Interspecific aggression in hermatypic corals from Bermuda

Abstract

Interspecific aggression between hermatypic corals on Bermudian reefs has been investigated by aquarium and field studies, the latter involving induced interactions, observations from 30 m-2 transects and random SCUBA traverses. Resultant hierarchies, constructed by ranking the abilities of species to damage competitors, show close similarities with each other and with the Jamaican hierarchy at the family level, notwithstanding some differences in the ranking of some species. Only 11% of natural-occurring interactions depart from the aquarium-derived results; in terms of species-pair combinations, 30% show partial or complete inversions from aquarium to field, with most changes involving species close together in the field hierarchy. Circular (intransitive) interactions occur mostly within a network of weakly-aggressive species in both aquarium- and fieldderived hierarchies. While number of potential interactions m-2 varies directly with density, frequency of aggression is positively correlated with coral diversity (species richness), while frequency of “no reactions” and conspecific fusion (combined) shows a correspondingly negative correlation with diversity. Frequency of aggression does not appear to be depth related. Comparison of aquarium and field hierarchies suggest that digestion by mesenterial filaments is the most important mechanism of aggression under natural conditions. Sweeper tentacle activity is the most likely cause of field reversals involving Madracis mirabilis and Montastrea cavernosa. Other factors, such as stress caused by seasonal environmental extremes, may be responsible for reversals or inconsistent behaviour in other species.