146 A BIOLOGY OF CRUSTACEA 



CRUSTACEA ON LAND 



Life on land presents difficulties which are not encountered when 

 living in water. Temperatures are liable to fluctuate more violently, 

 and there is the ever-present danger of drying-up, which can rapidly 

 prove fatal. A crustacean living on land must be capable of resisting 

 such adversities. Many crustaceans do this by means of their 

 behavioural responses, which tend to keep them in cool moist 

 places (p. 94). They also have some physiological adaptations which 

 enable them to conserve water and to delay death from desiccation. 



First there are those Crustacea which can be regarded as 

 amphibious; they spend some of their time in water and some on 

 land. The isopod Ligia oceanica is a good example of this group; it 

 wanders about on the seashore, and sometimes strays inland for a 

 short way. The concentration of salts in its blood shows compara- 

 tively large variations; it seems as if Ligia has developed a tolerance 

 to such variation, which enables it to withstand a certain amount 

 of drying, hence concentration of the blood, without ill effect. This 

 ability to allow a certain amount of evaporation to occur is probably 

 useful when the animal is exposed to the hot sun, because evapora- 

 tion of water cools the surface from which it is evaporating. In 

 this way Ligia can keep its temperature a few degrees below that 

 of its surroundings, which may make all the difference between 

 survival and death when temperatures are high. 



Some crayfishes habitually leave the water and make burrows in 

 soft earth. In these burrows they keep cool and moist, and so are 

 not exposed to the more extreme conditions of life above ground. 

 This is similar to the way in which many semi-aquatic species sur- 

 vive on the seashore; they make burrows in sand or mud, or hide 

 beneath seaweed and stones. In this way they are protected from 

 the heat of the sun until the tide returns to cover them. 



The land crabs are the most conspicuous of the terrestrial 

 Crustacea, and of these the Robber Crab, Birgns latro, is probably 

 the best known. This is a large crab, weighing up to 5 or 6 pounds, 

 which is widespread in the Indo-West-Pacific areas. It spends most 

 of its time on land, even climbing up trees, but the females return 

 to the sea to release their young, which hatch in the zoea stage 

 and spend the early part of their lives floating in the sea. In com- 

 mon with other land crabs the gills of this species are small, but 

 the gill chamber is modified for aerial respiration by having spongy 

 walls which are richly supplied with blood vessels. The problem of 



