Oblong Mojarra

A mojarra is a mirror, and that is a serious piece of engineering.

The flanks of these fish are covered in stacks of tiny reflecting plates made of guanine crystals, layered at a spacing tuned to visible light. What they do is reflect the water around them: over open sand with nothing to hide behind, a vertical mirror shows a predator approaching from the side an image of the water on the other side of the fish, and the fish disappears.

The hard part is that a plain mirror would give the game away instantly, because light underwater is polarized and a flat reflector changes the polarization in a way many marine predators can detect. Silvery fish get around it with crystal stacks of mixed orientation that preserve the polarization of what they reflect. It was worked out only in the last fifteen years, and it is one of the better pieces of optics in any animal.

So the least interesting-looking fish over the sand flats is wearing the most sophisticated camouflage on the site.

This one is the deep-bodied mojarra with a row of faint dark spots below the lateral line, out over sand and mud in sheltered water rather than on the reef itself.