ocypode albicans, which 

 dwells near high-tide level, displays well-marked osmoregulatory capacity 

 in air and in anisosmotic solutions. Ocypode is normally hypo-osmotic 

 and possesses a blood-chloride level of 378 mM when the external chloride 

 is 480 mM. Internal chloride levels are maintained constant for 24 hours 

 in air, or in solutions ranging from 120-600 mM (32). 



In warmer regions of the world some crabs (anomurans and brachyu- 

 rans) have taken to a terrestrial existence, although they must return to 

 the sea for reproduction. These animals are protected to a considerable 

 degree against water loss by their relatively impermeable exoskeleton. 

 Littoral and terrestrial crabs show much variation in their ability to with- 

 stand desiccation on exposure to air, but there is some tendency for crabs 



