Eye to the right: The deep-sea squid Histioteuthis bonnellii might see with its colour-changing skin David Shale/NaturePL
TWENTY metres underwater, off the coast of north-west Spain, biologist Roger Hanlon is stalking his prey. His camera is trained on a subject that has painted itself beige, grey and white to match the gravelly seabed. It perambulates towards a clump of kelp and, settling itself amid the fronds, quickly deepens its complexion to match their rich red-brown. This colour craft is impressive, but for Hanlon it is also baffling. He knows the common octopus is colour blind.
At least, that is what the textbooks tell us. In his own recent book, Hanlon lists multiple arguments for the cephalopods – octopuses, squid and cuttlefish – seeing in monochrome. Yet if you ask him casually, he remains unconvinced: “I would tend to think that cephalopods are able to sense and match colour somehow.”
Quite how they do it has confounded biologists for more than a century, though they have come up with some strange ideas to explain the conundrum. Now we are whittling down the spectrum of possibilities – in the hope of gaining an unprecedented insight into how these most alien of creatures see the world.
Like Hanlon, the Nobel prize-winning zoologist Karl von Frisch didn’t believe cephalopods were colour blind. In the 1910s, he and the ophthalmologist Carl von Hess got into a debate about it. Hess tested the vision of squid and cuttlefish by trapping the animals in tanks so small they could barely move and flashing coloured lights at them. He noted how their pupils …
