Behaviour of Two Species of Grapsid Crab During Intraspecific Encounters

1) Behaviour during intraspecific encounter is described for two species of grapsid crab. 2) Ritualized agonistic behaviour, in the form of shield posturing, display and fighting, is described for both species. In Goniopsis it occurs during encounters between similar sized adults of either sex but in Aratus it only occurs during encounters between similar sized adult males. This difference is linked with sexual dimorphism of the chelipeds which is marked in Aratus but not obvious in Goniopsis. 3) It is concluded that reactions to intraspecific encounter serve to maintain a dominance order and that cheliped size is a major indicator of status. 4) Previous work has shown that agonistic behaviour in grapsoid crabs is usually centred on defence of territory. Aratus and Goniopsis, however, are not territorial.