Endolithic fungi in reef-building corals (Order: Scleractinia) are common, cosmopolitan, and potentially pathogenic

Reef-building corals appear to exist in dynamic equilibria with four principal partners: interconnected polyps of a colonial coelenterate, endosymbiotic dinoflagellate zooxanthellae residing in the host's endoderm, endolithic algae that penetrate coral skeletons, and endolithic fungi that attack both endolithic algae and the polyps. Although reports of fungal and algal-like endoliths in corals date back almost 150 years (1) and evidence of a fossil history extends as far back as the Upper Devonian (~370 ma) (2), most attention has been paid to the structure (3), function (4, 5), and diversity (6) of the coral-zooxanthellae interactions, ignoring the endolithic members of the consortium. Recently, Le Campion-Alsumard et al. (1995) (7, 8) described an interrelationship between endolithic algae and fungi within the massive coral Porites lobata Dana 1846 (Poritidae), on Moorea island near Tahiti, French Polynesia. Fungi were also found to penetrate the most recently deposited skeletal material and to be associated with pearl-like skeletal deposits formed by polyps of P. lobata in response to attack by their heterotrophic, endolithic symbionts. Here we extend these observations to the pocilloporid coral Pocillopora eydouxi and to the acroporid corals Acropora cytherea, Acropora humulis, and Montipora cf. studeri, collected at Johnston Atoll, central Pacific Ocean. Our observations suggest that direct coral-fungal interaction is widespread, not only geographically, but taxonomically as well. Thus, fungal endoliths, acting as opportunistic pathogens, may play a greater role in the ecology of coral reef systems than previously recognized.