DISTRIBUTION 1 5 I 



two main ways in which dispersal can be achieved : it may be due 

 to the animals own locomotory activity, or it may be passive so 

 that the animals own locomotory efforts are not directly involved. 



The most spectacular of the migrations performed by the Crus- 

 tacea is the annual march to the sea by land crabs of the family 

 Gecarcinidae. The common West Indian species, Gecarcinus 

 ruricola, comes down from the hills in swarms during the rainy 

 season in May. The crabs spend a couple of weeks on the beaches 

 and the females enter the sea to allow the young to hatch from the 

 eggs. When this has been done the crabs migrate back up to the 

 hills until next year. They are followed a few weeks later by the 

 young crabs which have passed through their larval stages in the 

 sea. 



Marine crabs and shrimps show comparable migrations due to 

 their own locomotorv activities. Many species move offshore during 

 the winter and return to shallower water during the spring and 

 summer. Marking experiments with crabs and lobsters have 

 revealed that although these creatures indulge in their offshore 

 migrations they do not migrate very far away from their own 

 particular patch of coast. The really big migrations are those made 

 passively under the influence of the oceanic currents. 



Some species are distributed as adults by the ocean currents. 

 Crabs of the genus Planes cling on to drifting seaweed and have 

 become very widely distributed, particularly in the warmer parts 

 of the world. The isopod Idotea metallica arrives occasionally in 

 British waters, having been transported across the Atlantic by the 

 North Atlantic Drift. But it is particularly the larval stages that 

 are susceptible to dispersal by water currents, and which confer the 

 advantages of wide distribution on species which have limited 

 powers of locomotion when adult. 



One striking example is the Robber Crab, Birgus latro, which, 

 when adult, lives on land and is quite incapable of swimming. This 

 crab has become widely dispersed all over the Indo-West Pacific 

 region by virtue of its larvae, which are released into the sea by 

 the females. The crab has thus managed to colonise remote islands 

 that it could never have reached if it attempted to migrate as an 

 adult. 



The barnacles also provide a striking example, because their 

 adult powers of locomotion are non-existent, although some do 

 attach to driftwood and others to swimming animals (see p. 102). 



Although water currents are the most frequent agents of dis- 



