Why Deep-Sea Animals Are Red

Bring up a net from six hundred meters and a surprising number of the animals in it are bright red. [A] Shrimp, jellyfish and small fish that live nowhere near the surface are colored as loudly as a warning sign. [B] The explanation has nothing to do with being seen and everything to do with what light is left at that depth. [C] Seawater absorbs the long wavelengths first, so red light disappears within the first few tens of meters. [D]

An animal is visible only if some light bounces off it and reaches an eye. A red animal in deep water has nothing red to reflect, and the blue light that does reach it is absorbed by the same pigment. The result is an animal that looks black to any predator hunting by ambient light, while being cheap to produce: the pigments are common ones, and the animal is not paying for a complicated mirror or a layer of transparent tissue.

The arrangement fails against a predator that carries its own light. Several deep-sea fish produce a red beam from an organ below the eye, and against that beam a red shrimp glows as clearly as it would at the surface. Only a handful of species can both make red light and see it, which is why the strategy still works most of the time.