coral and counting individuals located within 5 cm intervals above the substrate. 

 A G-test of independence (Sokal and Rohlf, 1969) was used to compare distribu- 

 tions of urchins above and below 60 cm coral height on both sides of the reef. 



A comparison of urchin populations located in the two habitats was made by 

 examining the size class structure of Diadema . Haphazard samples of individuals 

 from each population were collected by hand from several areas east and west of 

 the study sites. Greatest test diameter was measured (to the closest 0.01 cm) 

 across the ventral surface using vernier calipers for 106 individuals from both 

 fore and back reefs. Mean test diameters were compared using a Student's t- 

 test, and size class structures were determined by the method of Harding (1949). 



A measure of the relative aggression of threespots toward Diadema in both 

 habitats was recorded. These data were obtained by placing an urchin representa- 

 tive of the mean size in the algal lawn of a haphazardly chosen damselfish and 

 counting the number of bites on urchin spines made by the damselfish in 1 min 

 or until the urchin was outside the algal lawn. Relative levels of aggression 

 were compared using a Student's t test. All statistical computations were 

 completed by using the Statistical Analysis System (SAS). 



RESULTS 



Stands of elkhorn coral were significantly smaller in total volume on the 

 fore reef than on the back reef due to significantly smaller width dimensions 

 (table 1). Densities of Eupomacentrus planifrons associated with the fore reef 

 stands were significantly higher than those of the back reef. Algal lawns were 

 significantly larger on the fore reef than on the back reef (table 1). Although 

 more coral substratum apparently was available in the back reef, fewer fish 

 maintained smaller territories in this region. 



The density of Diadema was higher on the fore reef than on the back reef 

 (table 1). Additionally, mean test diameter was_ significantly smaller (t = 

 18.64;_df = 191; p < 0.0001) on the fore reef (X = 47.94 mm) than on the back 

 reef (X = 74.48 mm). Population size class structure exhibited no apparent 

 differences between habitats (fig. 1). When all urchins from both habitats are 

 considered, a probable negative relationship may exist between the size and 

 density of Diadema . However, because of the limited data available on mean 

 test diameter and mean density of Diadema in each habitat (N = 2), correlation 

 analysis could not be completed. 



Although significance was not testable due to small sample sizes (N = 3), 

 an apparent negative correlation was demonstrated between the density of Diadema 

 and the density of E. planifrons on the back reef, but this was not the case on 

 the fore reef (fig. 2). Thus, in the back reef habitat where densities of E. 

 planifrons were low and algal lawns were small, small numbers of large Diadema 

 were found. 



Relative aggression levels of E. planifrons toward Diadema were found to 

 be much higher (t = 5.25, df = 13, p < 0.0002) on the back reef (X bites 

 min~l = 27.6) than those exhibited by threespots on the fore reef (X bites 

 min - * = 6.6) (fig. 3). Thus, intensity of the behavioral interaction between 



141 



