Effects of Low Ambient Calcium Levels on Whole-Body Ca2+ Flux Rates and Internal Calcium Pools in the Freshwater Cichlid Teleost,Oreochromis Mossambicus

Abstract

Calcium fluxes and internal calcium pools were measured in fed, rapidly growing, male tilapia, Oreochromis mossambicus, acclimated to 0.8 mmol l⁻¹ (FW) and 0.2mmoll⁻¹ (LFW) Ca²⁺. Plasma calcium levels were slightly and significantly higher in the LFW tilapia, but muscle calcium concentrations were independent of ambient Ca²⁺. At the time of the experiments, the LFW fish were growing and accumulated calcium, although the calcium content of their hard tissues was reduced. The LFW fish had higher Ca²⁺ influx and efflux rates than the FW fish. The increase in the influx of Ca²⁺ in LFW fish was, however, substantially greater than the increase in the efflux of Ca²⁺, giving these fish a more than four-fold increase in net Ca²⁺ influx from the water: for a 20-g tilapia net uptake rates of Ca²⁺ from the water were 390 and 1620 nmol Ca²⁺ h⁻¹ for FW- and LFW-adapted fish, respectively. These values were calculated to represent at least 69% of the total calcium accumulated by these growing fish. This indicates that even in low-calcium water, tilapia absorb a significant amount of their calcium requirement directly from the water. The pool of readily exchangeable calcium in the bone of FW fish was estimated to be about 7 % of the total hard tissue calcium. In the fish acclimated to LFW, this percentage increased to about 15 % as total hard tissue mineralization decreased. This may indicate that tilapia can increase the mobility of their hard tissue calcium during periods of low calcium stress.