1. Field studies indicate that low Ca concentrations limit the distribution and success of Ca-demanding freshwater crustaceans in soft-water localities. A long-lasting depletion of Ca caused by reversed acidification could thus be detrimental to such species.
2. Experimental studies of Gammarus lacustris and Astacus astacus revealed decreased survival and retarded growth at ambient Ca concentrations <5 mg l−1, compared with animals reared at 10 mg l−1.
3. A significant proportion of the total body Ca content was lost during the moult, but moulting frequencies were not affected by ambient Ca concentration. Complete postmoult calcification of the carapace could not be achieved at ambient Ca concentrations <5 mg l−1, and individuals reared at such conditions had a lower specific Ca content than individuals reared at sufficient Ca levels.
4. A juvenile bottleneck regarding the susceptibility to Ca deficiency was shown in G. lacustris, as neonates had a relatively higher mass-specific Ca content and a higher Ca threshold for survival than adults.
5. Because of a short postmoult calcification period of less than 2 days, G. lacustris had limited ability to compensate for low water Ca levels by obtaining Ca from food. However, food might be an important source of Ca for A. astacus living at low ambient Ca, because eating could be resumed during the relatively long calcification period (>15 days).