2,3,7,8-tetrachloro-dibenzo- p-dioxin is a potent toxin and induces cytochrome P450 in the crayfish, Pacifastacus leniusculus

2,3,7,8-tetrachloro-dibenzo- p-dioxin (TCDD) is the most potent congener of a family of environmental contaminants. We demonstrate that TCDD is a highly potent toxin in an invertebrate, the freshwater crayfish, Pacifastacus leniusculus, with an LD 50 of 30–100 μg kg −1 of body weight. Toxicity was characterised by delayed mortality, typically 15–40 days after dosing, and anergia. No gross histological changes were seen in hepatopancreas. Treatment of crayfish with 3 μg of TCDD kg −1 body weight significantly induced cytochrome P450, as measured spectrally, but we were unable to measure associated enzyme activities, presumably due to proteolysis of NADPH-P450 reductase. TCDD is thus a potent toxin and enzyme inducer in crayfish. We hypothesise that the nature of this pleiotropic response, together with its exceptional potency, suggests the presence of a receptor-mediated mechanism of TCDD toxicity in crayfish.