A COMPARISON OF FORE AND BACK REEF POPULATIONS OF DIADEMA ANTILLARUM PHILIPPI 

 AND EUPOMACENTRUS PLANIFRONS CUVIER AT ST. CROIX, U.S. VIRGIN ISLANDS 1 



C. Lance Robinson 2 and Ann Houston Williams 



Department of Zoology-Entomology 



Alabama Agricultural Experiment Station 



Auburn University 



Auburn, Alabama 36849 



ABSTRACT 



The community structure of populations of threespot damselfish ( Eupomacentrus 

 planifrons ) and the black sea urchin ( Diadema antillarum ) in fore reef and back 

 reef habitats of Teague Bay reef off the West Indies Laboratory, St. Croix, 

 U.S. Virgin Islands, was examined during July and August 1981. All populations 

 studied occupied patches of Acropora palmata at depths of 3-5 m and were observed 

 with SCUBA. 



Field observations indicated that populations of Diadema within back and 

 fore reef habitats differed in the fol lowing ways: 1) mean urchin test diameter 

 wa_s significantly larger on the back reef (X = 74.48 mm) than on the fore reef 

 (X = 47.94 mm),_2) mean urchin density, sampled during daylj_ght hours was lower on 

 the back reef ("X = 2.3 urchins m -3 ) than on the fore reef (X = 5.04 urchins m -3 ), 

 and 3) on the back reef 60.8% of the urchins were found on coral branches more 

 than 60 cm above the substrate, whereas on the fore reef 74.5% of the urchins 

 were found below that level. Damselfish densities were significantly lower on 

 the back reef (X = 0.18 m~ 3 vs. 0.31 m -3 ) and algal lawns were significantly smaller 

 (X = 146.1 cm 2 vs_;_ 349.1 cm 2 ) than on the fore reef. Back reef damselfish were 

 more aggressive (X bites min -1 = 27.6) than those on the fore reef (X bites 

 min"l = 6.6). 



These differences in community structure may indicate differing levels of 

 competitive interaction between threespot damselfish and Diadema . The higher 

 energy environment of the shallow fore reef may effectively reduce the size of 

 Diadema by reducing urchin grazing. Thus, competitive interactions between 

 threespots and Diadema may be lower on the fore reef than on the back reef. 



INTRODUCTION 



The community structure and competitive interactions between threespot 

 damselfish ( Eupomacentrus planifrons Cuvier) and the herbivorous sea urchin 

 (Diadema antillarum Philippi ) have been documented in the East Back Reef of 



1 Contribution No. 130 from the West Indies Laboratory, Fairleigh Dickinson 



Uni versity. 



2 Present address: 41 Wagon Wheel Drive, Semmes , Alabama 36575. 



139 



