The regulation of calcium in Daphnia magna reared in different calcium environments

The specific content, dissolved uptake rate, dietary assimilation efficiency (AE), and efflux rate constant (k(e)) of calcium (Ca) were quantified in juvenile (4-d) and adult (10-d) Daphnia magna cultured in low (0.5 mg L-1)- and high (50 mg L-1)-Ca environments using a radiotracer technique. Daphnids raised in the high-Ca environment had higher Ca contents than did those raised in the low-Ca environment, and juvenile daphnids had higher Ca contents than adults. Uptake from solution was the dominant source (97-100\%) of Ca for daphnids. The maximum influx rate (J(max)) was higher in juvenile daphnids (3.24-4.10 mg g dry weight [wt](-1) h(-1)) than in adults (1.51-1.62 mg g dry wt(-1) h(-1)), while the influx rates were comparable in different Ca environments. The half-saturation concentration (K-m) was 2.51-5.58 mg L-1. The AEs of Ca declined exponentially with increases in food concentrations, and lower AE was observed in the higher Ca environment. The k(e) of Ca (0.83-1.98 d(-1)) was the highest among the elements whose k(e) had been quantified in D. magna, and it was 1.8-2.4 times higher in the high-Ca environment. Excretion into water was the dominant route (60-85\%) of Ca release from daphnids; another 15-40\% of Ca was lost as molts. The regulation of Ca in daphnids is mainly accomplished through adjusting their efflux but not their influx of Ca; their regulation ability is very limited, which may lead to a poor performance in daphnids in Ca-deficient water