Tributyltin Biomonitoring in Portuguese Estuaries with the Portuguese Oyster (Crassostrea Angulata)

In Portugal the measurement of shell thickening (Shell Thickness Index or STI) in the Portuguese oyster Crassostrea angulata was used to evaluate bioavailable tributyltin (TBT) in estuaries. Valves from living C. angulata populations and relict valves were collected in 1989–90 over the entire Sado estuary and they showed shell thickening (low STI) increasing with time. Portuguese oyster reproduction had recently ceased in the Sado estuary. Relict oyster valves from the Tagus estuary showed no shell thickening. Caged cultchless Portuguese oyster spat showed valve thickening after two and four months exposure to TBT-painted panels, and when transplanted to the Sado estuary. Spat exposed to an increasing series of TBT-painted panels had increased mortality, TBT tissue concentrations and shell thickening. Oyster spat transplanted to the Sado estuary and a canal in the Aveiro estuary had low STI at four months, but not when transplanted to another Aveiro estuary site or the Viana do Costelo and Figuera da Foz estuaries. Adult oysters with low STI, including those for market sale, had lower tissue TBT concentrations than spat and were not considered hazardous to human health. The STI bioassay using transplanted oyster spat was easily measured in four months, related to TBT exposure, independent of overall growth and a better indicator of bioeffective TBT than tissue TBT concentration.