Interclonal variation in the acute and delayed toxicity of cadmium to the European prosobranch gastropod Potamopyrgus antipodarum (Gray)

The lethal responses of three European clones - A, B, and C - of the prosobranch snail Potamopyrgus antipodarum to acute cadmium exposure were examined by the use of a conventional LC₅₀ test and a delayed toxicity test. The questions addressed were: (1) Are there differences in susceptibility (LC₅₀ values and uniformity of response) among the three European clones of P. antipodarum? (2) Are the patterns of differences in susceptibility among clones observed in the LC₅₀ test also observed for the delayed toxicity test? (3) Is there concordance in the ranking of susceptibility among clones under acute cadmium exposure and under chronic cadmium exposure? The results showed that the widths of the tolerance distribution differed among clones. Clones A and B had a steeper slope than clone C (for clone A the difference was marginally significant), which indicates that individuals from clones A and B showed a more uniform response to acute lethal cadmium stress than individuals from clone C. On the basis of the measured differences in LC₅₀ values, clone A individuals showed the highest tolerance to acute cadmium (LC₅₀ value: 1.92 mg Cd L^-1) followed by clone B (LC₅₀ value: 1.29 mg Cd L^-1) and clone C (LC₅₀ value: 0.56 mg Cd L^-1). Clone C was significantly less tolerant than clones A and B. The delayed toxicity test showed a similar pattern to the LC₅₀ test with regard to tolerance differences among clones; however, mortality continued following transfer to clean water, indicating that cadmium was lethal at much lower concentrations than indicated by the conventional LC₅₀ test. Results of the LC₅₀ test and the delayed toxicity test in the present study were in general agreement with results from chronic cadmium exposure experiments (Jensen et al. [2000] Ecol Appl [submitted]), i.e., the least susceptible clone A in the acute cadmium exposure test was also the least susceptible clone in the chronic cadmium exposure test. Based on the dramatic differences between the LC₅₀ and the cadmium exposure concentrations causing delayed toxicity, we suggest that the potential for delayed toxicity should be given greater consideration in ecotoxicity testing.