Behavioural and siphonal tissue responses of Scrobicularia plana (bivalvia) to zinc

The effects of zinc being added to sea water, to final concentrations of 0·1–20 ppm, have been studied on the heart rate, valve movements, mortality of Scrobicularia and on both isolated and in situ inhalant siphon preparations.The acute toxicity threshold for added zinc was determined to be about 10 ppm at 10°C. The median lethal times for 10 and 20 ppm zinc were 143·5 and 116·1 h respectively.The responses of Scrobicularia to zinc concentrations in sea water of between 0·1 and 10 ppm were tested by exposure for 6 h. Above 1 ppm, valve closure and bradycardia occur within 10–15 min. Below 5 ppm the valves subsequently opened and heart rate increased over the 6 h period, but in 10 ppm closure and pronounced bradycardia were maintained throughout.Addition of zinc, to final concentrations of 1, 5 and 10 ppm, had no effect on the isolated inhalant siphon in contrast to copper at 0·25 and 0·5 ppm which caused very marked siphonal contractions. However, when zinc (1–10 ppm) was added to an in situ inhalant siphon preparation, contractions occurred which were lost on removal of zinc from the bathing medium. Cutting the cruciform muscles medially resulted in the loss or delay of the response to zinc. This suggests the possibility of the cruciform muscle complex (muscle strands and associated sense organs) responding to zinc. This contrasts with the direct action of copper on the neuromuscular system of an isolated siphon.