Imposex is a reproductive abnormality in which female snails begin to transform to males, but do not become functional. It was caused by tributyltin (TBT) used as an antifoulant in boat paints. Imposex was first recorded marine snails (Conus) (Mollusca: Caenogastropoda) at Rottnest Island, Western Australia, in January 1991, where 88% of individuals at the west end were affected. Most were at moderate Stages 3 and 4 on a scale of 0 (no affect) to 6 (death). TBT was banned on boats < 25 m long in late 1991 in WA. In 1996, imposex had declined to 69% of females with Stages 3 and 4 still the most common. By 2007 only 35% of females exhibited imposex; Stage 3 was the highest level recorded. TBT was below detection limits. TBT was banned on vessels > 25 m in September 2013. In February 2017 only 4% of Conus had imposex, at Stage 1.