Abstract
Several recent studies have demonstrated that the freshwater pulmonate snail Lymnaea stagnalis is extremely sensitive to metals (Co, Ni, Pb) in chronic exposures. The objective of the current study was to evaluate the acute and chronic sensitivity of L. stagnalis to Cu and investigate the underlying mechanism(s) of toxic action. A 96-h LC50 of 31μgL−1 Cu was estimated indicating L. stagnalis was moderately acutely sensitive to Cu relative to other aquatic organisms. However, in a 30-day chronic exposure using juvenile snails an EC20 of 1.8μgL−1 Cu was estimated for snail growth making L. stagnalis the most sensitive organism tested to date for Cu. Hardness-based and BLM-based water quality criteria for Cu at the water quality conditions used in this study were 7.8 and 1.5μgL−1, respectively, indicating L. stagnalis is significantly under-protected by hardness-based WQC. Investigations into the mechanism(s) of toxic action for Cu were conducted on young adult snails necessitating higher Cu exposures. Exposure to Cu at 12μgL−1 resulted in no detectable effects on hemolymph osmolality, net Ca2+ uptake, titratable acid excretion, or ammonia excretion. Exposure to 48μgL−1 Cu was shown to significantly reduce (91%) net Ca2+ uptake which is strongly correlated with shell deposition and corresponding snail growth. Snails exposed to 48μgL−1 Cu also exhibited reduced ammonia excretion, a marked hemolymph acidosis, and a compensatory increase in titratable acid excretion. The reduction in net Ca2+ uptake was hypothesized to be a secondary effect of Cu-induced inhibition of carbonic anhydrase, but no reduction in carbonic anhydrase activity was detected. Overall, it remains unclear whether inhibition of Ca2+ uptake is a direct result of Cu exposure or, along with the other observed physiological effects, is secondary to an unidentified primary mode of toxic action. Given the hypersensitivity of L. stagnalis to Cu, further study into the mechanisms of action and effects of varying water chemistry on Cu toxicity is clearly warranted.
