Modeling effects of cadmium on population growth of Palaemonetes pugio: Results of a full life cycle exposure

In an 8-month laboratory experiment, Palaemonetes pugio (grass shrimp) were exposed to aqueous cadmium (free cadmium ion concentrations of 1.51 or 2.51 μg Cd2+/L) for an entire life cycle, from larva to reproductive adult and through to production of second-generation larva. Individual-level effects on survival, life stage duration, and reproduction were measured, and population growth was projected using two models: a stage-based matrix model and a z-transformed life cycle graph analysis. Adult survival was significantly reduced at 2.51 μg Cd2+/L, but cadmium exposure had no effects on survival or stage duration of embryos, larvae, or juveniles. Survival of second-generation larvae was unaffected by maternal exposure. Brood size was reduced by 27% at 1.51 μg Cd2+/L and by 36% at 2.51 μg Cd2+/L. The percent of females in the population that was gravid was approximately 50% lower at 2.51 μg Cd2+/L, compared to controls. Both population models projected a dose-dependent decrease in population growth rate (λ), up to a 12% reduction at 2.51 μg Cd2+/L, which can be attributed mainly to contributions from reproductive effects. Elasticity analysis revealed that population growth rate was most sensitive to changes in survival of juveniles and adults. However, lethal effects of cadmium made only a small contribution to the effect on population growth rate. Even though both models project positive growth (λ > 1) of grass shrimp populations exposed to low concentrations of cadmium, the ability of populations to withstand predation pressure would be compromised.