Short-term sublethal effects of cadmium on the filter feeding chironomid larva Glyptotendipes pallens (Meigen) (Diptera)

Abstract

The effects of the trace metal cadmium on the behavior, biomass, bioaccumulation and survival of individual fourth instar larvae of the midge Glyptotendipes pallens (Meigen) (Diptera) were investigated in a series of laboratory experiments. Behavioral responses were quantified by means of the impedance conversion technique. A 96 h exposure to cadmium gave rise to two different, concentration dependent response mechanisms. At concentrations of 0.1 to 1.0 mg·1 −1 Cd, slight deviations from the normal feeding pattern of unexposed larvae were observed. In this concentration range, larval uptake of cadmium increased slowly with no demonstrable effects on the survival of the larvae. However, development was retarded as indicated by the lower biomass values observed. The second mechanism became operative at concentrations between approximately 2.5 and 10 mg·1 −1 Cd. At a concentration of 2.5 mg·1 −1, a large variability in behavioral responses to cadmium exposure was observed. Some exp osed larvae exhibited only slight deviations from the normal activity patterns, while others developed totally aberrant behavioral patterns. All larvae that were exposed to Cd concentrations of 5.0 and 10.0 mg·1 −1 demonstrated totally aberrant behaviors, characterized by long periods of inactivity. Further, food uptake ceased completely at these concentrations. Although the ultimate survival of larvae exposed to the highest concentrations tested was strongly affected, the exposure values were still substantially below the 144 h LC 50 level for cadmium. Behavioral effects resulting from a 96-h exposure to cadmium were detectable at concentrations as low as 0.5 mg·1 −1 Cd, a level that is 100 times lower than the 144 h LC 50 value. This fact indicates that sublethal, ecologically relevant effects can be demonstrated in time spans normally utilized for acute toxicity testing. The use of the impedance conversion technique for the assessment of behavioral responses to contaminant exposures appears to hold particular relevance for the monitoring of freshwater pollution.