Repeated Reversals During Spatial Competition Between Corals

On many tropical reefs, competition for space among sessile animals is common and is generally thought to influence the population and community ecology of reef corals. A 20—mo photographic study revealed frequent temporal reversals in the observable outcome of interspecific spatial competition between some of the most abundant Caribbean corals. These reversals apparently were not due to seasonal changes or to other environmental effects on relative competitive ability. Rather, they resulted from inherent differences between the competing species in response time and competitive mechanism employed. Specifically, encounters between individuals of Agaricia agaricites and Porites asteroides or between individuals of A. agaricites and Montastraea annularis are characterized by a repeated cycle of attack by one opponent using digestive filaments, and subsequent retaliation by the initially wounded coral when it develops and deploys fighting tentacles. The resulting bilateral injuries increase the distance separating competing colonies. Active competition then ceases temporarily until the corals grow back into proximity, and the cycle of reciprocal injury is re—initiated. This phenomenon has several important implications. Because the advances and retreats occurring along competing coral margins are temporary, short—term or infrequent observations might misleadingly suggest one or the other opponent was continuously "winning" these interactions. Similarly, the distances competing edges advance and retreat during these repeated reversals are small (millimetres). If observed at too great a spatial scale, these dynamic competitive interactions might incorrectly appear to be a case in which adjacent animals have simply ceased growth along their common margin (i.e., a standoff). Over time, interactions involving repeated reversals result in no or only a very slow exchange of spatial resources, and may potentially enhance the coexistence of competing colonies. Nevertheless, despite having little direct effect on the occupation of space by competitors, ultimately such interactions may indirectly influence coral distributions by costing competitors reduced fecundity, growth, or survivorship.