Using colour without seeing colour

I started off my research career with octopuses when I was in a landlocked prairie province. And my supervisor talked from time to time about the deep mystery that cephalopods (squids, octopus, cuttlefish) could use colour, but couldn't see colour. So this is the sort of paper I've been waiting a long while to read.

To get a tiny little taste of the remarkable behaviour of these animals, check out David Gallo's talk below; the relevant bits start about 1 minute 50 seconds in.



I like to show the TED talk first, because it's so great to hear the audience's response. But if you want to see the last bit alone...



And one more link, a PBS special on cuttlefish. The video clips here are almost as amazing as the vanishing octopus.

How do researchers start to get a handle on such glorious eye candy?

This new paper tests the idea that the colours that cuttlefish can make are similar enough to the colours found in their natural habitat that the animals don't need to see colour. That is, if you can change colour to many different shades of brown, you are not likely to stand out too much if you inhabit an area where most things are brown.

There's surprisingly little behaviour or physiology in this paper, which is unusual for this journal. The main technique the authors used was to photograph cuttlefish on various natural and artificial substrates. The digital revolution in photography is making feasible what would have been a nightmare not too long ago. It's now dead simple to take an image, put it into a computer, measure its red, green and blue values, brightness, and so on.

The team selected several spots on the cuttlefish body to measure the animal's colour, then selected several spots on the nearby substrate to measure the colour of the surrounding sand, shell grit, and so on. Some sand were commercially available sands that were... perhaps not quite natural looking (bright red), and others were collected nearby.

They then used computers to compare the cuttlefish colour and the surface. In general, it's quite a good match. Interestingly, however, the match seems to get better as the water gets deeper. If you've ever swam underwater, you know that colours change because the water filters out the light. Red light drops out really quickly, so the deeper you go, the more blue the water appears.

I had to say "seems to get better" in the previous paragraph, because the researchers didn't actually photograph in anything other than very shallow water. Instead, they calculated what they would expect to see at increasingly greater depths. Although I would normally grumble at this, the physics of light and water are probably sufficiently well understood by now that this is probably a pretty safe prediction.

The paper also presents some data on the colour pigments in the animal, but this basically confirms what people had seen by eye: there are three colours in the dorsal side of the animal, and two ventrally.

Thus, this paper hints that how to use colour while being colour blind is a problem with an evolutionary solution, rather than a physiological one. Those with the pigments that best match their local habitat will win out over generations, not in the few seconds where the animals are making those lightning fast colour changes.

Reference

Mäthger, L.M., Chiao, C., Barbosa, A., Hanlon, R.T. (2008). Color matching on natural substrates in cuttlefish, Sepia officinalis. Journal of Comparative Physiology A DOI: 10.1007/s00359-008-0332-4