CALCIUM BALANCE IN EMBRYOS AND LARVAE OF THE FRESH-WATER-ADAPTED TELEOST, OREOCHROMIS-MOSSAMBICUS

Changes in Ca2+ 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 degrees C. Total body Ca2+ content was maintained at a const ant level, about 4-8 nmol per individual, during embryonic development . However, a rapid increase in body Ca2+ level was observed after hatc hing, 12.8 to 575.3 nmol per individual from day 1 to day 10 after hat ching. A significant influx and efflux of Ca2+ occurred during develop ment, with the average influx rate for Ca2+ increasing from 5.9 pmol m g(-1) h(-1) at 48h postfertilization to 47.8 pmol mg(-1) h(-1) at 1 da y posthatching. The skin was proposed as the main site for Ca2+ influx before the development of gills, and the increased Ca2+ influx may be ascribed to gradual differentiation of skin surface and chloride cell s during embryonic development. Ca2+ efflux was 16-56 pmol mg(-1) h(-1 ) in 1-day-old larvae. The resulting net influx of Ca2+, 10-12 pmol mg (-1) h(-1), accounted for the increased Ca2+ content after hatching. W hen comparing the measured and estimated ratios of efflux and influx, active transport was suggested to be involved in the uptake of Ca2+ . Chloride cells, which may be responsible for the active uptake of Ca2, started to differentiate in the skin of embryos 48h after fertilizat ion, and the density of chloride cells increased following the develop ment. A possibility of active transport for Ca2+ in early developmenta l stages of tilapia is suggested.