Sensitivity to cadmium along a salinity gradient in populations of the periwinkle, Littorina littorea, using time-to-death analysis.

In this study, we assessed the combined effect of Cd concentration and salinity, on Cd uptake and mortality rate of Littorina littorea, collected along a salinity and pollution gradient in the Western Scheldt estuary (The Netherlands). Animals kept at their field salinity levels were exposed to three Cd concentrations (i.e. 10, 40 and 320 microM), while animals kept in 10 microM of Cd were subjected to five salinity treatments (i.e. 15, 20, 25, 30 and 35 per thousand). Mortality was recorded every 24h and Cd body burdens were measured with ICP-AES. Time-to-death data were analysed via Cox proportional hazard models, including the co-variates "site-Cd treatment" in the Cd experiment and "site-salinity treatment" in the salinity experiment. "Cd-treatment" and "field-salinity" affected mortality rates significantly in the Cd experiment, such that the mortality risk increased by 2.3 times when salinity was lowered from 35 to 15 per thousand, while it decreased by 19.7 times when Cd dropped from 320 to 10 microM. "Site" did not significantly affect the mortality risk in the salinity experiment but affected time-to-death via its interaction with the "salinity-treatment". Generally, mortality did not occur at a given threshold Cd tissue level, but changed over time and treatments, in function of the site. The results demonstrate the importance of the animals' environmental history and illustrate the usefulness of time-to-death analyses in ecotoxicological experiments.