Conflicting morphological and reproductive species boundaries in the coral genus Platygyra

In mass-spawning corals, potential exists for gametes of a number of species to mix in the water column. The existence of morphologically distinct sympatric coral populations despite such an event implies the presence of isolating mechanisms to prevent hybridization and maintain species boundaries. Over 380 fertilization trials were conducted to determine the extent of reproductive isolation among the seven morphologically defined species (morphospecies) of the scleractinian coral genus Platygyra, on the Great Barrier Reef. Results from these experiments demonstrate that fertilization between-morphospecies can occur at rates equivalent to within-morphospecies fertilizations, indicating that no gametic-level barriers to fertilization exist among these morphological species. Observations of spawning times both in the field and in the laboratory have shown that all seven morphospecies spawn on the same night and that there is considerable overlap in the hour of spawning among them. These data indicate that few, if any, temporal barriers to fertilization exist among morphospecies of Platygyra on the Great Barrier Reef. In addition, larvae resulting from between-morphospecies crosses are capable of settlement and subsequent growth equivalent to that of within-morphospecies larvae. Our results reveal a discontinuity between reproductive and morphological species boundaries within the scleractinian genus Platygyra and challenge species definitions within the Scleractinia. It is not yet clear what mechanisms might maintain morphological boundaries in Platygyra in the face of the clear potential for gamete mixing. The disjunct distributions of certain morphospecies along latitudinal and habitat boundaries, and the small levels of variation in reproduction may be two such mechanisms.