FISH GRAZING AND COMMUNITY STRUCTURE OF REEF CORALS AND ALGAE: 

 A SYNTHESIS OF RECENT STUDIES 



Mark A. Hixon 



Department of Ecology and Evolutionary Biology 



University of California 



Irvine, CA 92717 



ABSTRACT 



This paper reviews major studies published over the past 5 years on the impact 

 of fishes upon the major components of tropical reef benthos. With one notable 

 exception, virtually no studies have demonstrated a strong effect of fish 

 coral livores upon coral communities, although at least one coral genus is 

 certainly affected by fish predation. On the other hand, herbivorous fishes 

 strongly affect the distribution and abundance of reef algae, with recent studies 

 elucidating temporal and spatial variations in herbivore impact. Of all reef 

 fishes, territorial damselfish have especially widespread direct and indirect 

 effects. By establishing and defending distinct algal mats, they affect: (1) 

 coral growth, recruitment, and bioerosion; (2) algal abundance and local 

 diversity; (3) local microfaunal abundance; and (4) nitrogen fixation by blue- 

 green algae. These and other interactions are summarized on a synthetic flow- 

 chart. 



INTRODUCTION 



The dominant benthic organisms on tropical reefs are scleractinian corals and 

 various algae. That grazing fishes can affect the local distribution and 

 abundance of such organisms was first demonstrated by the caging experiments of 

 Stephenson and Searles (1960) and Randall (1961). More recently, ecologists have 

 focused their attention on the role of fishes and large invertebrates in deter- 

 mining the structure of benthic communities, that is, how differential foraging 

 can alter competitive relationships and thus the local species diversity of reef 

 benthos. Recent reviews have been published on the impact of sea urchins 

 (Lawrence and Sammarco 1982) and the crown-of-thorns seastar (Birkeland 1982). 

 The goal of this paper is to briefly review and synthesize major studies on the 

 effects of fishes upon the community structure of reef corals and algae published 

 during the past 5 years (1978 through part of 1983). Due to textual constraints, 

 this overview is necessarily of limited scope. Those seeking more comprehensive 

 reviews of marine plant-herbivore interactions are referred to Ogden and Lobel 

 (1978), Borowitzka (1981), and Lubchenco and Gaines (1981). Sheppard (1982) 

 includes a general taxonomic review of coral livores. 



FISHES AND CORALS 



Despite observations that members of about ten families of fishes eat coral 

 polyps (Randall 1974, Patton 1976), surprisingly few studies have demonstrated 

 that such coral livores influence the distribution and abundance of hermatypic 

 corals. For example, although butterf lyf ishes (Chaetodontidae) are apparently 

 the major reef -fish coral livores (e.g., Reese 1977), recent data from the Red Sea 

 suggest that these fishes consume a negligible portion of the total standing crop 

 of live coral (Harmel in-Vivien and Bouchan-Navaro 1981). However, at least one 

 coral genus, Pocillopora , is strongly affected by fish predation. Neudecker 

 (1979) found that colonies transplanted from a relatively fish-free lagoon (1 m 

 depth) to reef slopes (15-30 m depth) in Guam survived well when caged, but were 



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