Salt and water balance
Most marine vertbrates maintain lower concentrations of salts and other
chemicals in their blood than are found in seawater, and so face a
continuous problem of water loss to the environment, because of the tendency
of water to move through membranes from regions of low salt concentration to
regions of higher concentration. The marine cartilaginous fishes differ from
almost all of the bony fishes (except the coelacanths and aestivating
lungfishes) in being able to reabsorb in the renal (kidney) tubules most of
their nitrogenous waste products (urea and trimethylamine oxide) and to
accumulate these products in their tissues and blood, an ability termed the
urea retention habitus. The concentration within the body thus exceeds that
of the surrounding seawater, and water moves into the body with no
expenditure of energy. When any of these fishes moves into freshwater, as
many do, the urine flow to the outside increases; hence, the concentration
of urea in the blood decreases. In the sawfish, for example, the increase of
urine output is more than twentyfold; the blood urea concentration decreases
to less than one-third the amount observed in marine forms. Purely
freshwater elasmobranchs, such as the stingrays of the Orinoco and Amazon
drainage systems, seem to lack the urea retention habitus.
-- Jay DeLong Olympia, WA
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