Effects of flow on competitive superiority in scleractinian corals

An effective attack by a scleractinian coral on a subordinate neighbor typically results in the death of the subordinate’s margin along the intercoral gap. We tested the assumption that spatial variations observed for interactions between a highly “aggressive” coral, Galaxea fascicularis (Linne) and its subordinate neighbors can be partly explained by hydrodynamic conditions. For several months, we used submerged pumps to expose transplanted corals in situ to intermittently enhanced flow. This manipulation rendered aggression by G. fascicularis ineffective. Strong currents deflect the aggressor’s tentacles and possibly enhance the rate of tissue recovery in subordinate corals. Observations of naturally occurring pairs of G. fascicularis and subordinate neighbors indicated that the proportion of cases, in which the subordinate neighbor exhibited damaged margins was significantly lower at sites exposed to strong currents compared with protected reefs. If other aggressive corals are similarly affected by flow, then at nondisturbed reefs, corals using aggressive strategies should have an advantage at sites characterized by weak flow, whereas fast‐growing corals that compete by overtopping their neighbors should be superior at exposed sites. These conclusions agree with observations that the abundance of G. fascicularis and the percentage of unoccupied substratum are lower at the exposed sites.