An Examination of Interspecific Territoriality in the Dusky Damselfish, Eupomacentrus Dorsopunicans Poey

Interspecific territorial behavior in the dusky damselfish, Eupomacentrus dorsopunicans, was examined at St. Croix, U.S.V.I. Territory size was not constant, but varied as a function of the intruding species. Larger territories were defended against diurnally active fishes, while nocturnally active fishes only rarely elicited agonistic responses from E. dorsopunicans. A positive correlation was found between the size of the territory defended and the intruder's dietary overlap with E. dorsopunicans, although when only diurnal fishes were considered, this relationship was no longer statistically significant. Since nocturnal fishes on western Atlantic coral reefs have relatively low trophic overlap with the dusky damselfish, it was not possible to completely separate the effects of trophic similarity from activity pattern. Removal of dusky damselfish from their territories resulted in a rapid reduction in algal biomass within the territory owing to heavy grazing by acanthurids, scarids and labrids. These results indicate that one function of territorial behavior in E. dorsopunicans is the protection of food resources. The possible importance of this behavior in the protection of shelter sites will be examined in a future study.