With their gills exposed to sea water, all marine fishes are faced with the problem of conserving body water in a strongly hypertonic environment. Sea water is about 3.5% salt, over 3 times that of vertebrate blood. The cartilaginous fishes solve the problem by maintaining such a high concentration of urea in their blood (2.5% — far higher than the ~0.02% of other vertebrates) that it is in osmotic balance with — that is, is isotonic to — sea water.
This ability develops late in embryology, so the eggs of these species cannot simply be released in the sea. Two solutions are used:
Enclose the egg in an impervious case filled with isotonic fluid before depositing it in the sea.
Retain the eggs and embryos within the mother's body until they are capable of coping with the marine environment.