Shell thickening and chambering in the oyster Crassostrea gigas: natural and anthropogenic influence of tributyltin contamination.

Abnormal thickening and chambering in Crassostrea gigas oysters have been adopted for many years as bioindicators of available tributyltin (TBT) in coastal waters. Nevertheless, since natural causes can also induce the formation of multiple chambers, a field study and laboratory experimentation has been conducted with 72 examples of C. gigas in successive culture media. This work has enabled differences to be established between natural fine sediment-induced characteristics and the influence of TBT on the shells. External shell deformities have been assessed using three biometric indices, shell thickness index, weight index and volume index. Internal differences have been observed in longitudinal sections of the shell: retraction of growth, stagnation of the adductor muscle scar and thinning of the chambers in the TBT-polluted shell secretion. A new index, the opening chambers index, has been proposed, with a value of less than 1 in the TBT-polluted environment and greater than 1 in shells secreted in an unpolluted production site. These conclusions should be borne in mind when C. gigas is used in biomonitoring programmes.