When Domenico Fulgione placed Moorish geckos on dark surfaces, he saw what he had seen for years. These spiny, hand-sized lizards changed colour. Within an hour, their typical creamy white complexions transformed into blacker hues that better matched their environment.
And then Fulgione blindfolded the geckos.
They still changed colour. How does an animal adjust its colour to match its environment, when it can’t see that environment at all?
Fulgione’s team found an important clue when they repeated their experiment and bandaged the geckos’ torsos, rather than their heads. This time, their camouflage failed. They could see perfectly well but with their flanks covered, they were less effective at matching their surroundings than their unrestrained or blindfolded peers.
These bizarre results started to make more sense when the team analysed the gecko’s skin. They found that the skin is rife with opsins—light-sensitive proteins that are the basis of animal vision. When light enters your eyes, opsins in your retinas respond by triggering chemical reactions that send signals to your brain. That’s how you see. The Moorish gecko has plenty of opsins in its eyes too, but the team also found these proteins all over the skin of its torso. It’s especially common in the lizard’s flanks, and in cells called melanophores that are filled with dark pigments.
The researchers think that the flank opsins can respond to surrounding light levels and automatically adjust the gecko’s colour. If they’re right, the lizard has a kind of distributed vision that is independent of its eyes, and perhaps its brain. In other words, it can “see” with its skin.
This isn’t a new concept. Some cells in the skin of fish like tetras and tilapias can change colour independently, thanks to their own opsins. And in 2010, Lydia Mäthger and Roger Hanlon found that cuttlefish also have opsins all over their skin. These relatives of squid and octopuses excel at quickly matching the colour of their skin to their surroundings. And yet, they’re colour-blind. Perhaps cuttlefish are also using opsins in their skin to sense light without having to involve their eyes?
But the presence of opsins means little on its own. In all of these animals, scientists still need to show that the opsins are actually responding to light, that they are sending signals to other parts of the body, and that these signals are causing changes in colour. For the geckos, there’s another mystery: why are most of the opsins on the lizard’s flanks, when it’s the back that changes colour most dramatically?
Reference: Fulgione, Trapanese, Maselli, Rippa, Itri, Avallone, Van Damme, Monti & Raia. 2014. Seeing through the skin: dermal light sensitivity provides cryptism in moorish gecko. Journal of Zoology http://dx.doi.org/10.1111/jzo.12159