Feeding ecology of the mangrove oyster, <i>Crassostrea gasar</i> (Dautzenberg, 1891) in traditional farming at the coastal zone of Benin, West Africa

Abstract

Wild collection management and farming of the mangrove oyster ( Crassostrea gasar ) occurring widely at the Benin (West Africa) coastal zone require knowledge on the feeding ecology to explore energy sources and nutritional needs. Six hundred thirty (630) individuals of C. gasar have been sampled in the rearing site at the Benin coastal lagoon to investigate on the trophic ecology of this cultivated bivalve. The diet analysis revealed that C. gasar is a filter-feeder foraging mainly on phytoplankton (72.70%) and substrate particles (22.95%). This trophic specialization results from anatomical structure, mainly the presence of gills which facilitate the filtering of number of plankton taxa. Dominant phytoplanktons ingested comprised of Diatomophycea (33.52%), Chlorophycae (17.19%), Scenedesmacae (13.80%), Dictyosphaeriacae (3.79%), and Pleurococcacae (2.75%). Eight genuses of phytoplankton, Poly cystis , Coelosphaerium , Protococcus , Botryocco cus , Crucigenia , Melosira , Cyclotella , and Gyro sigma dominated the diet of C. gasar with aggreated volumetric proportions reaching 69.06% of the diet. Higher occurrences were recorded mainly for Melosira occurring in 263 (41.75%) stomachs, substrate particles in 211 (33.49%), and Polycystis in 151 (23.97%). C. gasar exhibited a high niche breadths varying from 4.54 to 5.78, suggesting that this bivalve consumed a high variety of food items, thus exhibiting a degree of trophic plasticity. Diet overlaps ( ? jk ) among different size classes were high and varied from 0.71 to 0.98, indicating an ontogenetic diet shift pattern in C. gasar . Probably, to adapt to the ben thic-muddy environment and to increase survi val, C. gasar has evolved a specialized feeding me chanism and strategy to retrieve only needed nu trients for growth and to reject awful and nondigestible foods. Also, at the oyster rearing grounds, there is an evidence of shift in the food web structure leading to an increase of the bio logical productivity at the coastal zone. The output from this study is a valuable documentation for the sustainable development of oyster aqua culture, wild stock management and conserva tion. However, further scientific knowledge on nu tritional needs, phytoplankton toxicity and habitat degradation, and improvement of farming techniques are required for an integrated oyster management.