Colorful Tropical Fish

Today, to celebrate the start of the spring break, I went to the New York Aquarium. It was pretty fun, we saw a lot of bizarre fish and water animals. In particular, it was a very good place for the whole-family-activity.

But then a natural question arises. Why are the fish so colorful? Since those tropical fish are usually small, they can be easily be spotted by their predators if they are colorful. Well, my first guess is that they live in the deep water, given the weak light, any color doesn’t matter. However, this is not true since tropical fish typically lives in the shallow water. Another explanation I come up with is that the water is not so colorless. There are many kinds of plants with colors, like corals, sea weeds, anemones and so on. So the fish, in particular the small ones, can hide in between the objects to escape from the predators. So their colorful appearances give them the chameleon effect.

My colleague thinks that we have to explain why there are so many colors. I think this is not the right question to ask. This is the question of naturalness. If we see many different colors, this is natural. While if there is only one color, we need to ask why there is only one.

But there is one thing I don’t understand. All the fish we see have the “aspect ratio” , which is the height of the body to the width, much larger than 1. Why not the other way around? That is why we don’t see fish that have aspect ratio much smaller than 1. Is it an accident or have an origin from hydrodynamics?

Update: In fact, stingrays are the type of fish with aspect much less than 1. I think the drawback of this kind of aspect ratio is the placement of eyes. They have to be placed on one side, so the fish can only see half of the world.