Ecological Aspects of Some Coral-Boring Gastropods and Bivalves of the Northwestern Red Sea

Abstract

More than 17 molluscan species were obtained from burrows in coral substrata at Al-Ghardaga (Hurghada on maps) on the Red Sea coast, six of which in particular bore into livingcolonies. The species reported in this paper belong to the families Mytilidae, Coralliophilidae, and Gastrochaenidae. The direction of boring in living corals is to the outside, the borers keeping pace with the growing coral layer to maintain their burrows open. Coral growth is generally of a higher rate than that of borers, and burrows are accordingly mostly much larger than their inhabitants. There is evidence in such cases that burrows form initially by growth of coral around the settling young. Boring of Lithophaga species is mostly due to the abrasive action of the shell which moves straight and posteroventrally without any rotation. Incoralliophilids, boring is also executed mechanically by the turning movements of the shell. Boring in dead coral is directed inwards, and burrows are nearly as large as the borers. The latter avoid the blocking of their burrows ( e.g. , by a living coral incrustation) either by great siphonal extension ( Rocellaria ) or the free ends of the shell may be strengthened to maintain the capability of boring in the opposite direction ( Lithophaga laevigata ). Both L. luevigata and Modiola chmamomeus bore mainly mechanically by the rocking movements of the shell. Chemical boring is still a possibility,particularly in the posterior narrow region of burrows of Modiola lodging the extended pallial siphons which are deprived of any effective mechanical devices for boring. Therole of boring algae in rarefying bored coral material has also to be included as an indirect chemical factor.