Cuttlefish Are Colorblind But Can Match Any Background

Octopuses, squid, and cuttlefish are colorblind. Their eyes contain a single type of opsin, the protein that lets photoreceptors respond to light. A human has three; a cuttlefish has one. By every textbook definition, they should be unable to tell red from green.

And yet a cuttlefish dropped onto a checkerboard of red and green pebbles will match the pattern in under a second, getting the colors right. Roger Hanlon at the Marine Biological Laboratory has been documenting this since the 1990s. The animal that cannot see color is producing it correctly, on demand, on a body it cannot even see.

In 2016, Christopher Stubbs and Alexander Stubbs proposed an explanation that has the elegance of an optics problem. Cephalopod pupils are weirdly shaped — W-curves in cuttlefish, dumbbells in octopuses — and their lenses suffer extreme chromatic aberration, splitting wavelengths to focus at different depths. By rapidly changing focus and pupil shape, the animal could in principle read which wavelengths are sharpest, sorting color out of blur.

There may be a second channel. In 2015, M. Desmond Ramirez and Todd Oakley showed that the same opsin used in the eye also lives in the skin of the California two-spot octopus. Chromatophores, the pigment cells that produce the color match, can sense light directly. The skin may not see the way the eye sees, but it doesn't have to. It only has to tell red from green.

Neither hypothesis is settled. What is settled is that the camouflage works.