5 - Regulation of Ions, Acid–Base, and Nitrogenous Wastes in Elasmobranchs

Abstract

1. Introduction 2. Ionoregulation 2.1. Overview 2.2. Seawater 2.3. Freshwater and Intermediate Salinities 3. Acid–Base Balance 3.1. Overview 3.2. Seawater 3.3. Freshwater and Intermediate Salinities 4. Nitrogenous Wastes 4.1. Overview 4.2. Seawater 4.3. Freshwater and Intermediate Salinities 5. Concluding Remarks Elasmobranch fishes osmoconform to the marine environment by synthesizing and retaining substantial levels of urea (∼350 mM) and other organic osmolytes. Urea is reabsorbed at the kidney and although renal urea transport proteins have been isolated and characterized there is still a limited understanding of the reabsorptive processes involved. Marine elasmobranches are nitrogen-limited and attention has now turned to ammonia retention mechanisms. Elasmobranch fishes regulate plasma ions below seawater levels but above what is typical of marine teleosts. The gill plays a primary role in ion regulation, with the kidney and rectal gland contributing as well. The gill also has a phenomenal capacity to eliminate acid or base loads. Recent discoveries of solute transporters in the gill, kidney, and gut of elasmobranchs bring an enhanced understanding of ion and acid–base regulation. There have been a series of exciting papers on the elasmobranch gut that demonstrate a dynamic tissue with multiple roles. This chapter discusses recent and earlier research on ion, acid–base, and nitrogen regulation in these magnificent fishes inhabiting marine, fresh water, and intermediate salinity environments. There is tremendous scope for novel discoveries in this captivating group of fishes and we have identified compelling research questions for fish biologists.