Introduction

Due to the physicochemical properties of the aquatic medium, the ability to regulate acid–base balance by manipulation of PCO2 is severely limited in fish. Instead, the transfer of acid–base relevant ions between the extracellular fluid and external medium is utilised as the primary process for pH regulation (Heisler, 1993). There are four potential sites for the transfer of acid–base relevant ions to occur in fish: the gills, kidney, skin and gut. The relative and potential roles of each of these are outlined below, with emphasis on the areas least known about, before focusing on the newly discovered role of the gut in piscine acid–base balance.

Sites for the transfer of acid–base relevant ions in fish

The gills

Traditionally, the gills are considered to be the principal site of acid–base regulation. This has been repeatedly demonstrated in both freshwater-adapted and seawater-adapted fishes (Cameron, 1976; Claiborne & Heisler, 1984; Wood, Wheatly & Hobe, 1984; Tang, McDonald & Boutilier, 1989; Goss et al., 1992), and overall the gills are normally considered to contribute more than 90 per cent to the transfer of acid–base relevant ions to and from the external environment (Heisler, 1984, 1993; Wood, 1988). This is thought to be effected through a combination of (i) apical ion exchange mechanisms (Na+/ acidic equivalents and Cl−/basic equivalents), and (ii) diffusion of acid–base relevant and ‘strong’ ions through the paracellular pathway (Wood, 1988; McDonald, Tang & Boutilier, 1989).