Table 2. --Comparison of proportional (%) vertical distributions of Diadema 

 within stands of A. palmata on fore and back reefs. A G-test compared 

 individuals found at heights of < 60 cm or > 65 cm (G = 10.76, p < 0.005, 

 df = 1). 



Ht. Fore Reef Back Reef 



(cm) 



15-30 13.9 19.6 



35-60 50.6 19.6 



65-90 21.5 12.2 



95-135 13.9 48.6 



N 79 107 



observation was made at 1030 hr; however, it did not differ from samples taken 

 later in the same area. Thus, differences in vertical distribution must have 

 been due to differing urchin behavior. 



DISCUSSION 



Although the damsel fish- Diadema assemblages in the fore and back reef 

 environments at St. Croix appear similar in population composition, the structures 

 of these two assemblages differ in population abundances, distributions, and 

 mean size. These differences may be the result of differing physical factors 

 and/or biological interactions. 



Coloni es of Acropora palmata were similar in height but significantly 

 smaller in volume on the fore reef (table 1). The difference in volume was 

 attributable to differences in width of coral stands. This structural difference 

 in the corals may be related to the higher wave energy on the fore reef. Waves 

 can cause considerable damage to fore reef corals, as evidenced by abundant 

 coral rubble found on the fore reef substratum (Robinson, pers. obs.; Van den 

 Hoek, et al . , 1978). Rogers, et al . (1982) documented that storm damage on the 

 reefs of St. Croix was due to fractures of the distal ends of A. palmata 

 branches. 



Mean test diameters of Diadema from the fore reef are smaller and those 

 from the back reef are larger in the present study than those reported by Ogden, 



146 



