Low chloride and high short-chain carboxylic acid concentrations in extracellular fluid: a common physiological feature of the Hirudinea

Abstract

The ionic blood composition of extracellular fluid was studied in 11 species of Hirudinea from various habitats (marine, limnic, semiterrestrial) and families. The major cation found was sodium. Chloride concentrations, however, were low in relation to cations. Extracellular concentrations of short-chain carboxylic acids (with malate as the most abundant acid) were responsible for a significant proportion of total anionic equivalents. An apparent anion deficit in limnic and semiterrestrial species and a pronounced gap in the osmolytes in a marine piscicolid suggest the presence of unidentified extracellular constituents. Irrespective of their systematic position and habitats, the investigated Hirudinea all share the physiological feature of low extracellular chloride and high short-chain carboxylic acid concentrations, which distinguishes them from the Polychaeta.