Water balance in embryos of Fundulus heteroclitus and F. bermudae in seawater

The purpose of this study is to examine the mechanisms of osmoregulation in embryos of Fundulus heteroclitus and F. bermudae. The turnover rate of tritiated water in intact embryos of F. heteroclitus increases from 0.05 to 0.013 h^-1 at 4 days to 0.12 ± 0.030 h^-1 at 7 days and to 0.18 ± 0.060 h^-1 at 10 days, corresponding to estimated diffusional water permeabilities of 0.4, 1.0, and 1.6 x 10^-6 cm/s^-1, respectively. These values are low when compared to other organisms but not sufficiently low to maintain embryos in water balance. The osmolality of the embryonic extracellular fluids decreases during development from 362 ± 9.2 mosmol/kg^-1 at 4 days to 344 ± 6.9 mosmol/kg^-1 at 10 days. Water loss is balanced by drinking seawater at a rate of 600 pl/mg^-1/h^-1 for 7- to 8-day embryos. The embryo possesses a pair of branchial chambers that communicate anteriorly with the pharynx through embryonic gill slits and posteriorly with the perivitelline space by means of pores. In the absence of a mouth it is probably through these pores that seawater is injested.