Do river sharks of the genus Glyphis have reduced or absent rectal glands for salt removal?
The rectal gland of marine sharks removes excess salts to maintain osmotic balance (Oguri 1964). In freshwater stingrays (Potamotrygon spp.), the rectal glands are greatly reduced in size (Thorson et al. 1978) but are apparently functionless (Goldstein and Forster 1971). This is not surprising because freshwater rays do not experience the regular influx of salt water.
I have been unable to determine whether such degeneracy or loss has been observed for the river sharks of Australia (genus Glyphis). Pillans et al. (2009) suggest that osmoregulation is probably similar to bull shark, which would not be surprising because they are in the same family. However, bull sharks that live in salt water but regularly enter fresh water have rectal glands that are normal size (Oguri 1964). River sharks are obligate freshwater species and thus have no need to maintain a large rectal gland for the same reason as the freshwater rays. From an evolutionary perspective, I would predict reduction or loss of the rectal gland.