Exposure of the oysters to 270 g L−1 of lead significantly reduced shell length growth (F 5.14, P 0.016 Fig. 2a) as compared with the control treatment. The mean length of shells actually decreased by 2 ± 1% (mean ± SE) because of shell weakening at the distal margin. Visually, shells from oysters cultured in the high lead treatment lacked the imbricate processes at the distal margin evident in oysters grown in other treatments, indicating shell stress (Fig. 3). Although there was no impact on the measured shell length in the 90 g L−1 treatment, Figure 3 shows that these oysters also lacked the imbricate processes. These two highest lead concentrations resulted in shells that were weaker and more brittle than control oysters. Exposure to zinc did not significantly affect shell length growth over the 2-month period (Fig. 2b). Exposure to hexadecane and octacosane at 30–810 ng L−1 did not have any significant effects on shell length (Fig. 2c, d).