Introduction

Metabolic energy production, indispensable for the maintenance of life, relies essentially on the regulation of pH as a central parameter for the involved processes. Deviation by more than a few tenths of a pH unit may considerably reduce metabolic flux or even completely inhibit further energy production on the basis of pronounced pH activity optima of certain key enzymes (such as phosphofructokinase) (see Heisler, 1990b). Tight regulation of pH is accordingly indispensable for conservation of homeostasis.

In fish, pH regulation is extensively challenged by changes of the environment: utilization of water as the gas exchange medium frequently subjects fish to large and rapid changes in water O2 and CO2 concentrations, in temperature and in environmental electrolytes, and to various other natural or human-induced stress factors that air breathers never encounter to any comparable extent. This chapter focuses on the description of characteristics, sites and mechanisms of fish integumentary acid–base regulation, with particular emphasis on the limitations of active and passive components contributing to the overall regulatory pattern. Due to limitations of space, the reader is referred to review articles wherever appropriate.

Regulatory mechanisms

Mechanisms for acid–base regulation are common to all living organisms and are essentially the same for fish as for terrestrial animals. The extent to which individual mechanisms are utilized, however, is quite different. The situation for fish is characterized by the intimate contact with an aqueous environment, generally including continuous immersion and utilization of water as the gas exchange medium.