FAUNAL COMPOSITION AND BEHAVIOR OF A CORAL ISLET IN THE SAND-FLAT 



Jean-Georges Harmelin 



Station Marine d'Endoume 



Marseille, France 



ABSTRACT 



Sketching the life history of one of the small coral patches of the 

 sand-flat was the purpose of this field study. Activity rhythms, 

 trophic relationships, and social behavior of the various sessile 

 and motile components of this community have been noted during the 

 24-hour cycle by visual and photographic recordings. The diversi- 

 fied fauna form an individualized "microcosm." Diurnal-nocturnal 

 activity changeover proceeds according to a chronological order 

 depending on specific critical light levels. Social behavior of 

 some motile components is directed towards maintenance of the com- 

 munity's equilibrium. Most of the inhabitants' feeding patterns 

 involve outer sources of food and the impact of the foreigners' 

 foraging seems to be weak, 



OBJECTIVE 



Owing to a lack of knowledge of the West Indies marine fauna and of the working 

 site, much of the planning of this research program was not defined accurately 

 until the first days of the mission. Its main objective was to report the 

 events happening during a 24-hour cycle in an animal community located in a 

 small patch-reef of the sand-flat. To this end, I have tried to define the 

 faunal stock, the trophic activities, the social behavior of some motile com- 

 ponents and the interaction with surrounding biotopes. The degree of similarity 

 existing between these small coral islands and artificial reefs is one of the 

 major concerns in this study. 



PROCEDURES 



This work was based upon visual observations. The most important tools were, 

 in addition to the breathing apparatus, plastic sheets for recording notes and 

 still-cameras. The unobtrusiveness and the length of the observations were 

 imperative, and the silence and lack of bubbles of the rebreathers were a great 

 advantage. Observations were carried out during dawn, day, dusk, and night. 

 Diurnal or crepuscular observations began at 6 to 7 meters from the islet in 

 order to note without disturbing it the activity of the most perceptible motile 

 fauna. After that, detailed observations were pursued at closer range. At 

 night, observations were made using artificial illumination. These periods were 

 kept short to avoid disturbing the organisms affected by light. 



RESULTS 



Site 



The small coral island chosen was lying on the sand- flat, SW of the habitat, at 

 50 meters from the main reef. Its proximity to "Way-Station" 3 allowed an easy 



VI-285 



