Interaction of Respiration, Ion Regulation, and Acid-Base Balance in the Everyday Life of Aquatic Crustaceans

Abstract

Extracellular and intracellular acid-base balance is necessary for the maintenance of normal metabolic processes. The primary source of acid is metabolically produced CO2, and the CO2/HCO3− system is the most significant buffer. The regulation of acid-base balance is complex, involving the interaction between respiratory gas exchange and ion transport. In aquatic crustaceans respiration is governed by the need to extract oxygen from water, an O2-poor medium; thus, acid-base balance is maintained primarily through ion transport mechanisms. These mechanisms include Na+/H+ and Cl−/HCO3− exchange processes that are sensitive to the extracellular acid-base status of the animal. In marine crabs, ion regulation and acid-base balance are accomplished by the posterior gills, while in freshwater species all gills and the antennal gland perform these functions. Intracellular acid-base balance appears to be maintained primarily by ion transport across the cell membrane. Hemolymph pH varies inversely with acclimation temperature and salinity. In both cases Pco2 remains nearly constant, and the pH change is a result of changes in hemolymph HCO3− concentrations brought about by ion exchange mechanisms. Environmental hypercapnia or hyperoxia induces a repiratory acidosis characterized by increased Pco2, low pH, and elevated HCO3−; this is partially compensated for by ion exchange processes that bring about a further increase in hemolymph HCO3−. Exercise causes a mixed respiratory and metabolic acidosis with compensation via H+ ion excretion and hyperventilation.