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, Polycystis, Coelosphaerium, Protococcus, Botryoccocus, Crucigenia, Melosira, Cyclotella, and Gyrosigma 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 benthic-muddy environment and to increase survival, C. gasar has evolved a specialized feeding mechanism and strategy to retrieve only needed nutrients 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 biological productivity at the coastal zone. The output from this study is a valuable documentation for the sustainable development of oyster aquaculture, wild stock management and conservation. However, further scientific knowledge on nutritional needs, phytoplankton toxicity and habitat degradation, and improvement of farming techniques are required for an integrated oyster management.
Highlights
Shellfish and oysters are valuable food for human health and contain about 80% of water, 17.2% of protein, and many vitamins such as A, D, E, B1, B2, B6, B12, C etc., and minerals that satisfy human nutritional needs [1]
The least consumed phytoplankton were Chloroococcacae (1.57%) represented by Coeulosphaerium (1.54%) and Dictyosphaerium (0.033%), Desmidiacae represented by Euastrium (0.03%) and Peridiniacae represented by Peridinium (0.04%)
Eight (8) genus of phytoplankton, Polycystis, Coelosphaerium, Protococcus Botryoccocus, Crucigenia, Melosira, Cyclotella, and Gyrosigma dominated the diet of C. gasar with aggregated volumetric proportions reaching 69.06% of the diet (Table 2)
Summary
Shellfish and oysters are valuable food for human health and contain about 80% of water, 17.2% of protein, and many vitamins such as A, D, E, B1, B2, B6, B12, C etc., and minerals that satisfy human nutritional needs [1]. In artisanal and commercial fisheries, Crassostrea is massively harvested by grass-roots for food, sales and their shells as ingredient for animal food industries [2,3,4]. A. Adite et al / Natural Science 5 (2013) 1238-1248 lusks to supply food needs and to provide sustainable resources for grass-roots [5]. With regards to its ecological importance, Crassostrea stands out as an indicator of environmental quality and measures the degree of contamination of aquatic ecosystems, since it accumulates polluting substances that lead to chromosomal changes and mutations [6]. In addition to improving the quality of polluted water, oysters contribute to reducing ocean and coastal waters acidification because of the continuous absorption of anthropogenic CO2 [6]
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