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Why do most fishes have a "vertical" body shape instead of a "flat" one ? Here "vertical" means that the body(from a front view) is roughly orthogonal to the horizontal plane, and a "flat" body shape means something like a frog's or a bird's with spreading wings.

PS: This question was raised when I was thinking how I can swim faster.

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  • $\begingroup$ Is you question really why is the fish's mass usually distributed into a truck-like shape (rather than into a tank-like shape) or whether they move their fins perpendicular to the direction of gravity? $\endgroup$
    – Probably
    Commented Jan 17, 2017 at 10:11
  • $\begingroup$ It is about body shape or mass distribution. $\endgroup$
    – booksee
    Commented Jan 17, 2017 at 19:15
  • $\begingroup$ I suspect it has a lot to do with the efficient arrangement of muscles for propulsion. The typical fish undulates from side to side, so considering leverage &c, a body that's deeper than it's wide probably works better. Wider fish like rays & anglerfish either use a different propulsion mechanism, or are sedentary. Whales & dolphins move their flukes vertically, and AFAIK are always more round than tall. $\endgroup$
    – jamesqf
    Commented Jan 17, 2017 at 19:51

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While the most popular image of a fish for many is as you say, a more "vertical" than horizontal body shape, however there are many different adaptations to body shape depending upon the lifestyle of the fish. An ultimate example would be the flat fishes, such as flounder and halibut, which lay on one side essentially flat on the ocean floor or other similar surface. These fish however have adapted to have both eyes on one lateral side. There do exist other fish, such as some angler fish, especially in the family Lophiidae, which are more wide than tall, and they sit on the ocean floor and use a lure like appendage to attract prey so that they can ambush it, using their fins more to walk along the bottom surface. The feeding/hunting style is also highly related to the body shape of a fish, some shapes are more suited for moderate constant speeds, while others are suited for short quick bursts. Generally though, flat fish are not well suited for swimming at the middle of the water column, giving them a smaller habitat in which to live, while the more commonly found fish body shapes are used by much more species throughout the water column.

More information from MEER:

More information regarding Fish locomotion on Wikipedia.

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  • $\begingroup$ Rays are flat too. Cetaceans also are broad and they are decent swimmers. $\endgroup$
    – WYSIWYG
    Commented Jan 17, 2017 at 12:44
  • $\begingroup$ The flat fishes seem to be to be sort of an extreme example of a "vertical" body orientation, at least in that their developmental dorsal-ventral axis is the long way, it's just that they live on their sides. Rays of course are quite the opposite. $\endgroup$
    – Bryan Krause ♦
    Commented Jan 17, 2017 at 21:06
  • $\begingroup$ I think we can all agree that cetaceans are not fish, but their body shapes and locomotion patterns are worth looking at. Unlike most fish, cetaceans use their horizontal caudal fins in an up and down movement, whereas most use their vertical caudal fins for a lateral to lateral movement. I agree that flatfish are an extreme example, however monkfish are a different example, although they are quite sedentary and their fins have developed more to propel them along bottom surfaces like feet than they are adapted for swimming. $\endgroup$
    – Sudachi
    Commented Jan 18, 2017 at 6:58
  • $\begingroup$ The link talks about the balance of stability and speed. I assume the most hydrodynamic shape (eels) is the best for speed so how does the tall shape help the stability? I'd guess "gravitational" stability (the ability to balance gravity) is what we talking about here. $\endgroup$
    – Probably
    Commented Jan 29, 2017 at 12:03
  • $\begingroup$ Some fish are built for short rapid movements while some are able sustain fast movements for long periods of time, like tuna. Gravity is not a factor in the sea, but buoyancy is. However buoyancy is controlled with a swim bladder in most fish. Here stability refers to a fish's ability to make angular turns and resist opposing currents etc. $\endgroup$
    – Sudachi
    Commented Jan 29, 2017 at 14:16
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Swimming below the water surface, when you can tune your body density to obtain neutral buoyancy (as fish do), the axis of the swimming motion with respect to vertical is indifferent.

And indeed, there is a very wide variety of orientations of body plan for fish, from the vertical axis dominated body plans and motions (e.g. bream) to the more horizontal body with vertical swimming axis (e.g. monkfish) of mixed motion (e.g. many rays) and down to flatfish. The evolutionary constraints are thus rather on other advantages awarded by specific body plans (e.g. camouflage, angle of vision,...)

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