Persistence and coexistence of a nonsymbiotic coral in open reef environments

In the Republic of Belau (western Caroline Islands), the nonsymbiotic stony coral Tubastraea micrantha (Ehrenberg) occurs commonly on open reef habitats that are dominated by symbiotic corals. This observation calls into question the often-held view that nonsymbiotic corals are restricted to cryptic environments because of their inability to compete successfully for space with rapidly growing symbiotic corals. In this study, we investigated aspects of the biology of Tubastraea that might account for its coexistence and persistence on the reef. Our results indicate that despite low calcification rates, linear skeletal extension for Tubastraea is surprisingly rapid for a nonsymbiotic species and comparable to those of many symbiotic corals. This factor, coupled with Tubastraea's resistance to direct competitive dominance (i.e., by aggressive interactions) and avoidance of predation, promotes its coexistence with symbiotic species. The relative importance of these biological interactions in different coral communities leads to predictions corroborated by observed patterns of local distribution and abundance.