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Based on pictures, it seems to me that a vast majority of fish species that have scales do NOT have scales on their heads.

Is that fact true?

To make this properly answerable:

  • lets' define a "majority" as >70% of fish species. But frankly, I'm more interested in actual numerical answer than whether it passes some arbitrary threshold or not.

  • The universe which I'm interested in measuring the percentage are fish species that have "normal" (Cycloid and ctenoid is the technical term, I believe?) scales on their bodies.

    If that's not specific enough, you can restrict the universe to species in Actinopterygii (ray-finned) that have scales.

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Shouldn't a majority be 50% + 1? Also are you referring to species or to total numbers? –  kmm Mar 12 '12 at 1:33
    
@Kevin - species, as the first bullet point notes. As far as 70% vs 50%+, the question came out of an argument, for which mere 50%+ would not be a convincing enough number, whereas 70% would. And without a specific number, this would be a bad SE question (too vague) –  DVK Mar 12 '12 at 1:49
    
@DVK: there is really no argument that 50%+1 > 50%-1, hence it is the majority. But I'm just nitpicking. –  nico Jun 6 '13 at 21:39
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(Should be comment, but rep=1.) This might be helpful (from Wikipedia): "The same genes involved in tooth and hair development in mammals are also involved in scale development." Mammals tend to have less hair on their face than on their bodies. If you can track down the reason for that, then you might have the same reason for fish, and then could use that to reason out your estimate. –  dmm Sep 26 '13 at 22:14
    
Would also be nice to know (or at least think about) whether if this is the case it's because of synapomorphy (i.e. shared heritage) or convergence. –  Oreotrephes Sep 27 '13 at 0:07
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1 Answer 1

I believe it is mainly due to hydrodynamics. Scales reduce drag while allowing for the body to be able to still move. Drag only occurs in the back part of the fish, so there is where you need scales.

Please refer to http://darwin.wcupa.edu/~biology/fish/pubs/pdf/ISSDR(drag%20reduction).pdf for other interesting biological solutions for drag reduction.

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