To rest in hydration: hatching phenology of resting eggs of Heterocypris incongruens (Crustacea: Ostracoda)

The ability of resting eggs to survive periods of drying and hydroperiod too short to allow reproduction affect the efficiency of a species in colonising and persisting in temporary waters. We previously described the formation of an egg bank of Heterocypris incongruens, a widespread and common freshwater ostracod. The egg bank was made up by genetically homogeneous resting eggs produced by a laboratory clonal lineage we used to colonise a pond with unpredictable hydroperiod. Here we report on laboratory experiments for the study of hatching phenology of resting eggs from the newly established egg bank. We measured the hatching percentage and eggs bank depletion during 4 successive inundations (followed by dry periods) without reproduction. In a first factorial experiment egg hatching was assessed under the following set of conditions: a) egg age (3 or 15 months), b) inundation photoperiod (12:12 L:D or 16:8 L:D), c) water quality (mineral water or adult-conditioned water). In a second experiment egg hatching in four successive inundations was assessed varying first inundation duration (6 or 30 days). Our estimation of resting egg density in the pond was 65x104 eggs m-2. On a total of 7133 hatchings recorded , about 80 % were observed in the first inundation while the rest in three subsequent inundations. Egg age and photoperiod had no effect on hatching percentage while metabolites produced by adults females in conditioned water had a negative effect. Short inundation did not affect hatching percentages, hence resting egg survivorship. Hatching in successive inundations did not depend on genetic differences among eggs and the process may be considered a stochastic switching of a single genotype among different phenotypes. Such a hatching phenology ensures a risk spreading response that is advantageous in highly stochastic environments. It prevents the population extinction in the case of very short hydroperiods and for at least four subsequent generations of failed reproduction