The feeding habits of two herbivorous pomacentrids (damselfishes) are examined in relation to coral reef community ecology. The subjects, Eupomacentrus planifrons from the Caribbean and E. nigricans from the central Pacific Ocean, selectively ate the epiphytic layer of
the algal mat growing inside their territories. This layer consisted of blue-green algae (Oscillatoria), diatoms, and bacteria with associated detritus. A thicket of a red alga (Gelidium sp.) formed the basal layer and the bulk mass of the algal mat which is not eaten. The fish
are dependent upon stomach acidity for digestion which functions to lyse the epiphytes' cell walls but does not significantly affect the red alga (Gelidium pulchellum). Foraging behavior of herbivorous pomacentrids on reefs has three basic effects. (1) The number of small motile invertebrates
is greater inside the territory than in comparable algal masses outside of it. The territory functions as a refuge for juvenile benthic invertebrates (e.g., crabs and sea stars) and demersal plankton. (2) Corals and coralline algae are excluded from inside territories by overgrowth of the
filamentous algal mat. Consequently, territories do not contain substrata which form part of the developing reef framework. This may promote eventual collapse. (3) Growth of blue-green algae is greater inside territories than elsewhere on reefs. Thus, pomacentrid territories are probably significant
sites for nitrogen fixation on the reef.
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