What does egg size tell us about hatching phenology in Heterocypris incongruens?

The timing of many phenological events (e.g. egg hatching, development and reproduction) is the result of the interaction of genotype, environmental factors and maternal effects. In many taxa it is known that egg size and development time are linked and that, especially in temporary habitats, mothers, anticipating the environment that their offspring will face, can determine egg size (and its variability) to spread the risk connected with environmental unpredictability. We investigated this phenomenon in a widespread microcrustacean: Heterocypris incongruens. We measured the size of eggs produced under laboratory-controlled conditions (24 °C and two photoperiods) by genetically identical females of an ecotype typical of Northern Italian vernal pools. We used 12:12 L:D as a proxy of incoming favourable but unpredictable winter-spring conditions in the Po river plain and 16:8 L:D as a proxy of incoming dry, predictable but unfavourable conditions (that we expected to induce resting egg production). The lack of genetic variation makes it easy to investigate the genetic basis of the effect of the photoperiod experienced by the mother on egg size. Mean diameter was larger in eggs produced at 12:12 L:D (N = 151, 133.16 µm) than at 16:8 L:D (N = 106 129.93 µm). Eggs that did not hatch within 100 days from deposition were classified as resting eggs: their percentage increased from 48% at 12:12 L:D to 73% at 16:8 L:D. Egg diameter at deposition was not significantly larger in non-resting (N = 42, 133.45 µm) than in resting eggs (N = 68, 130.63 µm). Development time of non-resting eggs was highly variable (from 2 to 100 days), it was not related to egg size at deposition and did not change with photoperiod. Any possible effect of egg size at deposition on development time was probably masked by the high variability of the latter variable. During our observation time (every 24 hours for at least 100 days), egg diameter of both resting and non-resting eggs (for a total of 1841 measurements) increased following an asymptotic model. Size increment was slower and eventually larger in eggs laid at 16:8 L:D than at 12:12 L:D and was probably linked to embryo growth in both resting and non-resting eggs. Final diameter of both resting and non-resting eggs did not show a threshold size and its variability might be linked to the hatching asynchrony observed in resting and non-resting eggs. Our work contributes to the budding field of egg focused mother controlled phenology.