Phenotypic plasticity and fitness consequences in nematodes exposed to toxicants

A prevailing view in ecotoxicology is that toxicants affect organisms by impairing lifecycle traits that arc most sensitive to these toxicants, a concept also adopted by national and international legislative authorities for deriving safe standards for contaminants in soil and water. For example, a classic approach in toxicity testing is to quantify juvenile survival or reproduction, since these are often known to be the most sensitive traits with respect to toxicants. However, organisms can tolerate stressors such as toxicants by having plastic life-cycle traits thus enabling them to maintain population growth rate or fitness.

The main objective was to test the classic premise on the most sensitive life-cycle trait by investigating toxicant-induced plasticity in life-cycle traits and fitness alterations.

Using theoretical life-cycle models, Chapter 2 showed that the impact of toxicants on fitness was determined by its relationship with plastic responses in life-cycle traits and the life-history strategy. These results were compared with experimental observations using nematodes as a case study. The nematode Plectus acuminatus was selected for chronic life-cycle investigations in Chapter 3 on the basis of its moderate sensitivity to cadmium and pentachlorophenol. Chapter 4 presented a mathematical life-cycle model for P.acuminatus to estimate fitness reductions from plastic responses in life-cycle traits to cadmium. This model was used in Chapter 5 to evaluate the impact of critical effect concentrations (EC ₂₀ ) for cadmium and pentachlorophenol on fitness. It was demonstrated that fitness in P.acuminatus was not determined by the most sensitive life-cycle trait.

Moreover, less sensitive traits appeared to have a stronger impact on fitness. In addition it was found that P.acuminatus was able to maintain fitness due to plasticity in life-cyle traits when exposed to cadmium. Finally Chapter 6 explored the effect of copper on

fitness in two nematode species with divergent life-history strategies ( P.acuminatus and Heterocephalobus pauciannulatus). From a toxicological point of view the reproductive period in P.acuminatus was 6 times more sensitive to copper than in H.pauciannulatus. However, the relationship between plasticity in the reproductive period and fitness showed that fitness was equally reduced in both species.

The present findings agree with the theoretical results obtained in Chapter 2 and it can be concluded that the impact of toxicants on fitness depends on: 1) the plasticity of lifecycle traits to toxicants, 2) the relationship between the plastic responses in life-cycle traits to fitness and 3) the life-history strategy of the organism. These conclusions do not support the general premise in ecotoxicology that the impact of toxicants on organisms is determined by the most sensitive life-cycle trait. It is therefore advocated that future ecotoxicological research with other organisms should put more emphasis on the ability of species to adapt to toxic stress by having plastic life-cycle traits in order to enhance the predictive value of toxicity tests which provide the basis for ecological risk assessment procedures.