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The common dogwhelk (Nucella lapillus) is a dioecious species. However, on exposure to low concentrations (<1 ng I-1, Gibbs et al., 1987) of tributyltin (TBT) females develop certain male sexual characteristics, notably a penis and vas deferens. This phenomenon, first reported by Blaber (1970) in dogwhelks from the Plymouth area, and later termed 'imposex' (Smith, 1981), is now recognised as the most sensitive and straightforward way to identify contamination of coastal areas by TBT. The degree of imposex may be readily quantified as the relative penis size index (RPSI), which expresses the relative bulk of the penes in females and males from a given sample. It is calculated as (mean cubed female penis length/the mean cubed male penis length) x 100% (Gibbs et al., 1987). Gibbs et al. (1987) have examined the development of imposex in greater detail, and have divided the process into six stages (known as the vas deferens sequence, or VDS) based upon the degree of development of the vas deferens and penis in the female animal. This classification enables an assessment to be made of the reproductive ability of an animal. Stages 1 to 4 show a progressive development of the penis and vas deferens. At a VDS of 5, vas deferens tissue proliferates over the opening of the reproductive tract, blocking it, and subsequently causing sterility due to the inability of the animal to release egg capsules. Aborted egg capsules then gradually accumulate in the capsule gland (stage 6). The vas deferens sequence index, VDSI, is the mean VDS in a given sample of females.

ABSTRACT. The sublethal effects of tributyltin oxide (TBT) at concentrations of 2 to 128 ng 1-' in sea water on imposex development, weight, activity and feeding rate of the adult common dogwhelk Nucella lapillus (L.) were investigated over 12 mo under conditions conforming to the UK Good Laboratory Practice standard. TBT promoted irnposex development, with a no-effect concentration of c 2 ng I-' for exposure of 6 mo or more. The degree of imposex d~d not exceed levels w h~c h caused sterility in any treatment group. Low levels of lmposex in controls showed seasonal changes in intensity. Seasonal patterns were overridden by 2 ng TBT 1-' or more. The low numbers of mortalities observed were not related to TBT concentration. The activity of dogwhelks (tendency to climb the tank walls) varied seasonally, reflecting natural behaviour patterns of wild populations. TBT exposure had no effect on this activity The feeding rate (predation on mussels Mytilus edulis) showed marked seasonal vanations in all tanks, but there was no correlation with TBT exposure. Exposure to TBT had no effect on the tissue weight of male dogwhelks, but the body weight of females was negatively correlated with TBT after 12 mo exposure. Mean bioconcentration factors (water to tissue of dogwhelks) ranged from 7400 to 25000, and decreased with increasing TBT exposure concentration. It is est~mated that the feed mussels accounted for approxinlately 40% of the TBT accumulated by the dogwhelks.

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