An experimental assessment of survival, re-attachment and fecundity of coral fragments

Abstract

Many branching corals are routinely broken apart into live fragments, mainly by physical disturbances. Fragments have the potential to survive independently and reproduce sexually, but until these processes are better understood, it is unclear when and where fragmentation is adaptive. In this study, we assess variation in rates of survivorship, reattachment and fecundity of fragments as a function of species, fragment size and habitat. We investigated three common Indo–Pacific corals (the staghorn Acropora intermedia, bushy A. millepora and tabular A. hyacinthus), using 900 artificially generated fragments of two sizes, deployed in the reef flat, crest and slope habitats. Fragments of A. intermedia had the highest survivorship, with 32% remaining alive after 17 months, compared to 15% for A. millepora and 8% for A. hyacinthus. Larger fragments survived substantially better than small ones (30 vs 8%). Fragments deployed on the reef flat also survived better (32%), compared to the reef crest (14%) and reef slope (10%). The amount of reattachment varied from 0 to 50% depending on species, size and habitat, and was greatest for staghorn A. intermedia, on the relatively protected reef flat, and for larger fragments. Experimental fragments had substantially reduced fecundities, relative to intact control colonies. Collections of natural fragments upon the reef crest revealed that bushy Acropora millepora produced the most fragments relative to the percent cover of established colonies, but these were small and (based on our experiments) have a high mortality schedule. Tabular Acropora hyacinthus produced the fewest fragments, which had the highest rate of mortality of the three species. The staghorn A. intermedia generated relatively large fragments at an intermediate rate, which survived well in all habitats. Consequently, asexual reproduction by fragmentation is likely to be a more important life-history trait for A. intermedia than A. millepora or A. hyacinthus. Depending on circumstances, the considerable costs of fragmentation (high mortality and low fecundity) may outweigh the advantages, in corals and other clonal organisms.