Tributyltin is a toxic antifouling compound banned on most U. S. vessels since 1988. TBT causes the development of imposex, a penis and vas deferens in female snails. We sampled Stramonita haemastoma from 1988–2000 in the Gulf of Mexico and Florida to determine the expression
of imposex and male:female ratios did not deviate from the expected 1:1 ratio. The percentage of females exhibiting imposex declined over time at Redfish Bay, Texas, Port Fourchon, Caminada Pass, Louisiana and Marineland, Florida, was 97–100% at South Padre Island, Texas and 86–100%
at Pensacola Bay, Florida. The relative penis size index (RPSI) was initially high at South Padre Island, Redfish Bay and Port Fourchon and declined over time at Redfish Bay and Port Fourchon. There were small significant increases in the RPSI at South Padre Island, Caminada Pass and
Pensacola Bay. The Vas Deferens Sequence Index (VDSI) was low and declined over time at Redfish Bay, Port Fourchon and Caminada Pass; declined at South Padre Island, but increased at Pensacola Bay. Very low RPSI and VDSI values were observed at Marineland. Intensity of
imposex declined at all locations over time but at Pensacola Bay they increased near large vessels exempt from TBT regulations.
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