Osmoregulation in elasmobranch fish — A review

Abstract

In recent years our understanding of the control of ion and urea metabolism in elasmobranch fish has increased with many more species being investigated. This has demonstrated that some species regarded previously as stenohaline marine are, at least partially, euryhaline and may survive in environments less concentrated than full seawater. In elasmobranch fish the success of the osmotic strategy depends on retaining large concentrations of plasma urea in addition to relatively high, in comparison to most other vertebrates, plasma concentrations of sodium and chloride. The strategy therefore depends on the integrated control of hepatic urea production and branchial and renal urea excretion and also on a balance of salt input at the gills and gut with salt output at the gills, kidney and via the unique elasmobranch rectal gland. This integrated response is, at least in part, controlled by a series of osmoregulatory hormones and their possible role in the control of osmoregulatory organs will be discussed. In this presentation recent findings will be reviewed and the osmoregulatory strategies of a partially euryhaline species, Scyliorhinus canicula, with a fully euryhaline migratory species, Carcharhinus leucas, will be compared. This will allow new models for the control of ion and urea metabolism in elasmobranchs to be proposed for future research.