THE ABSORPTION AND EXCRETION OF WATER AND SALTS BY THE ELASMOBRANCH FISHES

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Abstract
I. Analyses are reported on the composition of the body-fluids of the fresh-water Pristis microdon Latham, Dasyatis uarnak (Forskal), Carcharhinus melanopterus (Quoy and Gai-mard) and Dasyatis sephen (Forskal). The blood has typically a freezing point of[long dash]1.0[degree] C, a urea-N content of 200-300 mgm. % and a Cl content of 170 mM. per liter. The urine of Pristis is very dilute and resembles the urine of fresh-water teleosts. The urea content of the urine is invariably less than that of the blood and reasons are given for believing that the kidneys of the elasmobranch fishes actively conserve urea by reabsorbing it from the glomerular filtrate. The urine flow averages 250 cc. per kgm. per day. Considerably more than half the urea, ammonia and Cl excreted by Pristis escapes from the body by an extrarenal route. The evidence indicates that the extrarenal excretion of urea is a passive diffusion across the gills, while the extrarenal excretion of ammonia and Cl is a physiologically controlled process (probably also effected by the gills). Urea is apparently a normal end-product of the combustion of protein N in the elasmobranchs as in the lungfish, teleosts, amphibia, and mammals.[long dash]II. An examination of fresh water and marine elasmobranchs [including Squalus acanthias, Pristis microdon, Dasyatis uarnak, D. centrum, Carcharhinus melanopterus, Hypolophus sephen, Raja stabulijoris, R. diaphanes, and R. erinacea] indicates that the uremia which characterizes this sub-order results from the renal conservation of urea on the one hand and the relatively low permeability of the branchial and oral membranes to this substance on the other. Apparently this uremia, by raising the osmotic pressure of the blood, enables the marine elasmobranch to abstract water from its environment in the same way as does a fish in fresh-water, and to excrete a urine isotonic or hypotonic to the blood in compliance with the osmotic limitations of the fish kidney. This special function of urea in these fishes appears to be a feature which is added to the primitive method, shared by them and the teleosts, of absorbing sea-water from the gastro-intestinal tract and excreting the salt by an extrarenal route. The urea mechanism of the elasmobranchs frees them from the continuous drinking of sea water. In both groups it appears that the fundamental features which are physiologically regulated to a "steady state" are: (1) water content of the body relative to some as yet obscure reference[long dash]possibly the body proteins; (2) specific salt content of the organism. The total osmotic pressure and the urea content of the body fluids are merely incidental to these fundamental features and have no homoiostatic significance per se.

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