Combining Life-History and Toxicokinetic Parameters to Interpret Differences in Sensitivity to Cadmium between Populations of Polycelis tenuis (Platyhelminthes)

Five populations of the planarian Polycelis tenuis, collected from four locations in a metal contaminated stream and from one location in an unpolluted tributary stream, were compared for their sensitivity to cadmium by measuring survival, reproduction, and body size changes during 28 days of exposure via the water. Survival data and bioconcentration of cadmium measured at the end of the experiment enabled the use of a kinetic-based toxicity model, estimating the lethal body concentration, the uptake rate constant, the elimination rate constant, and the ultimate LC50 as estimated toxicokinetic parameters. Accurate body size selection and a one-month acclimatization period to standardized laboratory conditions were applied to enhance the comparability between populations. An increased elimination rate constant and a greater ultimate LC50 were estimated for one of the populations, located downstream from an ancient lead–zinc mine. The cadmium concentration causing an effect on reproduction varied considerably (a factor of 10) among populations but did not differ significantly between populations. For the number of reproducing individuals as a function of cadmium concentration, EC50 and steepness of slope of the concentration/effect relationship tended to be correlated positively (P<0.10), indicating that selection in the field may have occurred. Cadmium was found to enhance shrinkage of P. tenuis under conditions of food limitation.