Imposex occurs when male sex characteristics, such as the development of male sex organs, for example the penis and the vas deferens, are stimulated to form on normal female gastropods. One inducer of imposex is tributyltin (TBT), an anti-fouling agent for boats which affects many marine gastropods, including the female Nucella Lapillus (dog whelk). The penis growth of the imposex female dogwhelk blocks the oviduct, whilst ovule production continues.