Energetics of a Predator-Prey Interaction: Corals and Coral-feeding Fishes

Various hypotheses have been proposed to account for defense of a feeding territory by coral-feeding fishes. However, before the adaptive significance of feeding territories can be fully understood, energetics of the predator/prey relationship must be quantified . Energetics of the interaction between the coral Pocillopora meandrina and the territorial coral-feeding damselfish Plectroglyphidodonjohnstonianus were examined to determine the minimum energetic requirement of the predator and the effect of predation on productivity of the prey. Coral productivity for colonies exposed (experimental) and not exposed (control) to predation, and metabolic rates of the fish were determined. Fish required 240 cal /day, while corals produced 0.2 1 cal/cm 2 /day. A typica l colony of P. meandrina did not produce enough energy to sustain a fish. Data presented here indicate that territories of two or more colonies should provide sufficient energy. Predation by an individual P. johnstonianus did not have a measurable effect on coral primary productivity. TERRITORIALITY IS COMMON IN a wide range of taxa . Food, nest sites, shelter, and mates may be defended against members of the same species and of other species (Hixon 1980). Of these resources, food supply is particularly important since it is used to meet the energetic requirements for maintenance, growth, and reproduction (Tricas 1986). Therefore, food abundance is a major factor affecting territory size, and territory size should vary with food supply (Ebersole 1980, Tricas 1986). One hypothesis relating to optimal territory size states that animals defend terr itories that contain enough food to satisfy their shortterm energy requirements; any aditional area defended would yield no additional benefit at an increased cost of defense (Myers et al. 1979, Tricas 1986). This hypothesis predicts that territory size should vary as an inverse function oflocal food quality, quantity, and distribution. An alternative hypothesis states that animals defend as large an area as possible, I This paper is contribution no. 848 from the Hawaii Institute of Marine Biology. This work was funded by the Edwin Pauley Foundation. Manuscript accepted 15 September 1990. 2 Department of Zoology, 2538 The Mall , University of Hawaii at Manoa, Honolulu, Hawaii 96822. but food competitors constrain the size of the defensib le area. An area containing highquality food resources would attract more competitors; therefore, territory size would be adjusted as an inverse function of competitor density rather than food supply directly (Myers et al. 1979, Tricas 1986). Evidence in support of both of these hypotheses has been cited in man y studies (see references in Myers et al. 1979, Hixon 1980, and Tricas 1986). Although these two hypotheses are not mutually exclusive and may both playa role in determining territory size in coral-feeding fishes (Tricas 1986), only the first hypothesis was examined in this study. Hermatypic corals represent an economically defendable resou rce since they are immobi le, predictable in space and time, and continuously renewing (Brown 1964, Hixon 1980). For animals defending such a resou rce, selection should favor a territory occupant that maintains a maximum sustainable yield, since this would permit maximal long-term energy gain from the defended area (Hixon 1980). Territory size varies greatly among coralfeeding fish: from one to several hun dred square meters (Reese 1981, Sutton 1985,