Living outside the usual salinity of the open ocean poses considerable
ion regulatory problems for elasmobranchs. Typically, the fluids and
tissues of sharks and rays living in SWare hypertonic with respect to
the surrounding seawater which facilitates a slight osmotic influx of
water and the net diffusional gain of Na+ and Cl− ions (Smith, 1931;
Piermarini and Evans, 1998; Wood et al., 2002; Pillans et al., 2008).
This gradient means that sharks in SW generally do not need to
drink, and any net gain of ions is handled largely by a salt-secreting
rectal gland. The accumulation of high concentrations of urea in
cells and extracellular fluids accounts for a large proportion of the
high internal osmotic pressure of elasmobranchs and, while high
levels of urea can affect protein function in other organisms, some
unique elasmobranch proteins can only function in the presence of