The annual killifish Nothobranchius furzeri as a new fish model in long-term ecotoxicological research

Freshwater organisms are increasingly exposed to combinations of different stressors. However, since it is time-consuming and costly, research on the chronic effects of these stressors as well as on potential interactive effects between stressors, such as between pollution and global warming, is scarce, especially on vertebrates. Studies on transgenerational effects of these individual and combined stressors are almost non-existent.

The general aim of this thesis was to test the potential of the turquoise killifish Nothobranchius furzeri as a model in ecotoxicological research. We mainly focused on its use in long-term exposure experiments. Effects of stressors were measured by screening responses on a broad range of traits including survival, development, behaviour, thermal maximum, energy reserves, biomarkers and stress protein induction.

In chapters 1 and 2, we measured the acute sensitivity range of N. furzeri to several compounds. Compared to other commonly used model organisms, N. furzeri appeared to be more sensitive to cadmium whereas its sensitivity to copper, 3,4-DCA and chlorpyrifos was in range with or higher than the sensitivity of other fish models. In chapter 2, we also performed a full life span test on N. furzeri and found evidence that concentrations that did not affect fish in the first two weeks of exposure had adverse effects later in life, advocating for prolonged exposure tests to set safe concentrations of toxicants accurately.

In chapters 3, 4 and 5, we chronically exposed N. furzeri to a combination of pollution and warming. We show that even Nothobranchius killifish, which are exposed to large daily temperature fluctuations in their natural habitat, become habituated to a fixed environmental temperature and that even a modest diversion from this temperature may entail temperature stress that induces fitness related costs. Some pollutants reduced the thermal maximum of the fish, making it more vulnerable to the consequences of climate change. The joint effects of toxicants and an increased temperature on N. furzeri varied with the type of pollutant. Yet, we found indications of synergisms between the combined stressors for all tested pollutants. In general, our results served as a proof of principle for the usefulness of the Turquoise killifish N. furzeri in time-efficient chronic vertebrate exposure tests.

In chapter 6, we performed a multigenerational test lasting for two full life cycles. We found indications that a low concentration of chlorpyrifos, that merely affected fish of the first generations that were exposed during their whole life cycle, had severe effects on the fecundity of the second generation and thus severely impacted the fitness of the offspring. Future risk assessment would thus largely benefit from studies including transgenerational effects to fully account for potential increased sensitivity that only becomes apparent after prolonged periods of exposure.

Overall, our results imply that N. furzeri could be a valuable fish model in ecotoxicology. Advantages compared to other fish-test-organisms include the production of long-lived drought-resistant eggs and their maturation time of 18 days, among the fastest known in vertebrates. Combined, these traits facilitate time-efficient chronic exposure tests, which is essential for toxicants with delayed effects, but also to study long lasting effects within- and among generations.