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, 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. 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.

Survival and growth of mangrove oyster Crassostrea gasar (Dautzenberg, 1891) reared at different depths in ponds in Benin (West Africa)

“…At 90 cm, the oysters are close to the pond bottom and therefore may be influenced by the turbid nature of the bottom water during growth control or pond maintenance operations. Since oysters are known as filterfeeding organisms, they would die from gill plugging [10] .…”

“…Mangroves (Rhizophora racemosa and Avicennia germinans) are characteristic of mangrove ecosystems and constitute the natural habitat of the mangrove oyster [9] . In Benin, the mangrove oyster is subject to artisanal exploitation at the coastal water levels especially in Lake Nokoué and the coastal lagoon [8,10] .Several scientific studies have been conducted on the reproduction, feeding and growth of the mangrove oyster [6,10,11,12] , but there is a lack of scientific data on rearing in controlled systems of the species. Oyster research is absent so that currently at the coastal lagoon, oyster farming remains traditional without any improvement.…”

Specialization as a Personality Trait and Tendency: Introducing an Instrument of Measuring the Specialization Degree. Part 1

“…Systematic and wide use of the term is taking place especially in the frames of two main scientific fields; on the one hand that of Biology and on the other hand that of Finance. Specialization, both as a concept and as a procedure, constitutes main subject of research of Biology and it is approached, in specific, by two sides: 1) in relation to the degree and quality of the survival strategies, which an organism in a given environment selects, and 2) in reference to the specializing and specialized function in cellular, histological and organic level of a given living organism, animal, plant, fungus or bacteria (Bradshaw, 1989;Nurmi & Parvinen, 2008;Limor-Waisberg et al, 2011;Adite et al, 2013)…”

Ponds culture of juveniles of freshwater oyster Etheria elliptica Lamarck, 1807 in Benin (West Africa)

“…Their exploitation generates significant income for riparian populations [10,11] . However, consumers of oysters face a problem of food poisoning because of the way oysters are fed [12,13,14] . Indeed, oysters are filter feeders, and by doing so accumulate pollutants and heavy metals in their body [4,13,15] .…”