Timing matters: Sensitivity of Daphnia magna dormant eggs to fenoxycarb exposure depends on embryonic developmental stage

Although Daphnia magnais a key species in many lentic freshwater ecosystemsand is commonly used as model organismin ecology and ecotoxicology, very little is known about the effects of chemicals on their dormant life stages. Dormant eggs ( ephippia) are produced when environmental conditions deteriorate, and Daphniaswitch from clonal to sexual reproduction. Ephippiaproduced over different growing seasons can accumulate in the sediment of ponds and lakes, where they can be exposed to pesticides and other (anthropogenic) stressors. In the present study, we have investigated the effects of pesticide exposure on dormant eggs at different embryonic developmental stages and evaluated the degree of protection against pollution provided by the ephippial case. We therefore conducted a hatching experiment in which decapsulated and encapsulated dormant eggs were exposed to an insect growth regulator( fenoxycarb) at different stages during their development, both before and after activation of the eggs. In addition, we developed an analytical method to measure fenoxycarbconcentrations in the dormant eggs. Fenoxycarbnegatively affected development and hatching success and changed the timing of hatching in activated and in dormant eggs. Hatching characteristics as well as fenoxycarbconcentrations inside the eggs differed significantly between exposure treatments. Final stages of embryonic development were most sensitive to pesticide exposure and had the highest tissue concentrations of fenoxycarb. Tissue concentrations did not differ significantly between decapsulated and encapsulated eggs, suggesting that the ephippial case offers limited or no direct protection against pesticide exposure. With this study we provide new evidence showing that pesticides can bioconcentratein and affect D. magna dormant eggs. The severity of the effects on developing embryos depends on the timing of pesticide exposure. Our results stress the importance of considering the full life-cycle of model organismsused in ecotoxicologicalstudies, since these are ultimately aimed at assessing risks of chemical exposure on natural aquatic ecosystems.