During the 1980s a decline in populations of the bivalve Scrobicularia plana was noted in several U.K. estuaries: tributyltin (TBT) was suspected of being the cause although its toxicity to adults could not be demonstrated except at elevated concentrations; disappearance of clam populations has been also observed in other European countries. Laboratory tests have revealed that: i) D-larvae hatching from embryos after 48 hours in TBT concentrations of 188 ngSn/l amounted to < 50% of control values, and doses of 364 ngSn/l or 20 µgCu/l prevented normal development in = 90% of embryos. ii) planktonic veliger larvae exposed for 10 days to nominal TBT doses ≥ 50 ngSn/l grew at rates which, at maximum, were one third of that exhibited by controls. iii) settling pediveligers subjected for 30 days to levels of TBT ≥70 ngSn/l suffered significant mortalities, and postlarvae kept at 23 ngSn/l displayed some shell growth which was both substantially reduced and grossly abnormal. iv) exposure for 30 days to 2! 300 ngSn/l or ≥20 µgCu/l impaired the burying activity in sand of small spat, and juveniles reared in TBT solutions at ≥ 28 ngSn/l grew significantly less than those in the control treatment. v) while small spat held in heavy metal polluted sediment suffered massive mortalities in 12 days, juveniles exposed for 36 days to butyltin contaminated sediment (0.4 µgSn/g) did not display any limited survival, but both their growth and burying activity were significantly reduced relative to those of juveniles kept in control sediments. It is concluded that in U.K. coastal areas where TBT in water during the summer-autumn months ranged from = 20 ngSn/l to = 200 ngSn/1 (i.e. = 50-500 ngTBT/l) and Scrobicularia plana populations disappeared or declined markedly, a cause-effect relationship is most likely to exist between the former and the latter through the deleterious effects of the chemical on the early life stages of the clam; in addition, the continued presence of sediment-bound TBT may render mudflats unsuitable for the development of larval and juvenile bivalves.

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