Why do fish swim upright?


While running a design session this week I was suddenly struck by a strange thought.

Since fish live in a completely 3-dimensional environment and typically spend their time in a state of weightlessness (neutral buoyancy)…why do they swim upright (e.g. with their bellies facing towards Earth)?  

I posed this question to the team (a bit of a diversion from designing software I’ll admit) and, despite all being intelligent people with top-notch educations, none of them could come up with a plausible answer.

And then, while making some toast yesterday, another thought occurred to me.  Since I understand why land animals stay upright, perhaps the current thinking about life starting in water is incorrect and things actually started on land.  And would that mean that those creationist people actually have something right after all?

These were odd thoughts for several reasons:

  1. Well…they are just strange thoughts anyway.
  2. I normally get my strange thoughts in the shower.   (Hey!  You!  Get your mind out of the gutter)

Inquiring minds want to know…

Answers in comments below please.  A hearty handshake and a gold star to the first person with a plausible answer.



Filed under Life - or something like it

10 responses to “Why do fish swim upright?

  1. Humans have small stones called otoliths floating in a fluid in the inner ear. This allows us to orient ourselves to what position that we are in. I would imagine that fish have some sort of mechanism like this that allows them to swim right side up

  2. It’s tautological. Upright is how you define it, so if all fish swam ‘upside down’ you’d define that as upright! The alternative is to swim with the nose facing up (or down) which might make escape from a predator or progress to new feeding grounds more difficult, hence the evolutionary advantage. Anyway the sampling method is biased: we are more likely to see fish near the 2D surface of water rather than in 3D deep deep water – how do they swim down in the Mariana Trench?

    • But I did define upright as belly down…which is how most humans would define as ‘normal’. Typically if a fish is seem swimming nose up/down, or even belly up, this is considered a problem (and usually is because the fish is sick).

      But I still don’t get why belly down is the ‘right’ way to be in a weightless / three dimensional environment.

    • Looks like the fish of the Mariana Trench are mostly belly down too…but then the usual pictures of those are angler fish (ugly suckers) which makes sense since they often sit on the bottom and therefore need the eyes on top.

  3. I do love your blog man. I totally identify with the “epiphanies in the shower” phenomenon. I’m a software developer and do some of my best work there and while half-asleep in bed with my eyes closed.

    “in a state of weightlessness (neutral buoyancy)” <– This actually didn't strike me until I wrong all the following paragraphs and was considering the first comment but I decided to deliver it as a first point because I think it's erroneous and if I'm right and you fix that mental error, you might come up with other ideas of your own. As the first comment mentioned, there's also the inner ear and like vision, it's likely evolved to deal with certain consistent data (like not noticing our blind spot) and since gravity is still constant even if the fish are neutrally boyant, any orientation systems like the inner-ear would still feel the effect of gravity and nature does seem to prefer certain symmetry and consistency so their 3D world still has gravity and the following to cope with.

    Survival I'd say is the primary reason and that requires threat identification. Consider field-of-view, stereoscopic vision, the position of the eyes on the body (they had to go somewhere and virtually ALL living creatures with light-vision have them in the same place) and the greatest threats in shallow water; birds, bears and anything else that lives outside the water.

    If you do buy into evolution (i'm mixed but far from a creationist), it all started in some pretty shallow puddles and fish evolved their way out into the oceans (to account for those who might never even see the surface, but those are the minority) meaning that threats exist everywhere, but the nature of threats in the water (360 degrees) would explain why the eyes are on either side, offering a massive field of view and virtually no stereoscopic vision (read: http://library.thinkquest.org/28030/physio/stereo.htm). They're forward on the body because fish swim forward and need to see more in the direction of movement (also common in pretty much everything with eyes), and that only leaves the top of bottom of the head. I'd say they're on the top and swim upright to keep a consistent view in general (each eyes is processing a similar but mirrored scene) with the advantage slightly toward "up" to deal with threats outside the water which offer less warning. The advantage similar to having the eyes forward for the same of forward motion.

    Even if not for the sake of an upward threat, there's still the advantage of a consistent view. I've never seen anything to suggest that any creatures visual system was inherently incapable of processing stereoscopic vision but rather a variation of the eyes placement dictating (depending on need) whether stereoscopic information would be presented. If that's true and you understand how that data is processed, it makes sense that the systems are at least especially good, if not uniquely designed for processing scenes that are similar to some extent. If fish swam sideways (or had one eye on the top and one on bottom), one would present the surface and one the bottom and the scenes would have absolutely nothing in common in the case of animals with no stereoscopic vision.

    I think a snake would be a good way to amplify my point. I imagine they could get around in a less-than-upright manner if they wanted but don't. Their threats are overwhelmingly from above and living on the ground as most do, their eyes are positioned perfectly to see more above them than below them (where they're pretty well blind). Various primates also would provide more data since they can move about with so much agility, why are they prone to doing it an an upright manner? I'd say that all my points about survival vision about would apply. For that matter, loose at possum or other creatures that don't live upright, but almost always function perfectly upside-down when not properly upright; consistency and wide field-of-view of their short world (more width than height to be concerned with).

    Another thing to ponder: Why don't you walk around resting your head sideways instead of forcing it to be upright (a similar visual effect to walking other than upright)? I imagine that getting more or less information from the right/left side of your world at any moment would make functioning a bit different.

    • Wow…that was some comment. Thanks!

      Some interesting points but I especially liked “If fish swam sideways (or had one eye on the top and one on bottom), one would present the surface and one the bottom and the scenes would have absolutely nothing in common in the case of animals with no stereoscopic vision”. That actually makes sense to me as I could see that as reducing confusion and, when all other factors are even, that’s probably enough to tip the balance.

  4. pedro juarez

    Why ? in the world of FISH or the world of planets and stars what is UP and
    what may be DOWN is ? whatever u want it to be ?

  5. pedro juarez

    WHY enter when you can DIVE IN..?

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