Hybrid vigor in a tropical pacific soft-coral community

Although hybridization is a relatively widespread phenomenon in marine ecosystems, the ecological importance of hybrids is poorly understood. As crosses between two distinct genotypes, hybrids might express fitness characteristics similar to either parent species, or they might produce unique phenotypic attributes that make them more or less fit than either parent species. We identified a potential hybrid zone among soft corals in a back-reef community on Guam, where the broadcast-spawning species Sinularia maxima and S. polydactyla co-occur. Morphological and chemical traits confirmed the intermediate and unique status of the putative hybrid. Laboratory cross-fertilization experiments using S. maxima and S. polydactyla gametes demonstrated that barriers to hybridization are absent and that the laboratory-reared hybrids developed specific characteristics identified in adult hybrid field populations. Changes in the populations of each parent species and the hybrid were monitored over a decade. While populations of the parent species declined significantly, the hybrid population doubled in percent cover over this period. We assessed responses to abiotic (sedimentation) and biotic (competition and prédation) factors to determine whether the two parent species or the hybrids exhibited greater fitness. While sedimentation alone could not explain the decline in field populations of the parent species, sediment effects exact a significant cost to these soft corals over narrow spatial scales. Competition between the parent and hybrid soft corals explains some of the changes in the distribution and abundance of these species through time. The extracts of S. polydactyla had no effect against congeners, whereas those of S. maxima and the hybrid caused significant tissue necrosis. Field prédation was significantly lower on the hybrid than on either parent species. Laboratory feeding assays demonstrated that the extracts of the hybrid were significantly more deterrent than either S. maxima or S. polydactyla. Comparisons of the bioactive metabolites from the parent species, a hybrid compound, and a semi-purified mixture indicated that the hybrid is more deterrent than either of the parent species. Our results indicate that the hybrid Sinularia maxima × polydactyla exhibits greater fitness than the parent species and suggests that hybrid vigor may have implications for coral reef resilience.