Abstract
The sexual pattern, reproductive mode, and timing of reproduction of Isophyllia sinuosa and Isophyllia rigida, two Caribbean Mussids, were assessed by histological analysis of specimens collected monthly during 2000-2001. Both species are simultaneous hermaphroditic brooders characterized by a single annual gametogenetic cycle. Spermatocytes and oocytes of different stages were found to develop within the same mesentery indicating sequential maturation for extended planulation. Oogenesis took place during May through April in I. sinuosa and from August through June in I. rigida. Oocytes began development 7-8 months prior to spermaries but both sexes matured simultaneously. Zooxanthellate planulae were observed in I. sinuosa during April and in I. rigida from June through September. Higher polyp and mesenterial fecundity were found in I. rigida compared to I. sinuosa. Larger oocyte sizes were found in I. sinuosa than in I. rigida, however larger planula sizes were found in I. rigida. Hermaphroditism is the exclusive sexual pattern within the Mussidae while brooding has been documented within the related genera Mussa, Scolymia and Mycetophyllia. This study represents the first description of the sexual characteristics of I. rigida and provides an updated description of I. sinuosa.