CRITICAL-APPRAISAL OF THE EVIDENCE FOR TRIBUTYLTIN-MEDIATED ENDOCRINEDISRUPTION IN MOLLUSKS

This article reviews the field and laboratory evidence for endocrine d isruption in gastropod mollusks caused by tributyltin (TBT). Abundant and undisputed field data link TBT with an irreversible sexual abnorma lity of female neogastropod snails known as ''imposex.'' This phenomen on is a masculinization process involving the development of male sex organs, notably a penis and a vas deferens; in certain species the imp osition of a vas deferens disrupts oviducal structure and function, pr eventing normal breeding activity and causing population disappearance . In some species, oogenesis is supplanted by spermatogenesis. A relat ed condition referred to as ''intersex'' has been reported in littorin id mesogastropods, and these too become unable to lay eggs. Field evid ence clearly associates these syndromes with the use of TBT as an anti foulant, chiefly on boat hulls, and dose-related effects can be replic ated in laboratory exposures to environmentally relevant concentration s of TBT compounds. It has now been established that imposer and inter sex are forms of endocrine disruption caused by elevated testosterone titers that masculinize TBT-exposed females. The precise mechanism by which increased levels of testosterone are produced has not been fully described, but the weight of evidence suggests that TBT acts as a com petitive inhibitor of cytochrome P450-mediated aromatase. Some recent data suggest that TBT may also inhibit the formation of sulfur conjuga tes of testosterone and its active metabolites, thus interfering with its excretion. In summary, TBT-induced masculinization in gastropods, imposer and intersex, is the dearest example of endocrine disruption d escribed in invertebrates to date that is unequivocally linked to a sp ecific environmental pollutant.