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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 23 hours ago
    
@user137 Thanks. Why dont you convert this into an "answer" and add some references ? –  biogirl 22 hours ago

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