Water balance in developing eggs of the codGadus morhua L

Measurements of yolk osmolality from the embryo of codGadus morhua L. revealed significant variations in the hyposmolality during the embryonic development. The embryo proved to have an extremely low water permeability, protecting it from dehydration in the hyperosmotic seawater. The effect of temperature on the water permeability is high, expressed by an exceptionally high activation energy for water transfer. The agreement between embryonic volume decrease and diffusion permeability during the first 8-10 days after fertilization indicates that no water uptake mechanism is present at this time, thus leaving the embryo fully dependent on internal water stores. The cod egg is buoyant in seawater throughout development. The mechanism for providing hydrostatic lift is the large volume of diluted tissue water located in the yolk and subdermal spaces.