RELATIONSHIP BETWEEN THE CORAL PIT CRAB CRYPTOCHIRUS CORALLIODYTES HELLER AND ITS HOST CORAL

Individuals of the pit crabs Cryptochirus coralliodytes Heller inhabit massive corals of the family Faviidae. The nature of the association between Cryptochirus crabs and their host corals has not been studied thoroughly. In the coral reef in flat (Red Sea), at depths of 2-7 m, t he crabs inhabit about 25% of faviid corals of the genera Favia, Favit es, Goniastrea and Platygyra. Settlement of crabs at the megalopa stag e was observed in June and July (1994) in empty pits in Favites. Settl ement of adult crabs on living polyps, which resulted in the death of the polyps at a later stage and the creation of pits, was also observe d. The presence of spines and pointed projections on the crabs' body, suggests the existence of a mechanism of mechanical abrasion by which the pit is adjusted to the crabs' size. Many pits were found to occur at the bottom of depressions, and a positive correlation was found bet ween the size of the depression and the size of the inhabiting crab. E xamination of the corals' growth rate revealed that the crabs inhibit the growth of their hosts. In the present study, an attempt was made t o determine the nature of the association between the two partners by examining the crabs' distribution, their influence on the host growth rate and the mechanism of adjusting the pits to their size. The findin gs reported in the present study suggest that Cryptochirus coralliodyt es crabs are parasites on their coral hosts. (C) 1997 Elsevier Science B.V.