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Journal of Experimental Biology 97,289-299 (1982)
Published by Company of Biologists 1982

Mechanisms of Acid Extrusion by Two Marine Fishes: The Teleost, Opsanus Beta, and the Elasmobranch, Squalus Acanthias

DAVID H. EVANS ¹

¹ Department of Biology, University of Miami, Coral Gables, FL 33124, U.S.A. Mt. Desert Island Biological Laboratory, Salsbury Cove, ME 04672, U.S.A.; Department of Zoology, University of Florida, Gainesville, FL 32611, U.S.A.

1. Rates of efflux of H⁺ and ammonia from a marine teleost and an elasmobranch were measured.

2. Hypercapnia stimulated H⁺ efflux from both species, stimulated ammonia efflux from the elasmobranch, and inhibited ammonia efflux from the teleost.

3. In both species the H⁺ and ammonia efflux were predominantly across the branchial epithelium. In Na⁺-free sea water, the H⁺ efflux from both species was completely abolished and the ammonia efflux was inhibited by approximately 50%.

4. Injection of an acid load stimulated H⁺ efflux, which continued for 2–5 h until more than the injected acid load was excreted. It therefore appears that injection of an acid load also produces a metabolic acid load which must be excreted.

5. The H⁺ efflux from mineral/metabolic acidotic fish is entirely branchial and dependent upon external Na^-.

6. The data support the conclusion that marine teleosts and elasmobranchs possess branchial Na⁺/NH₄⁺ and Na⁺/H⁺ ionic exchange mechanisms and that Na⁺/H⁺ exchange plays a major role in the response to acidosis in both groups.

7. The possible evolution of these ionic exchange systems is discussed.

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