Oviduct malformation as a sterilising effect of tributyltin (TBT)-induced imposex in Ocenebra erinacea (Gastropoda: Muricidae)

In populations subject to high tributyltin (TBT) pollution, females of the neogastropod Ocenebra erinacea exhibit a characteristic malformation of the oviduct as an effect of advanced imposex. This abnormality is a longitudinal split of the oviduct wall, causing the bursa copulatrix and capsule gland to open directly into the pallial (mantle) cavity. When first discovered it was assumed such a gross malformation would preclude sperm transfer and capsule formation. Field evidence in terms of a general paucity of juveniles in affected populations supported this idea but direct evidence of a sterilising effect was lacking. Laboratory spawning experiments using normal and affected females have now been carried out. No capsule was produced by any female having a split oviduct. Thus, it is concluded that the net effect of the condition is sterilisation. The larvae of O. erinacea escape from capsules as veligers that assume a planktonic existence lasting for up to 5 days. The duration of this swimming phase is sufficient to permit extensive dispersion and subsequent colonisation over a wide area, including contaminated localities. Populations close to TBT sources could therefore be sustained by a supply of recruits from less-contaminated areas and their nonbreeding status thus masked. Juvenile females reared to one-year-old in water with a mean TBT concentration of 3 ng Sn 1(-1) exhibited the split oviduct condition seen in adults at contaminated sites, whereas those reared similarly at TBT levels of 0.2-0.3 ng Sn 1(-1) appeared normal. The disruption of the early ontogeny of the female reproductive tract, which leads to adult sterility, can be interpreted as an subtle effect of masculinization induced by TBT exposure.