Colour-blind octopuses and squid are masters of colour camouflage
Octopuses, squid and cuttlefish have just one type of light receptor in their eyes, so most of them are thought to be colour blind. Yet they can repaint their skin in milliseconds to match a reef or sandy floor, and their skin itself appears to contain the molecules needed to sense light.
The name cephalopod means head-feet, because the arms and tentacles grow from a modified version of the ancestral mollusc foot. About 800 living species exist, from a Thai squid only 10 millimetres long to the colossal squid, which can weigh 700 kilograms and is the largest invertebrate alive. They have ranged the oceans for over 400 million years, from the surface to the deepest trenches, yet have never moved into fresh water; the brief squid of Chesapeake Bay tolerates brackish water and goes no further.
The colour trick relies on chromatophores, pigment sacs in the skin that hold only red, yellow and brown and are stretched or squeezed by muscles. Reflective cells called iridophores and leucophores bounce back surrounding light, which may explain how the animals match colours they cannot see, and small bumps called papillae alter skin texture. A 2015 study found light-sensing proteins in the skin of squid and cuttlefish. Another idea is that their oddly shaped pupils use the blurring of different wavelengths to pick out colour. A jetting octopus often turns pale, because the brain region steering its streamlined shape also runs the chromatophores and cannot do both jobs at once.
Camouflage can become impersonation. The Caribbean reef squid has been seen disguising itself as a harmless parrotfish to sneak up on prey, and some species shine light downward from glowing bacteria to hide their shadow from predators below. Stripes preserved in fossils from the Silurian suggest camouflage is ancient.
They are widely considered the smartest invertebrates, with brain-to-body ratios between those of cold-blooded and warm-blooded vertebrates. Captive animals have climbed out of their tanks, crossed a lab floor to raid a crab tank and gone home again. Some squid even fly, jetting water mid-air and spreading fins and tentacles into wings to glide up to 50 metres.
Source: Cephalopod
