Relatively few experiments have investigated the toxicity of dioxins to invertebrates. Where work has been carried out the results generally indicate no susceptibility to dioxins. In one case, even where field evidence showed a correlation between exposure to dioxins and mortality in sediment-dwelling amphipods, laboratory experiments with spiked sediments resulted in no adverse effects and suggested that other factors contributed to mortality in situ. The few available studies have shown some evidence of toxic effects in invertebrates, including reduced reproductive success in sediment-dwelling worms and snails, and possible gene expression effects in clams. Soft-shelled clams were found to accumulate dioxins in gonad tissues after acute exposure in water, and showed possible alteration in gene expression associated with increased cell cycling. Acute toxicity and cytochrome P450 induction have been demonstrated in crayfish.

Studies have shown that many invertebrate species do not possess a functional AhR, which may account for the apparent lack of susceptibility to dioxins reported in most invertebrate studies. Cytochrome P450 has been measured in the digestive organs of sea star and, in one study, CYP1A induction was found to be significantly related to environmental exposure to dioxin. However, before reaching any final conclusions about invertebrate sensitivity, additional long-term toxicity tests on a diverse set of aquatic invertebrates is needed.

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