Skulking around in the murky depths of the ocean means you need eyesight that's as good as it possibly can be, and scientists have discovered that the octopus has a great trick to visualise its surroundings. New research indicates that the creature's skin contains the same pigment proteins found in its eyes, so it responds to light and can help it 'see' what's nearby.
University of California evolutionary biologists Desmond Ramirez and Todd Oakley have published their findings in the Journal of Experimental Biology, building on previous theories that had been put forward but never properly verified.
As Sandhya Sekar reports for the BBC, the researchers removed patches of skin from 11 octopuses before using light-emitting diodes to test their reactions. Sure enough, the skin adapted differently to different types of light.
These deep-sea cephalopods are well known for changing colour to match their surroundings, thus evading predators and sneaking up on food sources. What the new findings prove is that the octopus' skin isn't just responding to instructions from the brain and eyes - it's actually reacting to light and changing colour itself.
It's all thanks to the chromatophores under the skin of an octopus: very small, pigmented organs packed with chemicals. As the muscles around them expand and contract, the colour they display changes. Thousands of these chromatophores are packed just below the top layer of skin.
Ramirez and Oakley found that the chromatophores reacted most strongly to blue light, and hardly responded at all when red light was used. The fact that this happened after the skin had been removed from the octopus proves its ability to operate independently.
What's more, Ramirez and Oakley found specialised proteins in the skin called opsins that matched those found in octopus eyes, providing further proof that an octopus doesn't just have to rely on its sight to know what's happening.
The findings suggest that these opsins are loosely arranged under the skin - that means they can detect brightness changes, but can't build up a detailed picture of what's around the octopus. Nevertheless, they can be crucial in detecting and responding to an underwater environment.
A second study in the same journal by biologist Thomas Cronin from the University of Maryland in the US suggests the same kind of skin perception might be happening in other cephalopods too. In other words, these creatures can feel changes in light as well as see them.
"What we do not yet know is how these two inputs come together to control chromatophores in the whole animal," Ramirez told the BBC.