Calcium balance in embryos and larvae of the freshwater-adapted teleost, Oreochromis mossambicus

Changes in Ca(2+) content and flux, and the development of skin chloride cells in embryos and larvae of tilapia, Oreochromis mossambicus, were studied. Tilapia embryos hatched within 96h at an ambient temperature of 26-28°C. Total body Ca(2+) content was maintained at a constant level, about 4-8 nmol per individual, during embryonic development. However, a rapid increase in body Ca(2+) level was observed after hatching, 12.8 to 575.3 nmol per individual from day 1 to day 10 after hatching. A significant influx and efflux of Ca(2+) occurred during development, with the average influx rate for Ca(2+) increasing from 5.9 pmol mg(-1) h(-1) at 48h postfertilization to 47.8 pmol mg(-1) h(-1) at 1 day posthatching. The skin was proposed as the main site for Ca(2+) influx before the development of gills, and the increased Ca(2+) influx may be ascribed to gradual differentiation of skin surface and chloride cells during embryonic development. Ca(2+) efflux was 16-56 pmol mg(-1) h(-1) in 1-day-old larvae. The resulting net influx of Ca(2+), 10-12 pmol mg(-1) h(-1), accounted for the increased Ca(2+) content after hatching. When comparing the measured and estimated ratios of efflux and influx, active transport was suggested to be involved in the uptake of Ca(2+). Chloride cells, which may be responsible for the active uptake of Ca(2+), started to differentiate in the skin of embryos 48h after fertilization, and the density of chloride cells increased following the development. A possibility of active transport for Ca(2+) in early developmental stages of tilapia is suggested.