Schooling as a Mechanism for Circumventing the Territoriality of Competitors

Abstract

The herbivorous striped parrotfish Scarus croicensis exhibits pronounced within—site variability in its social behavior: some individuals hold permanent feeding territories while other form feeding schools. Members of both of these classes are subordinate to the omnivorous damselfish Eupomacentrus planifrons which strongly inhibits the feeding of parrotfish in its own feeding territories. To a lesser degree, territorial S. croicensis, whose territories are superimposed on those of E. planifrons, also inhibit the feeding of nonterritorial conspecifics. Data show that nonterritorial S. croicensis in schools feed at higher rates, and are attacked by territory owners less often than nonschooling nonterritorials. This supports the hypothesis that schooling enables those individuals to circumvent the territoriality of their competitors. Nonterritorial S. croicensis apparently constitute that proportion of the population that cannot obtain feeding territories, probably primarily as a result of the aggressive activities of E. planifrons, and schooling thus promotes the coexistence of these territorial and nonterritorial forms. A number of other teleost species, benthic—browsing omnivores, and herbivores, that associate with S. croicensis schools and are subject to strong aggression from E. planifrons, derive benefits from this association in the same way as the schooling parrotfish do. Predators that associate with S. croicensis schools, and that are little attacked by E. planifrons, benefit instead by feeding on organisms disturbed by the feeding school.