Resurrection of Daphnia clones and culture conditions
Daphnia magna clones were hatched from dormant eggs of three sediment layers of a shallow lake in Belgium (50°50’ N, 4°39’ E, Oud-Heverlee, Belgium). The three sediment layers match specific periods differing in fish predation pressure (Cousyn et al. 2001): the pre-fish period (1970 - 1972) with no fish present, the high-fish period (1976 - 1979) with high fish predation pressure because of intensive fish stocking (> 300 kg/ha), and the reduced-fish period (1988 - 1990) with relaxed fish predation pressure because of reduced fish stocking. An analysis of microsatellite variation indicated that the Daphnia from different periods can be considered as belonging to different subpopulations of one continuous population (Cousynet al. 2001). For current study, six clones per period were used. To minimize interference from maternal effects, all clones were cultured under standard conditions for three generations prior to the experiment: up to 18 adults in 500 mL glass vials, no fish kairomones, 20 °C, photoperiod 14:10 light:dark, daily fed S. obliquus (1.5 × 105 cells mL-1, ~1.25 mg C L-1) and with renewal of the culture medium every other day.