FOOD SELECTIVITY OF TRIPNEUSTES GRATILLA (L.) (ECHINODERMATA: ECHINOIDEA) IN OLIGOTROPHIC AND NUTRIENT-ENRICHED CORAL REEFS AT LA REUNION (INDIAN OCEAN)

摘要

Herbivorous echinoids can control algal populations and structure their composition in coral reef ecosystems. Food selectivity might play an important role in this structuration. The gut contents of a total number of 40 individuals of the edible sea urchin Tripneustes gratilla were analysed at two coral reefs of La Reunion. The sites differed in eutrophica-tion level, one being oligotrophic, the other being nutrient enriched. Available algal food was quantified in each site and food selectivity of T. gratilla in the field was determined by the use of an electivity index. The diet of T. gratilla relied mainly on algae. However, a significant proportion of detritus was found in the digestive tracts of individuals at both sites. A strong food selectivity of T. gratilla was found at both sites. The tough leathery macroalga, Turbinaria ornata, was the most selected species at both sites. Calcareous algae were strongly avoided. Algal species were generally either highly selected, or highly avoided, and rarely consumed in proportion to their availability in the field. Significant differences in food selectivity were found between sites, some species being selected/ avoided at one site but not at the other. These differences might be linked to the nutrient contents of algae at both sites and/or related to different levels of herbivory between sites.