Extreme tolerance to environmental stress of sexual and parthenogenetic resting eggs of Eucypris virens (Crustacea, Ostracoda)

1. The freshwater ostracod (Ostracoda), Eucypris virens, is commonly found in European temporary pools, where its long-term persistence completely relies on the build-up of resting egg banks. Extreme tolerance of dormant eggs and seeds is widely assumed, but freshwater ostracod eggs are relatively poorly studied. The study of ostracod resting eggs is of particular relevance as it may yield the key to understanding the distribution of the sexes in many species capable of both sexual and asexual reproduction. 2. We assessed the tolerance of dried resting eggs produced by females originating from three populations with males and three all-female Eucypris virens populations. Hatching time and success was compared between control eggs and eggs exposed to one of seven ecologically relevant stressors: digestive enzymes, high salinity, deep freezing, hydration, UV-B radiation, hypoxia, and insecticide treatment. 3. None of the stressors reduced statistically the viability of either sexual or asexual eggs. When compared with the reproductive mode specific controls, exposure to UVB radiation had a mild impact on the survival of sexual and asexual eggs (-16.8% and -22.4%, respectively), but this was only significant for asexual eggs. These results point to an extreme tolerance of E. virens resting eggs, and have important implications for the ecology and evolution of the species. 4. The timing of hatching was not affected by the stress treatment, except for UV-B radiation. A marginally significant delay in hatching response was observed for UV-B radiated eggs when compared to the overall mean, but this treatment effect was absent when compared with the reproductive mode specific controls. 5. The populations with males produced eggs that hatched on average earlier (-1.5 days at 17˚C) and were more successful (+26%) than asexual eggs. Due to the limited number of populations and the population-specific origin and age of the eggs, the possibility due to differences in age and origin of the resting eggs, or to variations in local conditions, cannot be ruled out.