Some like it cold: the effects of diapause conditions on reactivation of ephippial eggs of Daphnia magna

Abstract Survival of aquatic invertebrates relies on the resistance of dormant stages to environmental stress and the synchronization of dormancy with seasonal changes. We investigated hatching patterns in Daphnia magna from four Mediterranean populations that experience seasonal drying. Ephippial eggs were either transferred directly to hatching-stimulation conditions or first exposed to one of the following storage treatments: wet, dark storage at 20 °C or 1 °C; or dry, dark storage at 20, 40, or 60 °C. Surprisingly, the most successful and synchronous hatching occurred after 2-month dark, cold (1 °C) storage at the temperature that rarely, if ever, occurs in their natural habitats. Dry storage led to more gradual hatching. Early diapause termination was observed in two of the studied populations, with part of the eggs hatching prior to storage and within a few days of ephippium production. Across all populations, ephippial eggs demonstrated high heat resistance and dry storage temperature had no significant effect on hatching rates or egg mortality. Despite similar natural environmental pressures, populations differed significantly in their hatching timing, suggesting fine-scale local or temporal adaptation in dormancy regulation.