The effects of temperature and photoperiod on life history parameters of Daphnia magna (Crustacea: Cladocera)

SUMMARY. 1. The life history responses of two common and three rare Daphnia magna clones to constant conditions of temperature and photoperiod were investigated, and intraspecific variation in the genes controlling growth, survivorship and reproduction was observed.
2. Rearing temperature had a significant effect on growth, survivorship and on all reproductive parameters measured (generation time, intrinsic rate of increase and male production), while photoperiod influenced significantly only survivorship and male production.
3. The maximum proportion of male offspring was produced at a low temperature (14°C) and at a mid-range photoperiod (L:D16:8).
4. The likely mechanism whereby clones with different life histories are maintained in the population is discussed.