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

Abstract

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°C. Total body Ca²⁺ content was maintained at a constant level, about 4–8 nmol per individual, during embryonic development. However, a rapid increase in body Ca²⁺ 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 Ca²⁺ occurred during development, with the average influx rate for Ca²⁺ 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 Ca²⁺ influx before the development of gills, and the increased Ca²⁺ influx may be ascribed to gradual differentiation of skin surface and chloride cells 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. When 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 fertilization, and the density of chloride cells increased following the development. A possibility of active transport for Ca²⁺ in early developmental stages of tilapia is suggested.