Two studies were conducted to observe effects of dibutyltin (DBT) and tributyltin (TBT) on larvae of Mytilus edulis for an exposure period of 25 d. Endpoints for evaluation were shell growth and mortality measured at 33 d. Larvae were cultured in a new laboratory assay chamber in a recirculating static test. The control, 2, 20, and 200 gg/1 DBT-treated populations had mean shell lengths of 527, 523, 417, and 180 gm, respectively. Survival was 1% for the 200 gg/1 DBT-treated population, but ranged from 73 to 83 % for controls, 2, and 20 gg/1 treatments.
The no-observed-effect concentration (NOEC) was 2 gg/1 for DBT, while the lowest-observedeffect concentration (LOEC) was 20 gg/1. The chronic toxicity value was 6.3 gg/1. In the TBT bioassay, mean shell lengths for the control, 0.006, 0.050, and 0.130 gg/1treated populations were 565, 437, 385, and 292 ~tm, respectively. Control survival was 74%, whereas TBTtreated populations survival ranged from 52 to 58 %. The NOEC for TBT was 0.006 gg/1 TBT and the LOEC was 0.050. A chronic toxicity value of 0.017 gg/1 was calculated. The results of this study indicated that the toxicity of DBT was less than that of TBT. It was concluded that shell length was inversely related to exposure level in both DBT and TBT bioassays. In this study, we have observed TBT effects at lower exposure levels in the laboratory than previously reported, and also report the first data for DBT effects on mussel larvae.