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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.

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I contacted Richard Pillans, the primary author of the article linked to in the question. He told me that he did not know whether the rectal gland was reduced. Researchers are hesitant to capture and dissect the freshwater sharks because of their endangered status.  

He did tell me that they have gathered much more data on the movement of the sharks since the 2009 paper was published.  Apparently, river shark is a bit misleading (my words, not Dr. Pillans). Glyphis does occur in freshwater (no salt) but tends to be found in the middle range of brackish water. Brackish water is considered to be anything that is not purely freshwater nor purely salt water, with typical values between 0.5 and 30 parts per thousand (ppt). Freshwater is 0 ppt and open ocean salt water is 35 ppt, the equivalent of 35 grams of salt in one liter of water. 

If Glyphis is found primarily in the mid-range of brackish water, then I would expect the rectal gland not show signs of reduction. Pillans and Franklin (2004) showed that rectal gland mass of brackish water bullsharks (Carcharhinus leucas) in Australia did not differ significantly from either freshwater or saltwater individuals.  So, pending further evidence, I would conclude that Glyphis, which in the same family as bull sharks, probably does not show rectal gland reduction.

Pillans, R.D. and C.E. Franklin, 2004.  Plasma osmolyte concentrations and rectal gland mass of bull sharks Carcharhinus leucas, captured along a salinity gradient.  Comparative Biochemistry and Physiology, Part A 138: 363-371.

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  • $\begingroup$ Your answers are always impressive and supported by several references! Even better when you make both the question and the answer! It is good to have you on this website $\ddot \smile$. $\endgroup$ – Remi.b Sep 13 '14 at 21:20
  • $\begingroup$ @Remi.b Thanks, I am happy to be here. I keep trying to get to some of your questions but I keep getting diverted with the other questions that to me have an interesting bit of biological trivia (like the one nostril thing). Besides, your questions take more thought and research! :) $\endgroup$ – Michael S Taylor Sep 13 '14 at 21:23

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