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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
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