Use of embryo-larval development stage of the oyster Crassostrea gigas as a toxicity assessment tool for pure substances and complex mixtures
Embryos and larva of bivalves are used frequently as sentinel organisms in ecotoxicological bioassays for the evaluation of seawater quality. Indeed, they are very sensitive to pollutants and provide a fast response. The embryo toxicity bioassay with the bivalve mollusc Crassostrea gigas was used to evaluate toxicity of three metals (mercury, copper, cadmium) and three effluents emanating from Moroccan industries which discharge directly on the Atlantic coast near Casablanca (electrochemical and mixed industrial waste waters) and on the El Jadida coast (phosphate treatment waste water). After 24-h exposures to the various media, the effects, expressed as CE50 values (effective concentrations affecting 50% of the test population), were compared. The results of the tests show that mercury is the most toxic metal (CE50 = 4.4 μg Hg∙L-1) followed by copper (CE50 = 16.4 μg Cu∙L-1) and finally cadmium (CE50 = 46.9 μg Cd∙L-1). The effects of the industrial wastewaters, tested with increasing dilutions, indicate a strong risk of seawater quality deterioration, especially in the discharge zones with CE50 values corresponding to effluent concentrations of less than 1%. The effluents decreased in toxicity according to the following order: electrochemical effluent &gt; mixed industrial effluent &gt; phosphate treatment effluent. This study demonstrated the usefulness of oyster embryos and larvae as a sensitive tool for assessing environmental quality.