So recently I read the question why are most fishes vertical (in the sense of distribution of their body mass) and it got me thinking what is the reason behind the direction they move their fins. My explanation is that they need to catch the image they several times to truly percieve it, so the eyes have to be orthogonal on the direction they move their fins. Am I correct, therefore they swim like this fish swimming axis rather than this fish swimming vertically because there is more to see on the horizontal dimension in shallow waters?

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    $\begingroup$ Can't see a difference in the two pics? $\endgroup$ – AliceD Jan 23 '17 at 9:19
  • $\begingroup$ @AliceD The waves illustrate the direction of gravity $\endgroup$ – Probably Jan 23 '17 at 11:09
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    $\begingroup$ I think it's much more a consequence of the way their muscles are arranged. Dolphins & whales work perfectly well by moving their flukes vertically. $\endgroup$ – jamesqf Jan 23 '17 at 18:50
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    $\begingroup$ @Probably: I really don't know enough about the evolution to do more than guess, but if I had to, I'd guess that somewhere along the line the ancestors of mammals somehow evolved in such a way that it was easier to bend up and down rather than side to side - think of walking movement, then swimming with fins. Thus when dolphin ancestors started back to the sea, their natural movement was up & down, and that evolved in the direction of improved up & down... $\endgroup$ – jamesqf Jan 24 '17 at 4:18
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    $\begingroup$ @Probably: Not quite what I meant. Think of Tiktaalik tiktaalik.uchicago.edu/meetTik2.html crawling out of the water and scrabbling around on its fins. Carry on through amphibians & reptiles, and you see legs awkwardly splayed out to the sides, with a lot of side-to-side movement (carried to the extreme in snakes). But somewhere along the line to becoming mammals, creatures got their legs directly underneath them, which allowed more efficient locomotion - think of a horse or dog running. Now mammals have this new body plan... $\endgroup$ – jamesqf Jan 25 '17 at 6:41

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