Roles of the Rectal Gland and the Kidneys in Salt and Water Excretion in the Spiny Dogfish

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Abstract
Continuing previous studies, comparisons were made between intact dogfish and dogfish with the rectal gland excised under conditions of full sea water, 72-82% sea water, and of salt-loading. Glandless fish maintain a normal plasma composition for 21 days of study. Diluted fish, intact or glandless stabilize their plasma with a lowered osmolarity and chloride, an increased volume, aid an increased excretion of water and salt. With dilution the rectal gland is active; its stimulation clearly seems due to a volume effect. With salt-loading, glandless fish do not excrete salt as effectively as do intact fish. In all situations, the kidneys are unable to substitute for the rectal gland; they can not specifically concentrate chloride above plasma levels. In line with the natural physical gradients, the dogfish seems perfused inwardly by a saline solution. Glandless fish seem able to decrease the influx of salt. Plasma osmolarity is not raised by 500 gr of herring, force-fed and proven digested. Apparently, dietary protein is not convereted massively into urea.

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