Imposex is a descriptive term applied to some sea snails, marine gastropod mollusks which, under the toxic effects of pollutants, develop sex organs that are in contrast to their actual gender.

Imposex is a pathological condition where male sex characteristics, such as the development of male sex organs, (for example the penis and the vas deferens) form in female gastropods.

The only inducer of imposex is tributyltin (TBT), an anti-fouling agent for boats which affects females of the species Nucella lapillus (dog whelk) and more than 200 other marine gastropods. The penis growth of the imposex female dogwhelk blocks the oviduct, whilst ovule production continues. An imposex female dog whelk passes through several stages of penis growth, before it is unable to contain its constant production of ovules, resulting in death.

The imposex stages of female dog whelks (Nucella lapillus) and other molluscs (including Nucella lima) are used in the UK and worldwide to monitor levels of tributyltin (TBT). The RPSI (Relative Penis Size Index) of females to males, and the VDSI (Vas Deferens Sequence Index) are used to monitor levels of TBT in marine environments.

On September 29th, 2007, the Halifax, Nova Scotia based newspaper, The Chronicle Herald featured a story on female dogwhelks on the shores of Halifax Harbour and other parts of Nova Scotia (where they live on the beach in intertidal zones between high and low tides). The report stated that female dogwhelks still grow penises even after a ban on TBT.


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