Direct evidence for reproductive isolation among the three species of the Montastraea annularis complex in Central America (Panama and Honduras)

Montastraea annularis, M. faveolata, and M. franksi are three recently separated species that together dominate reefs of the tropical western Atlantic. Despite morphological, life-history, ecological and genetic differences, the legitimacy of their status as separate species has been questioned. This controversy stems from both the scarcity of unambiguous, diagnostic differences among them, and from the possibility of extensive hybridization associated with their approximately synchronous reproduction in sympatry. Here we report on fertilization trials and the timing of spawning, both of which suggest that the potential for hybridization may be limited. Crosses between M. faveolata (the most genetically distinctive taxon) and the other two species were largely unsuccessful, as were selfed matings for all three species. M. annularis is and M. franksi showed no evidence of fertilization barriers, but the timing of spawning typically differed between them by 1 to 2 h. We also found that spawning times in the field of M. annularis and M. faveolata were non-overlapping in 1995, and that the timing of spawning in M. annularis could be experimentally shifted forward by simulating earlier than natural sunsets. These findings from Panama and Honduras, particularly given their consistency with comparable observations and experiments elsewhere, provide evidence of reproductive isolation and support the separate species status of these three taxa. Some hybridization may occur under natural conditions at these sites, particularly between M. annularis and M. franksi. Rampant genetic interchange among these taxa in Central America seems unlikely, however, in the light of the concordant associations between morphology, reproductive biology and other characters which are observed.