Hatching of cladoceran resting eggs: Temperature and photoperiod

1. We identified temperature and photoperiod conditions under which the hatching of 45 cladoceran species could be elicited. Identification of appropriate hatching cues is of primary importance for the exploration of the ties between active and diapausing stages. 2. Incubation temperature affected the hatching success of resting eggs isolated from Danish, Belgian/Dutch and Spanish sediments. In general, most hatchlings and species were retrieved at 15̈C. Danish and Belgian/Dutch resting eggs hatched more successfully under a long day photoperiod than in continuous illumination. 3. Most species could be retrieved after incubation of resting eggs isolated from a limited amount of sediment (0.4 kg) under a single, well chosen combination of temperature and photoperiod. Processing additional sediment samples under seven more incubation regimes only allowed detection of 21% (Spain) to 34% (Denmark) more species. 4. The incubation period for resting eggs to hatch was strongly influenced by incubation temperature. Our results show that hatching experiments aimed at assessing cladoceran species richness and conducted at 15°C should be continued for a period of at least 2 weeks, after which a random subset of hatchlings (e.g. n = 100) can be selected from the total hatchling assemblage.