Ion and Osmoregulation in Prenatal Elasmobranchs: Evolutionary Implications

Abstract

Synopsis. The elasmobranchs represent a fascinating series of experiments in the evolution of maternal support for developing embryos. In oviparous species, eggs are enclosed in a tough, fibrous capsule. The capsule is very permeable and the embryonic tissues are bathed in a solution ionically similar to sea water within hours of oviposition. In the primitively viviparous species Squalus acanthias, early embryos in egg capsules are retained in utero and are bathed in a solution similar to maternal plasma. Several months into the 22 month gestation period the embryos are capable of independent iono- and osmoregulation in a uterine solution that resembles sea water. Embryos of more advanced viviparous species develop in a solution that is similar to maternal plasma. Iono- and osmoregulation by these embryos would appear to be minimal. It is clear that in the oviparous elasmobranchs, the ability of the egg/embryo to maintain salts and urea at appropriate levels is present at the earliest stage of development. The ability of prenatal elasmobranch embryos to iono- and osmoregulate would allow the evolution of a diverse array of reproductive strategies in the elasmobranchs.