Two Puerto Rican species of the encrusting red alga Metapeyssonnelia are known to overgrow and kill the hydrocoral Millepora complanata. The overgrowth is accompanied by bleaching or tissue lightening regions surrounding the growing margin of the algae on the coral tissue. Lipophilic and hydrophilic extracts from Metapeyssonnelia corallepida and M. milleporoides were obtained and analyzed by GC-MS. The following compounds were identified from M. corallepida: squalene and

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... ethoxymethyl)-furan3methoxycarbonyl-1,1-diethyl-2-buten-4-olide and from M. milleporoides: 2,4-dit-butyl phenol, 2(3H)-furanone and dihydro-4,4-dimethyl-(CAS), terpenes, furanones and phenol. Aliquots of crude extracts of both Metapeyssonnelia species applied to coral fragments resulted in visible changes to the coral tissue as well as unexpected change in numbers of zooxanthellae. The highest concentration of M. corallepida lipophilic extracts assayed generated bleaching halos, death of coral tissue and a reduction of the zooxanthellae number while the two lower concentrations of lipophilic extracts of M. milleporoides resulted in obvious tissue lightening, but without decline in zooxanthellae number.