Elasmobranchs are predominantly marine, although some 10% are estuarine, 2% are euryhaline and 1% are obligate in fresh water (Martin 2005). Studies of osmoregulation in elasmobranchs have been reported in the literature over the last seventy-five years; however, there have been significant advances in our understanding of the mechanisms underlying elasmobranch osmoregulation in the past decade. Although there have been several recent reviews of elasmobranch osmoregulation (Hazon et al. 2003; Evans et al. 2004, 2005), these have restricted focus to selected aspects of osmoregulation and tended to be highly technical. This article provides a broad review of osmoregulation in elasmobranchs for non-specialists, particularly fish biologists, behaviourists and ecologists with limited training in the biochemistry and physiology of osmoregulation.

Osmoregulation depends on the relationship between the solute-to-solvent concentrations of the internal body fluids (extracellular and intracellular) and the outside medium that surrounds the animal. Unless the internal body fluids and the outside medium have the same solute concentration, water will enter the body when its fluids contain a higher concentration of solute and leave the body when the surrounding medium contains a higher concentration. Electrolytes will similarly diffuse through the body down concentration gradients. These considerations hold true at both the extracellular and intracellular level. Thus, marine animals face problems of dehydration and the elimination of excess salts, while freshwater animals must conserve their salts and eliminate excess water.