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I have heard that sharks excrete $\ce{Na+}$ and $\ce{Cl-}$ by their gill surfaces but divalent ions like magnesium are excreted through feces. What could be the reason behind this?

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You want the reason as in why such a behaviour would have been evolutionary beneficial or adaptively useful or as to what anatomical mechanisms make divalent ion removal through gills (or vice versa) not possible? –  Satwik Pasani Sep 25 '13 at 4:58
    
@SatwikPasani I actually want the physiological mechanism but I'll love to learn any evolutionary advantage. –  biogirl Sep 27 '13 at 13:08
    
divalent cations tend to form less soluble salts than monovalents, such as calcium phosphate. If you tried to excrete these through the gills it would probably lead to the build up of calcified deposits on the gills and damage the tissue and decrease gas transfer surface area. This isn't a problem if you dump the cations into solid waste. –  user137 Jul 27 at 7:05
    
@user137 Thanks. Why dont you convert this into an "answer" and add some references ? –  biogirl Jul 27 at 7:54

1 Answer 1

up vote 4 down vote accepted

The main reason seems to be a mechanism to save water and allow the excretion of higher concentrations of the ions. See the image (from here):

enter image description here

The urine enters the glomeruli at a relatively low rate and with a low magnesium concentration. The paper cited below mentions a concentration of 1.5mM. First organic compounds as glucose are recovered actively, water follows the molecules passively. Then more ions as Ca$^{2+}$ enter in the proximal segment II. In the distal segment finally sodium and chloride are actively transported out while water passively leaves the segment. The divalent ions (magnesium, calcium) are left in high concentrations (the paper below says that the Mg concentration is now around 130mM). This helps concentrating these ions for excretion while loosing as little water as possible at the same time. Sodium and chloride are actively regulated in the gills.

For a short overview, see the link at the image above, for a detailed view, have a look at the following publication: "Kidneys sans glomeruli".

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Could there be a reason why this happens for calcium and magnesium not for sodium and chlorine ? –  biogirl Aug 2 at 8:55
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I have only found a hypothesis. First it seems a matter of energy (the transporters actively use ATP) and the nutrition. There is a paper about this, but I haven'T got hands on it. –  Chris Aug 2 at 20:49

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