Effect of thallium on phototactic behaviour in Daphnia magna

Thallium (Tl) is a trace metal enriched in wastewaters associated with mining and smelting of base metals. The toxicity of Tl to aquatic biota is poorly understood, particularly with respect to its sublethal effects. In this study, phototactic behavioural responses of naive (i.e. no previous exposure to Tl) Daphnia magna, a key regulatory freshwater crustacean species, were examined in waters containing Tl. Fed and fasted neonate daphnids (< 24 h old) and fed adults (10-15 days old) showed no significant response at any tested water Tl concentration. However, in fasted adults, an increase in the positive phototactic response (measured as a greater number of daphnids closer to the light source after a 5-min exposure) was seen at Tl concentrations of 917 and 2099 mu g L-1, values representative of extreme environmental Tl concentrations. The presence of Tl also decreased the swimming speed of adult Daphnia towards a light source. In the presence of cimetidine, a histamine receptor blocker, the increase in positive phototaxis induced by Tl disappeared, suggesting that Tl acts to perturb the phototaxis response through sensory inhibition. Conversely, although there was a trend towards enhanced activity, Tl had no significant effect on acetylcholinesterase, a marker of locomotor capacity.

Automated summary

This study investigated the impact of thallium on the phototactic behavioral response of Daphnia magna. The results showed that high concentrations of thallium increased the attraction of Daphnia to a light source and decreased its swimming speed. Thallium also perturbed phototaxis response through sensory inhibition. However, thallium had no significant effect on the locomotor capacity of Daphnia.