Niche Separation of Two Damselfish Species by Aggression and Differential Microhabitat Utilization

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Two herbivorous, interspecifically territorial West Indian damselfishes, the beaugregory (Eupomacentrus leucostictus) and the three—spot damselfish (E. planifrons), both inhabit the back—reef area of coral reefs in a narrow depth band. A survey of coral heads in this habitat revealed that beaugregories utilize all types of heads, while three—spots are found only on branching or creviced heads with relatively complex internal structure. Translocations of individual fish and heterospecific pairs were performed to determine whether this differential microhabitat utilization is the result of differences in microhabitat preference between the two species, or aggressive interactions between them, or both. As indicated by duration of residence of translocated fish on experimental heads, three—spot damselfishes showed significant preference for heads with complex internal structure, while beaugregories showed no preference. In translocations of heterospecific pairs, three—spots won aggressive contests more often on internally complex heads, and beaugregories won more often on heads without internal structure, then would be expected on a random basis. Since this difference fell just short of significance (P = .053), translocations of heterospecific pairs were also done in aquaria with artificial heads. In these more controlled conditions, the preponderance of wins by three—spots on complex heads and beaugregories on simple heads reappeared as a significant difference, with occasional co—occupation of the complex head by both species. The observed differences in microhabitat utilization are thus due both to different preferences and aggressive interaction. It is probable that the aggressive interactions, a form of interference competition, and responsible for the evolution of the different habitat preferences. However, for both the three—spot damselfish and the beaugregory, it has been shown that a principal function of aggressive interspecific territoriality is the protection of food resources from exploitation competition.