Acute toxicity of organic chemicals to Gammarus pulex correlates with sensitivity of Daphnia magna across most modes of action.

We investigated the sensitivity of the freshwater crustacean amphipod Gammarus pulex towards organic xenobiotic compounds in comparison to the sensitivity of the crustacean cladoceran Daphnia magna. In addition we studied the influence of the chemical's mode of action on the relationship between the sensitivity of G. pulex and that of D. magna. We tested the acute toxicity of twelve compounds (Malathion, Aldicarb, Carbofuran, 2,4-dichloroaniline, 2,4-dichlorophenol, 1,2,3-trichlorobenzene, 4,6-dinitro-o-cresol, 2,4,5-trichlorophenol, Ethylacrylate, 4-nitrobenzyl-chloride, Sea-nine, Imidacloprid) with different modes of action and physicochemical properties towards the freshwater amphipod G. pulex in laboratory experiments. Additional toxicity data was collected from the peer-reviewed literature and databases (data pairs for 44 chemicals in total). The chemicals were assigned to seven mode of action groups. The relationship between the sensitivity of G. pulex (48h-LC50s and 96h-LC50s) and that of D. magna (48h-EC50s) was investigated using regression analysis and correlation plots. G. pulex is two to three orders of magnitude more sensitive towards neonicotinoids than D. magna (P=0.0046, n=3). For organophosphates we found that D. magna is more sensitive than G. pulex by approximately a factor of six (P=0.0256, n=6). There was no significant difference between the sensitivity of D. magna and that of G. pulex in any of the other mode of action groups; however chemicals with the same mode of action grouped together in the same area of the correlation plot. Without the neonicotinoids 75% of all G. pulex toxicity data were within one order of magnitude of the D. magna data and 100% within two orders of magnitude. The regressions with all data and with all data minus neonicotinoids were both significant linear relationships with slopes around one and intercept around zero. Thus, G. pulex is generally equally sensitive towards organic xenobiotics as D. magna.