Pit structure and trophic relationship of the coral pit crab Cryptochirus coralliodytes

Individuals of the pit crabs Cryptochirus coralliodytes Heller inhabit mass ive corals of the family Faviidae. Their pit walls were observed to be cove red by blue-green algae and fungi. We suggest that the crabs enhance the gr owth of these algae and fungi with their metabolic excretions, which contai n ammonium. The endolithic algae and the fungi may facilitate the abrasion of the coral skeleton by the crabs, by perforating it and thus weakening th e skeletal structure. Computerized tomography analysis revealed dense skele tal material around the pits. Transverse sections showed that the calcifica tion around the pit was similar to other parts of the colony, whereas the m acro-architecture was different. Such a difference is the result of the cra bs' influence on the corals' living tissue, possibly on the calicoblast whi ch deposits the coenosteum Crabs, which were exposed to carbon-labeled cora ls for 1, 7 and 18 d, accumulated labeled carbon, indicating transfer of ca rbon from the coral tissue to the crabs. Histochemical examination of the s tomach and gut of crabs revealed the presence of mucopolysaccharids in the gut, supporting the hypothesis that the crabs eat coral products. The findi ngs of this study provide additional evidence that C. coralliodytes are par asites and support the general hypothesis that a nutritional relationship m ay have served as a basis for selection.