Tolerance to low ambient calcium shows inter-population differences in Daphnia galeata

The tolerance to low ambient calcium (Ca) was assessed in two populations of Daphnia galeata. There was a threshold for survival between 0 and 2 mg Ca l(-1). However the ability to cope with low Ca concentrations clearly differed between the two populations, as the population from the low-Ca locality was less tolerant to low Ca. Additionally, neonate individuals had poorer survival than adults when reared at similar to0 mg Ca l(-1), supporting a suggested juvenile bottleneck regarding the tolerance to low Ca concentrations. The mean specific Ca content in adult D. galeata was not different between the two populations, but individuals reared in medium with 1 mg Ca l(-1) only had two-thirds of the Ca content of those reared in medium with 10 mg Ca l(-1). The significant differences between the two populations investigated suggest that inter-population variation in tolerance to low ambient Ca concentrations might be important to explain the success of Ca-demanding crustaceans in soft-water lakes.