Why don't fish swim upside down?

Ok - having a son of 2, I’ve been watching Finding Nemo non-stop for the last two weeks. Fun. But it kinda bothers me with the question most fish and birds have their eyes opposite sides of their heads but what’s to stop fish from swimming upside-down for a year or two? Pier pressure?

Or am I mistaken and half the fish (the ones in the southern hemisphere) already swimming upside down and liking it? Kinda makes me think an upside-down swimming fish would have the jump on other fish, but I just don’t know.

I also noticed in Finding Nemo that the water in the Dentist’s spit sink spirals the same way as it does in America - man, I hope someone was fired over that!


Repeat after me: “The water does NOT necesarily go down the drain the opposite way in the southern hemisphere”. Yes, hurricanes and other large cyclonic storms rotate in opposite directions in each hemisphere, due to the action of the Coriolis force. However, over the small distances involved in sinks, toilets and bathtubs, the magnitude of this force is far too small to have any meaningful effect. Which way the water spirals is much more infuenced by the geometry of the container than the rotation of the Earth.

And if you don’t want to take Q.E.D. 's word for it, here’s a by some newspaper guy.cite

A fish swimming upside down would be fighting the effect of its swim bladder. That catfish must have a swim bladder located more centrally than other fish.

Like EvilGhandi pointed out, many fishes can swim upside down. In most teleost (bony) fishes, the swim bladder is located above the digestive tract, but this may or may not place it above the center of gravity of the fish.

Only if the swim bladder is above the center of gravity of the fish will it have to ‘fight’ the effects of its swim bladder in order to swim upside down. Even then, most fishes can control the gases in their swim bladder and could release gases to make this easier.

There’s a whole bunch of other reasons fishes don’t typically swim upside-down, though. They do often have semicircular canals, which help them detect which direction the force of gravity operates in. Swimming upside-down for long periods of time may result in the same sort of discomfort a terrestrial animal would feel doing headstands for a long period of time. Fishes often show countershading which enables them to hide better when they are swimming rightside-up, but will make them more easily visible when upside-down. Fishes also often have a mouth position which makes feeding easier in the rightside-up position; for example, surface feeders may have a mouth on the dorsal side of the head while bottom feeders may have a mouth on the ventral side. Swimming upside-down would make using this advantage more difficult. Schooling fish might have trouble following and imitating the actions of the school if they are inverted relative to their schoolmates, in much the same way Calvin has trouble when inverted relative to Mrs. Wormwood and her class… or not. Finally, swimming upside-down is, like, soooo '90s. All the cool fish these days are learning how to hula-hoop.