Comparisons of Calcium Regulation in Fish Larvae

The purpose of the present study was to compare the ability of larvae of different species, goldfish (Carassius auratus), zebrafish (Danio rerio), and ayu (Plecoglossus altivelis), to regulate their calcium balance. Whole-body Ca²⁺ content and Ca²⁺ influx in the larvae of the three species, which were incubated in low- (0.02 mM), mid- (0.2 mM), and high- (2.0 mM) Ca²⁺ artificial fresh water from embryonic stages, were compared. The Ca²⁺ uptake kinetics were determined in zebrafish and goldfish incubated in high- or low-Ca²⁺ artificial fresh water. Ca ²⁺ content of both zebrafish and ayu acclimated to low-Ca ²⁺ media were significantly lower than those acclimated to mid- or high-Ca²⁺ media. However, Ca²⁺ contents of goldfish in low-, mid-, and high-Ca²⁺ groups showed no significant differences. In goldfish, Ca²⁺ influx in the low-Ca²⁺ group was significantly higher than those of the mid- and high-Ca²⁺ groups. In contrast, the Ca²⁺ influx rate in the low-Ca²⁺ group was significantly lower than those in the mid- and high-Ca²⁺ groups in zebrafish and ayu. Compared to the high-Ca²⁺ group, the low-Ca ²⁺ group of goldfish showed a 13% increase in the maximal velocity (J_max) and an 84% decrease in the Michaelis constant (K_m) for Ca²⁺ influx. Smaller changes, i.e., an 8% increase in J _max and a 67% decrease in K_m, were found in zebrafish larvae. Goldfish possess a more effective Ca²⁺ regulatory capacity than do zebrafish and ayu. Differences in the strategies for Ca²⁺ balance may be associated with different development patterns and environments in which these fish naturally occur.