Relationship between the coral pit crab Cryptochirus coralliodytes Heller and its host coral

Abstract

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 Elat (Red Sea), at depths of 2–7 m, the crabs inhabit about 25% of faviid corals of the genera Favia, Favites, Goniastrea and Platygyra. Settlement of crabs at the megalopa stage was observed in June and July (1994) in empty pits in Favites. Settlement 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 observed. 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 between the size of the depression and the size of the inhabiting crab. Examination of the corals' growth rate revealed that the crabs inhibit the growth of their hosts. In the present study, an attempt was made to 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 findings reported in the present study suggest that Cryptochirus coralliodytes crabs are parasites on their coral hosts.