SUBTIDAL ENVIRONMENTS AND ECOLOGY 



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that a significant amount of algal production must be avail- 

 able for them to survive for any period. Population size of 

 irregular urchins seems influenced more by recruitment 

 success than by food availability (V. S. Frey, unpublished 

 data). 



There are many other organisms living in the sediment 

 which ultimately make their living from the algal produc- 

 tion occurring on sediment surfaces or passing over the 

 sediment. The callianassid crustaceans, mentioned previ- 

 ously for their bioturbational activities, almost certainly 

 process prodigeous quantities of sediment to winnow the 

 organics present on the surface of the sediment grains. 

 They may additionally exploit the algal fragments which 

 enter their burrow systems. 



On hard substratum, some herbivores at Enewetak live 

 within a limited area which they maintain as a territory. 

 Some damselfishes, particularly S(egas(es nigricans, estab- 

 lish and maintain an "algal lawn" of filamentous algae. The 

 algal lawn is often found on basal dead parts of the fine 



branches of Acropora spp. corals and is strongly defended 

 against intruding herbivores. This action by S. nigricans is 

 identical to the western Atlantic Stegasfes planifrons, the 

 first species for which algal lawn maintenance was 

 described (Kaufmann, 1977). It is likely that S. nigricans 

 can kill coral polyps in expanding its algal plot, and large 

 numbers can significantly damage Acropora spp. corals. 

 The darkened areas of damselfish algal plots are common 

 features of Acropora spp. thickets at Enewetak (Fig. 24). 

 The long-range effect of these areas of dead coral has not 

 been examined, though areas a few meters square of dead 

 Acropora are often found in the midst of dense thickets 

 (Fig. 24). These may potentially represent old algal plots 

 eroded away by other herbivores. 



The general lack of herbivores as significant as fishes 

 at Enewetak presents an interesting contrast to reefs in 

 some other areas of the world. In the tropical western 

 Atlantic, sea urchins, particularly Diadema antillarum, play 

 a role as herbivores equal to or superior to that of fishes 



Fig. 24 Algal "lawns" on Acropora sp. corals produced by the herbivorous damselfish Stegastes nigricans in the Enewetak Island 

 quarry. Upper left: Large dead area in the Acropora sp. coral possibly produced by the presence of an algal lawn. Upper right: 

 Aerial view of Enewetak quarry Acropora sp. with many algal lawns (dark spots) established on the coral. Lower left: Acropora 

 sp. coral in the quarry with algal lawns. Lower right: Stegastes nigricans with its algal lawn (dark area to right of fish). 



