Exoskeleton calcification in Norwegian populations of the crayfish Astacus Astacus (Linnaeus, 1758) (Decapoda: Astacidae) varies with size, gender, and ambient calcium concentration

Declining ambient calcium (Ca²⁺) concentrations in boreal, soft-water lakes of North America and Europe is one of many threats facing their biotic assemblages such as crayfish populations. We examined the specific exoskeleton calcium (Ca) concentration in Astacus astacus (Linnaeus, 1758) populations from a wide range of ambient Ca²⁺ concentrations to determine a possible correlation between the amount of Ca accumulated in their carapaces and the ambient Ca²⁺ concentrations. Exoskeleton Ca was the major constituent of the crayfish A. astacus carapaces in this survey (21.2 to 25.8% Ca of dry weight (DW)), whereas magnesium (Mg) displayed a disproportionately low constituent. The strong correlation between mineral contents of dry weight (DW) and ash weight (AW) (r=0.98) allowed us to refer mineral contents consequently to DW. A linear model using gender, length and ambient Ca²⁺ concentration (log transformed) explained 82% of the variation in carapace Ca content (as % DW). Astacus astacus females were slightly more calcified than males (0.4% of DW, when adjusted for ambient Ca²⁺ and body length). Large-bodied populations were slightly, but significantly more heavily calcified than those with smaller bodies: carapace Ca content increased by 0.2% DW for each cm increase in body length. The strong logarithmic effect of ambient Ca²⁺ implies that carapace Ca content increases by 1.7 × log(2) = 1.2% DW for every doubling of the Ca²⁺ concentration in the water.