Wellington discovered an interactive feedback loop whereby the damselfish 

 Eupomacentrus acapulcoensis may directly and indirectly cause this zonation. 

 When establishing territories in the shallow zone, damselfish differentially kill 

 Pavona by polyp removal and maintain their algal mats on the exposed substrate; 

 Pocillopora is apparently protected by its tightly branched morphology and rapid 

 polyp regeneration. Moreover, as discussed above, Pocillopora colonies within 

 the periphery of territories are protected from fish coral livores. These factors 

 enhance the ability of Pocillopora to competitively dominate Pavona in shallow 

 areas. The Pocillopora framework, in turn, provides the damselfish with shelter, 

 a necessary requisite for a territory. In the deep zone, shelter sites and thus 

 damselfish densities are low because overall coral cover (and thus inter-coral 

 competition) is low, apparently due to physical factors. Here, transient fish 

 corallivores (mostly puffers) differentially eat Pocillopora , whose branches they 

 can ingest and masticate, leaving Pavona as the dominant coral. Overall, it 

 seems that territorial damselfishes in general have a far greater impact on 

 hermatypic corals than do true fish corallivores per se. 



FISHES AND ALGAE 



Considerably more data have been gathered on the effects of fishes on reef 

 algae than on corals. This may reflect the fact that herbivorous fishes, 

 especially parrotfishes (Scaridae), surgeonf ishes (Acanthuridae) and territorial 

 damselfishes (Pomacentridae), are together among the most diverse and abundant of 

 reef fishes (e.g., Ogden and Lobel 1978). Indeed, Hatcher (1981) has empirically 

 estimated that about half the net algal production on One Tree Reef, Australia, 

 is consumed by fishes. Although some excellent studies have investigated the 

 impact of these fishes on subtropical rocky reefs (e.g., Montgomery 1980a, b), I 

 will necessarily limit this discussion to true coral reefs. 



Recent studies have substantiated earlier caging experiments (e.g., Stephenson 

 and Searles 1960; Randall 1961) showing that herbivorous fishes strongly affect 

 the distribution and abundance of reef algae. Typically, heavily grazed dead 

 coral surfaces become dominated by grazer-resistant crustose coralline algae 

 (e.g., Vine 1974; Wanders 1977; Brock 1979; Hixon and Brostoff 1981, 1982), while 

 caged but otherwise identical surfaces become covered by high standing crops of 

 erect algae (e.g., above studies plus: Lassuy 1980, Miller 1982, Sammarco 1983), 

 which apparently competitively exclude corallines. While grazing algae, fishes 

 also affect interspersed assemblages of sessile animals (e.g., Day 1977, 1983). 

 Attention has shifted lately to temporal and spatial variations in these general 

 patterns. Hatcher and Larkum (1983) demonstrated that algal standing crops at 

 One Tree Reef were controlled by grazing fishes all year (autumn and spring) on 

 the reef slope (10 m depth), but only during spring in the lagoon (2 m depth). 

 In autumn, inorganic nitrogen limited the standing crop of lagoon algae despite 

 the continued presence of fishes. 



In addition to seasonal variations, an apparently general trend within reefs 

 is that the spatial distribution of fish grazing intensity varies inversely with 

 tidal exposure and/or wave action (e.g., Van den Hoek et aj. 1978) and directly 

 with the availability of shelter for the herbivores from predatory fishes (e.g., 

 Hay 1981a, Hay et al. 1983). Thus, as documented in Guam (Nelson and Tsutsui 

 1981) and the Caribbean (Hay et al. 1983), the depth distribution of grazing 

 intensity may often be unimodal: low in yery shallow water due to limited 

 accessibility by fishes, high at intermediate depths due to high accessibility 

 and shelter, and low in deep reef areas, where the abundance of coral shelter 

 typically decreases. This pattern may explain the bimodal zonation of erect 

 algal cover found on reefs such as those in Curacao (Van den Hoek 1978): high 

 cover in the eulittoral zone (0-1 m depth), low on the upper reef slope (1-30 m), 



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