Synchronous multispecific spawning on coral reefs: Potential for hybridization and roles of gamete recognition

Problems of gamete recognition in corals and other mass-spawning invertebrates are potentially great. In 'mass spawnings', closely-related species or genera commonly spawn at the same time, or within 1 or 2 h of each other, increasing the potential for hybridization. Among mass-spawning corals, most of the species involved are hermaphrodites that package the gametes in buoyant bundles that float to the sea surface before breaking up. Local hydrodynamic features frequently act to aggregate gametes from many different species into slicks where both eggs and sperm can be viable for extended periods. Other mass-spawning invertebrate taxa, such as molluscs, polychaetes and various echinoderms, do not have buoyant gametes but also spawn with a high level of synchrony. Gametes of organisms participating in these spawning events must be able to successfully recognize conspecifics. If they cannot do this, either through sperm chemotaxis or by mechanisms at the level of sperm binding and penetration, there may be high levels of gamete wastage through hybridization. Alternatively, viable hybrids may be formed, a factor that could have contributed to the evolutionary history of mass-spawning taxa, as well as to the taxonomic difficulties that have plagued the taxonomy of groups such as reef-building corals. Within some mass-spawning taxa, pre-zygotic barriers to fertilization suggest relatively recent molecular evolution at gamete recognition loci.