FUNGI IN CORALS - SYMBIOSIS OR DISEASE - INTERACTION BETWEEN POLYPS AND FUNGI CAUSES PEARL-LIKE SKELETON BIOMINERALIZATION

The skeleton of live coral Porites lobata is regularly bored by euendo lithic algae (mostly Ostreobium quekettii) and fungi, both commonly ex tending up to the very tips of newly produced skeletal spines. The Liv e polyp tissue of P. lobata occupies a 4 to 5 mm thick surface layer o f the corallum, within which new skeletal material is deposited. Thus, the endoliths do not constitute a separate zone beneath the live poly ps; rather, the polyp tissue and populations of endolithic algae and f ungi of significant densities co-exist and interact within the same la yer. Aragonitic, hemispherical to conical outgrowths protruding from t he walls of structural pores were observed in skeletons of P. lobata f rom the barrier reef of Moorea Island, near Tahiti, French Polynesia. These protrusions were always associated with endolithic fungal hyphae attempting to exit from the skeleton into the space occupied by polyp s. The polyps responded to such intrusions in a manner similar to the response of mollusks to foreign bodies: by local deposition of dense s keletal material. As the fungus continued to penetrate through this re pair deposit, new layers of aragonite were added by the polyp, contrib uting to the growth of the protrusions. Fungal hyphae rarely entered t he pore spaces while these were still occupied by coral polyps. More o ften, the polyps escaped the fungus by moving upward, as a part of the ir normal growth rhythm, evacuating the previously occupied skeletal p ores. Deprived of resistance, fungal hyphae penetrated through the con es and exited into emptied pore spaces. The conical structures were af fected by diagenesis differently than the intact skeletal carbonate. B oth skeletal carbonate and repair carbonate were subject to syntaxial diagenetic crystal growth, but they resulted in cements with disparate crystal sizes. Septate fungal hyphae are common in coral skeletons as euendoliths, as cryptoendoliths in structural voids, and as endophyte s inside filaments of endolithic algae. They were also found inside so ft coral tissue. Fungi may be opportunistic pathogens in corals under environmental stress. Their activity, recorded and preserved in the co ral skeleton, provides information on changes in past conditions of co ral growth.