Ebook Osmoregulation in elasmobranchs: a review for fish biologists, behaviourists and ecologists
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.
Marine and euryhaline elasmobranchs in salt water reabsorb and retain urea and other body fluid solutes (Table I) such that osmolarity remains hyper-osmotic to their surrounding seawater; consequently they experience little or no osmotic loss of water (Smith 1931; Thorson 1962). Any water that is gained by osmosis across the gills is quickly balanced by renal excretion. The continuous inward diffusion of Na þ and Cl À (salt) from the environment is compensated for by salt excretory mechanisms in the rectal gland and kidney (Burger and Hess 1960; Burger 1965; Haywood 1973; Piermarini and Evans 2000). In contrast, marine teleosts remain slightly hypo-osmotic to the surrounding sea water, experiencing some water loss, and maintain osmotic consistency by actively drinking seawater and secreting excess salts via the gills and kidney (see reviews by Evans 1993 and Evans et al. 2005).
Freshwater and euryhaline elasmobranchs in fresh water, balance osmotic water gain by increased urinary excretion (Thorson et al. 1967; Goldstein and Forster 1971a, reviewed by Evans et al. 2004). They also synthesise less urea as well as retain less urea, Na þ and Cl À than marine individuals such that osmolarity remains relatively low (Table I), but still greater than the surrounding fresh water (Thorson et al. 1967; Thorson 1970; Goldstein and Forster 1971a, 1971b; Poulsen 1981; Wood et al. 2002; Tam et al. 2003; Anderson et al. 2005). Diffusional losses of Na þ and Cl À are balanced by electrolyte uptake at the gills, and salt reabsorption by kidney tubules (Goldstein and Forster 1971a; Gerst and Thorson 1977; Piermarini and Evans 2000; Piermarini et al. 2002; Wood et al. 2002; Tresguerrers et al. 2005). In contrast, freshwater teleosts remain slightly hyper-osmotic to the surrounding fresh water and osmoregulate by drinking relatively little water, excreting large amounts of dilute urine, obtaining water and salts via the gills and also deriving some salts from their diet (see reviews by Evans 1993 and Evans et al. 2005).
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