On the rocky shores of the Lorient coastline in Brittany, France, female bigorneaux—common marine snails—have been displaying a male organ, a penis, for several years. This unusual phenomenon, known as imposex, is not a rare occurrence but a widespread issue affecting large areas of the Breton coast and entire populations of these marine gastropods. The cause of this strange transformation lies in a chemical compound that has been used for decades to keep boat hulls clean and free of marine growth. Tributyltin (TBT), a component of antifouling paints, has been responsible for this biological anomaly. Although TBT is now banned in marine paints, it remains in the sediments of ports and continues to affect the environment for many years. Tributyltin causes imposex by interfering with the endocrine system of marine invertebrates, particularly the production and regulation of steroid hormones. In female bigorneaux, this disruption leads to the development of male sexual characteristics, including a penis, while the original female reproductive organs remain. The most serious consequence of this transformation is reproductive failure. In many affected females, the development of the male organ blocks the female genital opening, preventing them from laying eggs. The shell, which typically serves as a protective refuge, becomes a kind of biological prison, trapping the snails in a state where they can no longer reproduce. In the most contaminated areas, the reproductive blockage has led to a significant decline in local populations of gastropods, with some species disappearing entirely from parts of the coastline. The situation in Lorient is not unique; similar cases of imposex have been documented in other European regions where naval activity has concentrated around drydock areas. European regulations eventually banned the use of tributyltin in antifouling paints, a measure that has allowed, in some areas, the beginning of a recovery in gastropod populations. However, the persistence of this molecule in port sediments highlights the long-lasting nature of chemical pollution, which does not disappear immediately even when banned by law.