Abstract.In the Azores the gastropodThais haemastomashows varying degrees of imposex, the induction of male sex characters in the female. It is inferred that imposex inT. haemastomais caused by tributyltin (TBT) associated with boats using organotin‐based antifouling paints. The two indices ‐ relative penis size and vas deferens sequence ‐ employed previously forNucella lapillus, were used without alteration.T. haemastomais only the third species reported as exhibiting ‘effective sterilisation’ of the female. UnlikeNucella lapillus, T. haemastomahas a planktonic larva, allowing populations to be sustained even when local TBT pollution is high. This makesT. haemastomapotentially a better indicator species, as monitored populations can be maintained by the influx of larvae.T. haemastomamay thus provide a cheap method of assessing the extent of TBT contamination in warm waters, particularly in the Mediterranean and Atlantic Islands. Preliminary examination of material from a marina in Spain showed a high incidence of ‘steri
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