SALT-GLAND FUNCTION IN THE COMMON EIDER DUCK (SOMATERIA-MOLLISSIMA)

The function of the supra-orbital salt gland was studied in the common elder duck (Somateria mollissima). The maximum salt-secreting capacity was determined in (1) wild ducks which had been living in a marine environment, (2) ducks reared in captivity on fresh water, and (3) ducks from group 2 adapted to salt water. The maximum secreting capacity was found by infusing a solution of NaCl (1000 mosmol . kg(-1)) at increasing rates, from 0.691 to 1.67l mosmol . min(-1). Freshwater-adapted ducks secreted at a maximum rate of 0.785 mosmol . min(-1) (1500 mosmol . kg(-1)). Adapted to salt water they increased their capacity, and the best duck secreted at a rate of 1215 mosmol . min(-1) (1600 mosmol . kg(-1)). The best wild duck secreted at a rate of 1516 mosmol . min(-1). Ducks in group 3 were used to examine the response to a hyperosmotic or an isoosmotic infusion. The amount of salt (NaCl) given per unit time was the same. Given a hyperosmotic solution their salt glands secreted at a high rate: 30 min after the infusion had stopped the ducks had excreted 94% of the sodium infused, 92.9% via the salt gland. Given an isoosmotic solution they secreted at a rate about half the infusion rate: 30 min after cessation of infusion they had excreted 73% of the sodium, 42.9% via the salt gland and the rest by the kidneys.