Osmotic and ionic regulation in different populations of the new zealand freshwater crayfish Paranephrops zealandicus

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Abstract
Some features of the osmoregulatory mechanism are compared in four populations of Paranephrops zealandicus White collected from freshwaters of different ionic concentrations. Crayfish from freshwaters of ca. 2·0 mm-NaCl concentration show a sustained decrease in blood concentration of ca. 8% when placed in 0·2 mm-NaCl. Populations from freshwaters of ca. o·2−0·4 mm-NaCl show lower rates of net salt loss in distilled water and higher rates of net salt uptake from dilute NaCl solutions than do populations from freshwaters of ca. o·8−2·0 mm-NaCl. Renal salt losses over the first 24 h in distilled water account for ca. 18% of the total salt loss. It is suggested that P. zealandicus from environments of lowest concentration shows a similar degree of adaptation to freshwater as do crayfish of the northern hemisphere. It differs in possessing a substantially higher blood concentration.