Competition among small colonies of Agaricia: the importance of size asymmetry in determining competitive outcome

Scleractinian corals may experience competition at early life history stage s, particularly where aggregated settlement decreases the space available f or growth, The outcome of such interactions can affect post recruitment suc cess and the relative abundance of coral species, The purpose of this study was to determine the importance of competition among small colonies (less than or equal to 4 cm diameter) of Agaricia on shallow (less than or equal to 10 m depth) reefs in St. John (US Virgin Islands) and Jamaica. The assoc iation between the density of small Agaricia and the frequency of congeneri c encounters was examined, together with the relationship between density o f Agaricia and reef rugosity (topographic complexity). Rugosity was analyze d because it indirectly affects competitive interactions by providing crypt ic habitats that promote aggregated settlement and coral-coral interactions . To test one possible mechanism determining the outcome of such interactio ns, the relationship between size asymmetry of competing colonies and compe titive outcome was assessed. The results demonstrate that competition among small Agaricia is common, particularly where aggregated settlement is favo red by high topographic complexity. In other words, competitive interaction s among small Agaricia are affected by both physical (e.g. topographic comp lexity) and biological (e.g., settlement patterns) factors. The outcome of such interactions is influenced by the difference in size of competing colo nies, with dominant colonies generally being larger than subordinate coloni es. Further research is required to identify the mechanistic basis of domin ance, but currently the most likely hypotheses are that large colonies are dominant because they have greater energy resources than small colonies, or intrinsically faster growth rates. The high frequency of competition among Agaricia colonies in St. John and Jamaica suggests that further studies of congeneric competition within the Scleractinia may prove valuable in under standing the population biology of this taxon.