Effect of Size on the Acute Toxicity of Crude Oil to the Mangrove Oyster, Carasostrea gasar

Abstract

The acute toxic effects of a Nigerian crude oil (Egbogoro Liner II) to the mangrove oyster Carasostrea gasar were studied in bioassays. Tests were conducted over a 96-hr period after acclimatization of individuals in the laboratory, initially with a population comprising a broad size categories (11 - 60 mm) in range finding tests. The tests were semi-static bioassays in which the exposure media were replaced every 24 hours, at which the oysters were also examined for mortality. Subsequent experimentation was designed to compare the toxicities of oil to small (10 - 30 mm) and large (41 - 60 mm) oysters. LC50 values were obtained by graphical interpolation. Significant differences in effects at each time interval between small and large oysters were determined using the Man-Whitney tests on percentage mortality values. Relative toxicity tests showed consistently lower mortalities for small oysters at 24-hr, 48-hr and 96-hr periods, with Mann-Withney tests showing significant differences (p