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. LC
50 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<0.05) in mortality in 48 and 96 hr bioassays. The 96-hr LC
50 value for the larger oysters (135 ppm) was lower than that of the smaller ones (545 ppm), implying that smaller oysters (in the size ranges used) were more tolerant to crude oil than the larger ones.
@JASEM
