The association between the hermit crab Dardanus arrosor (Herbst) and the sea anemone Calliactis parasitica (Couch)

Abstract

The association between Calliactis parasitica and the Mediterranean hermit crab, Dardanus arrosor , has been re-examined. It was confirmed that, unlike the Atlantic crab, Pagurus bernhardus , Dardanus arrosor can display an active behaviour pattern which assists the transfer of Calliactis to its shell. Extensive trials showed, however, a marked tendency for crabs to divide into two groups: ‘performers’, which show this behaviour almost without fail; ‘non-performers’, which almost never show this activity. Most of the ‘performers’ were females and the ‘non-performers’ males. Calliactis frequently transfers to Dardanus -occupied shells without the crab’s help. Actively ‘performing’ Dardanus often fail to transfer Calliactis to their shells. Observations on these cases and experiments with inactivated shells show that for successful crab-aided transfers the co-operation of the anemone is necessary. This co-operation depends on a response of the tentacles to the shell. Apparently it is similar to the ‘clinging’ response to Buccinum shells at Plymouth, now known to depend on a molluscan shell-factor. Crabs and anemones from the Atlantic and the Mediterranean are able to form associations and display their normal behaviour patterns in response to individual partners from the other population. The results are discussed in relation to the possible origins and advantages of the association to the two partners in the two cases.