et al . (1973) on patch reefs at St. Croix (X = 59 mm). They also are slightly 

 larger than mean diameters measured by Bauer (1980) on the back reef at St. 

 Croix (X = 52.3 mm). Such differences may indicate differences in the availability 

 of food, mortality, or adaptations to energy levels in the three different 

 habitats. 



The differences between fore and back reef observed in this study also may 

 reflect differences in the availability of food, mortality, and/or adaptations 

 to differing wave energy levels. Preliminary data indicate that smaller urchins, 

 such as those found on the fore reef, may feed at a faster rate than the larger 

 urchins of the back reef (Robinson, unpub. data). However, the larger algal 

 lawns also found on the fore reef indicate a reduced grazing pressure by urchins 

 (table 1). Van den Hoek, et al . (1975, 1978) attributed a higher biomass of 

 algal turf to lower grazing pressure from Diadema in similar high energy regimes. 

 Thus, the smaller body size of urchin populations found on the fore reef may be 

 due to food limitation (Carpenter, 1981) even though abundant food is present 

 (i.e., larger damselfish lawns, table 1). High wave energy may force urchins 

 to remain lower in the coral matrix, preventing them from reaching this potential 

 food source (table 2). High energy regimes either may select for individuals 

 with smaller test diameters (i.e., smaller surface area for exposure to wave 

 energy) or may result in smaller urchins by reducing their effective feeding 

 periods. Lissner (1980) reported an inverse relationship between turbulence 

 and activity of the urchin Centrostephanus coronatus . 



Williams (1981) demonstrated that Diadema and threespot damselfish in 

 Jamaica were actively involved in competition for either food or space in the 

 back reef. This competition affected the amount of algae accumulating outside 

 of algal lawns. The relative level of competition in differing environmental 

 conditions was not measured in that study. We believe that the lower levels of 

 aggression in damselfish (fig. 3) and their larger lawns on the fore reef 

 provide evidence of reduced competitive interactions in this environment as 

 compared to the back reef. Additionally, larger lawns and a greater food 

 supply may be necessary for damselfish to maintain themselves in this high 

 energy regime. Reduced competition may be the result of environmental mediation, 

 whereby high wave energy reduces urchin grazing and thus increases the size of 

 damselfish territories. 



Sammarco, et al . (1974) and Sammarco (1980) indicated that, at high 

 densities of Diadema , successful settlement of corals was inhibited; in contrast, 

 maximum diversity of successfully settled coral spat was achieved at lower 

 densities of Diadema . Sammarco and Williams (1982) also indicated that settlement 

 success varied within and outside of damselfish algal lawns due to alterations 

 in urchin grazing levels. Similar factors affecting the settlement success of 

 corals will vary between fore and back reef habitats at St. Croix and may 

 account for differences in adult coral diversities in these two environments. 



ACKNOWLEDGMENTS 



We wish to thank the staff and researchers of the West Indies Laboratory 

 of Fairleigh Dickinson University. Special thanks go to Drs. J. C. Ogden, J. 

 Ebersole, and J. Morin for their advice and helpful comments. We thank R. 



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