Depth Zonation of Corals in the Gulf of Panama: Control and Facilitation by Resident Reef Fishes

Abstract

The mechanism that control the distribution and abundance of major benthic organisms in subtidal coral reef communities are poorly understood. Through field experimentation and manipulations, this study investigated the factors that account for the vertical zonation patterns found on a fringing reef in the Gulf of Panama (Pacific). In this community, pocilloporid corals form a near—monotypic stand (80—85% live coral cover) in shallow water (0—6 m depth) while massive corals, particularly Pavona gigantea, predominate in low density (°18% cover) in the deeper areas of the reef (6—10 m depth). The paucity of massive corals in shallow water results mainly from chronic and severe mortality caused by the omnivorous damselfish Eupomacentrus acapulcoensis (Pomacentridae). These highly territorial fish establish algal mats (used as a source of food and nesting site) on hard substrata, including coral. The effect of this activity was found to be differential with respect to (1) species (i.e., Pavona suffered higher mortality compared to Pocillopora) and (2) depth (i.e., the frequency and extent of damaged Pavona colonies were higher in shallow compared to deep areas). Pavona suffered higher mortality due to differences in coral morphology: damselfish are more effective at removing tissue from a massive (open) surface than from a tightly branched pocilloporid coral. Pavona was most severely damaged in shallow water because of depth—related differences in topographic complexity. By removing the topographic complexity surrounding colonies with damselfish and by adding complex substratum around colonies without damselfish, it was found that damselfish abandon territories when substratum complexity is reduced and immigrate to colonies when complexity is increased. Immigration eventually led to coral mortality. Hence, shelter is a necessary requisite for the establishment of a damselfish territory. Censuses revealed that shelter site availability surrounding Pavona colonies and damselfish abundance (both overall and per individual Pavona colony) were greater in shallow compared to deep water. Damselfish were conspicuously absent from small colonies (<1.5 m2 surface area) that lacked holes or crevices within the colony or immediately adjacent to it; such colonies were only present in deep water. Controlled experiments using live coral fragments (simulating 1—1.5 yr old juveniles) attached to hard substratum demonstrated that pocilloporid corals suffer high mortality and substantially reduced growth rates in deep reef areas due to grazing by pufferfish and parrotfish. The effectiveness of this predation/grazing activity was related to the position of the coral fragments relative to defended damselfish territories. Outside the territory, growth and survivorship of fragments were low for Pocillopora but high for Pavona. Within areas defended by damselfish, Pocillopora and Pavona fragments grew equally well both caged and uncaged. On the algal mat itself, however, Pavona fragments suffered high mortality from damselfish: Pocillopora growth under these conditions was only temporarily reduced. The data in this study indicate that damselfish territories provide a refuge against juvenile mortality for Pocillopora within defended areas, thus facilitating the establishment of branching corals in shallow water and concomitantly reducing or eliminating its potential space competitor, Pavona. A proposed successional model suggests that Pocillopora abundance may ultimately control the abundance of damselfish. This study indicates that community structure on some eastern Pacific reefs may be controlled by the direct and indirect behavioral interactions between damselfish and transient fish corallivores, and physical factors (availability of shelters) limiting the distribution of damselfish.