Caribbean acroporid coral hybrids are viable across life history stages

The Caribbean corals, Acropora palmata (elkhorn) and A. cervicornis (staghorn), are abundant in fossil records but recent disease outbreaks have led to unprecedented losses. Fused staghorn coral, A. prolifera, is an F1 hybrid with an intermediate morphology to the parental species. The hybrid is absent in the fossil record and has generally been documented as rarer than the parental species. Currently, however, hybrid abundance varies across geographic locations with some sites having equivalent or higher hybrid densities than at least one of the parental species. This apparent change in hybrid abundance may be a result of increased absolute hybrid abundance (i.e. hybrid embryo formation), increased relative abundance from increased asexual reproduction (i.e. fragmentation), or hybrids recently suffering less mortality than the parental species (i.e. hybrids lack the postzygotic barrier of inviability). Other studies have shown that prezygotic barriers are weak and many sampled hybrids have unique genotypes suggesting multiple distinct hybridization events. Here, postzygotic barriers of intrinsic and extrinsic inviability were examined across life history stages. Hybrids were not found to be inferior to the parental species at any examined life history stage, and in a few cases hybrid viability exceeded the parental species. Current evidence of weak reproductive isolation, an increase in the hybrid's relative abundance, and hybrid expansion into reef zones typically occupied by the parental species suggest that recent ecological changes in this system (i.e. dramatic declines in the parental species coupled with changes in the environment) may be affecting the frequency of hybridization.