The blue-ringed octopus is tiny, cute — and incredibly deadly. One bite from its beak could kill a human, and most sea creatures wouldn't fare much better. But the blue-ringed octopus shares a trait with poisonous butterflies. It has coloration that warns predators not to eat its deadly flesh. In the case of the blue-ringed octopus, this coloration takes the form of glowing blue circles all over its body, which it can light up in miliseconds.
Lydia Mäthger, a marine biologist at Woods Hole Lab, wanted to find out how this little cephalopod was able to alter its coloration so fast. So she took this incredible, super-slo-mo video of a blue-ringed octopus lighting up. What she discovered was that the creatures have a very specialized system for getting luminescent — one that is, as far as we know, unique in the octopus world.
Over at Scientific American, Katherine Harmon gives a good backgrounder on how these color displays work:
The octopus generally relies on three structures in its skin to create its elaborate displays. Chromatophores are pigment-filled sacs that are controlled by surrounding muscles. Flexing and contracting these muscles can expand or shrink the sacs, changing the overall appearance in a complicated choreography of color droplets. Beneath these are iridophores, which are firmer, iridescent sheets whose color is controlled by a shifting in the arrangement of proteins and cytoplasm to reflect different wavelengths of light or UV waves. And finally, leucophores are more passive, white reflectors that add luminosity and contrast to the overall display. The chromatophore sacs that contribute to color displays can start to change in a matter of milliseconds, but standard iridophores, which rely on physiological changes to shift color or luminosity, can take seconds or even minutes.
The researchers found that this little octopus has developed iridophores that are un-obscured by chromatophores. These rings of iridophores are lodged in pockets of muscular skin, which can quickly relax or contract, exposing more or less of the iridescent structures, respectively.