Toxicity tests were performed exposing the polychaete Dinophilus gyrociliatus to seawater samples containing the toxic dinoflagellate Ostreopsis cf. ovata. Experiments were carried out twice in mid summer, when O. cf. ovata bloomed, and twice in late summer, when the dinoflagellate density declined. Each time, four treatments were considered: original sample; reference sample, which always contained 200 O. cf. ovata cells ml -1; filtered-seawater sample; artificial seawater (control). All the time, almost all worms survived in the controls and filtered-seawater treatments. In mid summer, the mortality of worms in the treatments containing O. cf. ovata was high and density-dependent. On the other hand, mortality in groups exposed to the original and reference samples collected in late summer was negligible. The high per cell toxicity of O. cf. ovata during mid summer could represent an allelopathic response. The test with D. gyrociliatus appears to be a promising tool for assessing the ecological risk of toxic dinoflagellate blooms. © 2011 Elsevier Ltd.

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