Salt balance in embryos of Fundulus heteroclitus and F. bermudae adapted to seawater

²²Na⁺ and ³⁶Cl^- tracer flux experiments of embryos of Fundulus heteroclitus show that a significant influx and efflux of Na⁺ and Cl^- occur during development. The average influx for Na⁺ and Cl^-, respectively, are 5.0 ± 0.76 and 1.1 ± 0.1 nmol/mg^-1/h^-1. The average effluxes are 4.9 ± 0.95 and 3.0 ± 0.01 nmol/mg^-1/h^-1 for Na⁺ and Cl^-, respectively. The electrical potential across the embryonic cavity epithelium of F. heteroclitus and F. bermudae is 50 ± 1.7 mV and 40 ± 1.3 mV, respectively (inside positive). Diluting the artificial seawater bathing medium with distilled water decreases the potential across the embryonic cavity of F. bermudae to -11 ± 5.0 mV at 5% artificial seawater. Because in seawater the potential across the embryonic cavity epithelium is inside positive, the efflux of Cl^- is against its electrical potential gradient indicating an active extrusion mechanism for Cl^-. The site of the active extrusion mechanism for chloride is located in the yolk sac where chloride cells associated with blood vessels are common.