Calcium regulation in wild populations of a freshwater cartilaginous fish, the lake sturgeon Acipenser fulvescens

Lake sturgeon, Acipenser fulvescens, are one of a few species of cartilaginous fishes that complete their life cycle entirely in freshwater. Sturgeons maintain very low concentrations of circulating calcium (Ca²⁺) compared with other vertebrates, and therefore, face unique challenges in regard to Ca²⁺ regulation, which are likely to be magnified during vitellogenic stages of the reproductive cycle. In the present study, Ca²⁺ concentrations and associated hormones of female and male lake sturgeon were examined in two wild populations, and were related to reproductive stage. In both populations, free, bound and total Ca²⁺ were low, peaking in mid-late vitellogenic females. Internal Ca²⁺ and phosphate (PO₄³⁻) concentrations were inversely related to environmental concentrations, suggesting that these ions are preferentially retained and that mechanisms for mobilization are up-regulated under diminished environmental concentrations. Plasma 17β-estradiol, 11-ketotestosterone and testosterone, peaked in mid-late vitellogenic females, while the androgens peaked in spawning males. Urine Ca²⁺ was more tightly regulated than other divalent ions and decreased in spawning fish. Therefore, the increases in free plasma Ca²⁺, the very low circulating concentrations of free and total Ca²⁺, and the increase in PO₄³⁻ and bound Ca²⁺ in low Ca²⁺ environments indicate unique adaptations to Ca²⁺ regulation in the lake sturgeon.