I mean fish tend to swim with their top facing the surface and their bottom facing the sea floor. I know it aligns with gravity, but why? Is there no advantage to living your life upside down or vertically?
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Quick answer : they are used to it, and it works.
Species won’t change habits if it doesn’t give an advantage.
Long answer : https://www.smithsonianmag.com/science-nature/why-dont-fish-swim-upside-down-180967192/
I won’t claim to know the whole answer to this but there are a couple clear things that I can see regarding up/down orientation.
The first is the anus. I would argue that an anus positioned to not cover one’s self in its own feces is advantageous. For example, if an anus for a fish was placed on the top of its body then the feces would cover it’s body, that would invite disease and would significantly reduce a fishes ability to pass on its genes. So the anus on the bottom is clearly a dominating evolutionary outcome. You can look at other parts of an ocean organism and establish similar orientations that are dominant.
Not every aspect of a fish’s anatomy will be up/down positionally dominant. Eye placement for many fish is actually on the sides of its body. This is typically seen as an advantage for creatures that have to avoid predators as it allows closer to a 360 degree field of vision.
gravity may have less influence underwater but buoyancy has more. additionallym “up” is where light (and falling food) comes from while “down” is where the ground (and resting food) is so they remain important orientation points. moving between the surface and the depth changes the environment more than moving the same distance sideways
the direction of the light matters because predators tend to sneak up on prey along that axis so their camoflage works. a fish with a pale bottom is harder to see when it’s above you, a fish with a dark top is harder to see when it’s below you
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