Interaction between density and Cu toxicity for Enchytraeus crypticus--comparing first and second generation effects.

Density of organisms varies considerably in nature depending e.g. on seasonality or food availability. A recent investigation on interaction between Cu and density using Enchytraeus crypticus showed that density itself (5-50 per 20 gr dry soil) had an impact on population and individual growth [up to 3000 individuals per test vessel], but the interaction between density and Cu toxicity was not significant. Here, a follow-up study was performed, in which the interactions between density and Cu-exposure were investigated along a two-generation exposure using E. crypticus (three factorial: 1. density (5-50), 2. Cu (0-300 mg/kg) and generation (G1-G2)). After G1, the juveniles were retrieved and further exposed under the same conditions along a G2 (using a refined density set - 10 and 50). Results showed an interaction between density and Cu in the reproduction of E. crypticus, this being significant in G2, showing lower toxicity for higher density of organisms whereas in the 1st generation the opposite occurred. Hence, there was an interaction seen along G1 to G2, i.e. animals from density 50 in G1 when further exposed at density 50 in G2 had lower Cu toxicity compared to when further exposed at density 10. Possible explanations include the hydra effect (overcompensation in G2 due to stress in G1) or that for density 50 the organisms are exposed to less Cu than at lower densities, this by e.g. (1) organisms avoid exposure by lumping/clustering which would limit exposure to Cu and (2) there would be less available Cu contaminated soil per individual at high density hence less exposure.