Sodium balance in eggs and dechorionated embryos of the Atlantic salmon Salmo salar L. exposed to zinc, aluminium and acid waters.

Ionic regulation of eyed eggs of Atlantic salmon was studied using intact eggs and embryos from which the chorion and perivitelline fluid had been removed, a preparation which remained in good condition for at least 2 weeks. Both intact eggs and embryos in freshwater showed similar Na+ influx rates (0.0064 and 0.01 microM g-1 hr-1, respectively) but Na+ efflux rates (0.0024 and 0.0064 microM g-1 hr-1, respectively) were greater in embryos, suggesting that the perivitelline fluid which is known to bind cations has an important function in preventing Na+ loss from the yolk and embryo. Over 90% of the egg Na+ is located in the yolk and embryo but only about 10% is exchangeable while the chorion contains about 8% Na+ which is non-exchangeable. Both eggs and embryos in acid water at pH 4, in Zn2+ 5 mM l-1 and in aluminium 1 mM l-1 showed greatly reduced Na+ uptake but eggs in 10 microM l-1 aluminium or 100 microM l-1 Zn2+ showed normal Na+ balance while embryos were normal in 10 microM l-1 aluminium but showed reduced Na+ uptake in 100 microM l-1 Zn2+. It is concluded that the chorion and perivitelline fluid have a capacity to protect the embryo from metal ions and acid water.