Osmoregulation

A REVIEW OF OSMOREGULATION IN FRESHWATER AND MARINE ELASMOBRANCHS Neil Hammerschlag PEW Institute for Ocean Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA and ReefQuest Centre for Shark Research, P.O. Box 48561, 595 Burrard Street, Vancouver, BC, V7X 1A3, Canada neil@reefquest.com EXTENDED ABSTRACT ONLY – DO NOT CITE Introduction Studies of osmoregulation in marine and particularly fresh water elasmobranchs have been intermittently reported in the literature over the last 80 years. Although there has been significant research on elasmobranch osmoregulation, no study exists uniting the previous work into a single comprehensive report. This study examines previous research in elasmobranch osmoregulation and presents the results in a single, comprehensive review, covering topics including body fluid (solute and solvent) volume and concentration variations, body fluid synthesis, retention and secretion, in different elasmobranch species, in different habitats, having varying nutritional states, and in different life history stages. In elasmobranchs, blood and other body fluids are separated from the surrounding aqueous environment by permeable surfaces. Osmoregulation depends on the relationship between the solute to solvent concentrations of both the internal body fluids and the outside medium that surrounds the animal (Pang et al, 1977). Unless internal and external fluids have the same solute to solvent concentration, water will enter the body when its fluids contain a higher concentration of solute to solvent than does the water comprising the environment. In contrast, water will leave the body when the surrounding medium contains a higher concentration (Pang et al, 1977). Thus, marine animals face problems of dehydration and the elimination of excess salts while freshwater animals must conserve their salts and eliminate excess water (Pang et al, 1977). Marine elasmobranchs have evolved the technique of reabsorbing and retaining urea...