We already knew cephalopods are amazing creatures — how many of us can go from coloured to invisible in a blink? Hell, octopuses can even operate cameras. Still, sensing light with one's skin is a pretty impressive trick, even for the rock stars of the invertebrate world.
And yet, as we're now discovering, the octopus can do just that. In effect, these animals seem to have evolved a body-wide eye.
That's the extraordinary conclusion of a study published this week in the Journal of Experimental Biology, which shows that the California two-spot octopus (Octopus bimaculoides) senses light with its skin directly, no input from the central nervous system needed. The skin gets its light-sensing ability from opsins, the same family of proteins found in the retinas of many animals. Embedded in the skin of the two-spot octopus, opsins seem to work in concert with pigmented structures called chromatophores, which the octopus can stretch or contract to alter its colour.
Chromatophores in their contracted state (left) and at maximum expansion (right). Image: UCSB
"Octopus skin doesn't sense light in the same amount of detail as the animal does when it uses its eyes and brain," said lead study author Desmond Ramirez of UC Santa Barbara. "But it can sense an increase or change in light. Its skin is not detecting contrast and edge but rather brightness."
A press release describes the study's experiments:
As part of the experiment, Ramirez shone white light on the tissue, which caused the chromatophores to expand and change colour. When the light was turned off, the chromatophores relaxed and the skin returned to its original hue. This process, Ramirez noted, suggests that light sensors are connected to the chromatophores and that this enables a response without input from the brain or eyes. He and his co-author, Todd Oakley, an EEMB professor, dubbed the process Light-Activated Chromatophore Expansion (LACE).
In order to record the skin's sensitivity across the spectrum, Ramirez exposed octopus skin to different wavelengths of light from violet to orange and found that chromatophore response time was quickest under blue light. Molecular experiments to determine which proteins were expressed in the skin followed. Ramirez found rhodopsin — usually produced in the eye — in the sensory neurons on the tissue's surface.
There's no magic to the octopus's light-sensing skin — just evolution. It's well known that octopuses can masterfully change their colour based on light conditions, with certain species becoming transparent in the sunlight and darkening at night. Having light sensors embedded directly in its skin may help the octopus camouflage itself just a hair faster — and that rapid reflex could be the difference between living another day and becoming shark lunch.
Now, if we could only figure out how to steal this ability and integrate it into a synthetic material, we might really have ourselves the makings of an invisibility cloak.
Read the full scientific paper at the Journal of Experimental Biology.
Top image: California two-spot octopus (hatchling shown) can detect light with its skin alone, and respond by changing colour. Credit: UCSB