Discovery Bay, Jamaica, West Indies (Williams, 1978, 1979, 1980, 1981). The 

 damselfish maintains algal lawn territories on dead branches of staghorn coral 

 ( Acropora cervicornis Lamarck) and actively excludes herbivores, such as Diadema , 

 from these territories (Myrberg and Thresher, 1974; Williams, 1979). Through 

 these competitive interactions, habitat partitioning occurs which affects the 

 distribution and abundance of Diadema on the reef (Williams, 1979, 1981). 

 Because of its influence on the distribution of grazing sea urchins, the 

 damselfish is considered to be a keystone species (Williams, 1980). Sammarco 

 and Williams (1982) also have suggested that E. planifrons indirectly affects 

 shallow reef coral community structure by influencing the differential success 

 of coral recruitment and algal growth both within and outside damselfish 

 territories. 



This preliminary study examines the structure of two similar communities 

 and the interactions of these species on the reefs of St. Croix, U.S. Virgin 

 Islands, to determine whether or not damselfish occupy a similar role in reef 

 communities other than Jamaica. Both fore and back reef communities were 

 examined for comparison with each other and with previous observations from 

 Jamaica. 



MATERIALS AND METHODS 



Field studies were conducted on the bank barrier reef (Adey, 1978) located 

 north of West Indies Laboratory on the northeast coast of St. Croix, U.S. V.I. 

 (17°45.5'N, 64°35.5'W). The reef studied was representative of the area, with 

 the back reef composed of elkhorn coral ( Acropora palmata ) in the shallower 

 areas and various head corals ( Montastrea annularis , Diploria spp., etc.) found 

 in deeper water. Mi 1 lepora complanata and A^_ palmata were found in the high 

 energy areas of the crest, while A. palmata , A. cervicornis , and several species 

 of head corals prevailed on the fore reef (Ogden, 1974). A study site was 

 established west of the WIL pier at the site of solar panels on both fore and 

 back reefs in comparable water depths. This area was chosen because of 

 accessibility of the fore reef through a swim cut in the reef crest. All 

 observations and experiments were carried out between 1030 and 1545 hr using 

 SCUBA. 



Study of the interactions between threespot damselfish and Diadema was 

 restricted to those which took place among branches of A. palmata , an apparent 

 preferential substrate for threespots in the absence of A. cervicornis (Williams, 

 pers. obs.; Itzkowitz, 1974). Individual stands of A. palmata were selected for 

 observation by the following criteria: at least two threespot damselfish per 

 coral stand; coral stand boundaries easily discernible; and coral stands located 

 in -3 to -5m of water. Aluminum tags were used to identify the A^_ palmata 

 stands. Length, width, and height of each stand were recorded and used in 

 calculating densities of damselfish and urchins (number of individuals m~ 3 ). 

 Wilcoxon rank sum tests were used to compare densities and coral stand measure- 

 ments between fore and back reefs. Correlation analysis (Sokal and Rohlf, 

 1969) was utilized to examine relationships between the species' densities. 



Vertical distributions of urchins within coral stands were recorded by 

 suspending a leaded line, calibrated in centimeters, among the branches of 



140 



