Following treatment of the shells of adult Nucella lapillus with different marine paints under field conditions, the development of 'imposex' (the growth of a penis and vas deferens in females) was promoted by three TBT-based antifouling paints but not by a tin-free copper-based paint nor by yacht enamels.When adult animals were exposed to a range of sea-water leachates from TBT copolymer antifouling paint the initial rate of tin accumulation was approximately proportional to the ambient concentration over the range < 1-107 ng/1 of tin in the TBT fraction. In experiments of between 8 and 14 months duration, concentration factors (dry tissue/water) for tin (as TBT) reached about 10 5 at sea water concentrations of 1-5-18-7 ng/1. At 107 ng/1, however, the factor fell to about 3 x 10 4 owing probably to the accelerated degradation of TBT to DBT by the tissues. The bioaccumulation data from the laboratory experiments compare favourably with field data from populations around south-west England.At all experimental sea water concentrations, the bioaccumulation of tin by adult females was accompanied by increased development of imposex. However, the sterilisation of adult females through blockage of the oviduct by the vas deferens (as observed in declining field populations) occurred only after at least 12 months exposure to 107 ng/1 of tin as TBT: it also occurred after more than 18 months in some adults transplanted to a rocky shore exposed to a mean concentration of about 28 ng/1. Growing juveniles, on the other hand, are far more sensitive and may be sterilized at concentrations of only a few ng/1 before reaching maturity.The depuration of tin was followed for up to 9 months and, depending on the conditions, half-times for loss of the TBT fraction from the tissues ranged between about 50 days and more than 100 days. There was no evidence that loss of tin leads to any remission of imposex. I N T R O D U C T I O NPopulations of the gastropod, Nucella lapillus (L.), normally one of the commonest of rocky-shore macroinvertebrates, have recently suffered from declining numbers at many sites along the south coast of England. Bryan et al. (1986) showed that these declining populations exhibited a high degree of imposex (the induction of a penis and vas deferens in females) and that imposex was almost certainly caused by tributyltin (TBT) compounds leached from ships' antifouling paints. Evidence implicating TBT compounds in the development of imposex included: (1) a good relationship between the degree of imposex and the proximity of affected populations to harbours and marinas; (2) in Plymouth Sound, the degree of imposex increased dramatically between its discovery in 18-2

Between 1984 and 1993, levels of imposex (the induction of male characters including a penis on females) were measured in gastropods Nassarius (Hinia) reticulatus (L.) at localities in south-west England. Since tributyltin (TBT) was thought to cause imposex, concentrations in tissues and sea-water were also determined. Measurements made prior to the restriction of TBT usage in 1987 showed that intensities of imposex were related to TBT levels in females. Tissue concentrations were also related to those of sea-water and concentration factors (dry tissue/water) were approximately 30,000 at 10 ng Sn I 1 and 75,000 at 1 ng Sn I 1 . Penis development in females was initiated at about 1 ng Sn I 1 and in this respect JV. reticulatus appears less sensitive to TBT than the dog-whelk Nucella lapillus.As result of the TBT restrictions, concentrations in sea-water and tissues at some of the more polluted sites decreased by factors of 5-10 times between 1987 and 1993. However, population imposex declined very slowly. This was attributed to the longevity of the snails, the slow decline of penis-length in older females and the limited recruitment of less-affected females.It was concluded that N. reticulatus is a useful alternative to N. lapillus as an imposexbased TBT indicator at contaminated sites. However, when environmental TBT concentrations are declining fairly rapidly, analysis of N. reticulatus tissues provides a far better indication of change than measurements of population imposex.

The annual cycles of the Abra tennis populations in the Plym Estuary and the Fleet (Dorset) were studied from March 1980 to June 1982. In the Plym Estuary the highest numbers occurred in autumn following juvenile recruitment and the lowest numbers were recorded in mid-summer. In the Fleet such an annual cycle in numbers was not apparent since juvenile recruitment in 1980 was very high but in 1981 it was very low. The recruitment failure in 1981 may have resulted, at least in part, from the heavy summer infection of A. tenuis by digenean parasites that destroy the gonads of the host, thereby reducing the population fecundity.A. tenuis spawns in July and August: the mature egg has a diameter of 140 μm and is enclosed in a sticky gelatinous envelope. Females produce 450–1800 eggs which, when fertilized, are laid in a mass within the sediment and, following a direct development, the juveniles hatch as miniature adults after 2–3 weeks. This non-pelagic, lecithotrophic development is interpreted as being a means of ensuring the maintenance of populations within restricted estuarine habitats. Data suggest A. tenuis lives for 1 or 2 years and that the species is monotelic.