Effect of Cover on Agonistic Behavior of a Reef Fish: A Possible Spacing Mechanism

Each Dascyllus aruanus shelters in a colony of coral on a reef. The fish restricts its movements to the vicinity of this colony, which may be shared with several other individuals. In areas where suitable coral colonies are in short supply, the fish are efficiently spaced over the available coral (Sale 1972). The mechanism by which this dispersion might be attained is examined in the present paper. If the tendency of a fish to be aggressive were controlled by the degree to which it was crowded while sheltering in its coral, the pattern of emigration and immigration among neighboring groups would be influenced. Groups in one area would approach a common ratio between the number of fish they contained and the size of the coral they occupied. An experiment tested whether the frequency or intensity of agonistic behavior of a group of D. aruanus was affected by the amount of shelter provided for them. Agonistic behavior was observed when groups of two, four or six fish were placed with one, two, or three units of synthetic coral. In addition to the expected effect of group size on the level of agonistic behavior (P < 0.005), a significant effect of the amount of shelter (P < 0.005) and a significant interaction (P < 0.005) were detected. The results are interpreted as supporting the proposed mechanism.