Osmoregulation in chondrichthyan fishes

Effective regulation of water and solute concentration is a life-sustaining physiological process in all organisms. Aquatic organisms use very different physiological strategies to deal with the solute-poor environments of freshwater versus the solute-rich environments of seawater. Chondrichthyan or cartilaginous fishes (elasmobranchs and holocephalans) have evolved a ureosmotic strategy where high concentrations of urea are retained such that the fish osmoconforms to the marine environment but notably, ions are regulated. Here we discuss the functional role of the major organs involved in this process, how their actions may be integrated, and how the physiology differs in a freshwater environment.