Octopus Deep Water Camouflage - YouTube

If you're a snack-sized squid or octopus living in the ocean zone where the last bit of daylight gives way, having some control over your reflection could be a matter of life and death.

Most predators cruising 600 to 1,000 meters below the surface spot the silhouette of their prey against the light background above them. But others use searchlights mounted on their heads.

Being transparent and a little bit reflective is a good defense against the silhouette-spotters, but it would be deadly against the "headlight fish," says Duke postdoctoral researcher Sarah Zylinski.

Transparency is the default state of both Japetella heathi, a bulbous, short-armed, 3-inch octopus, and Onychoteuthis banksii, a 5-inch squid found at these depths.

Viewed from below against the light background, these animals are as invisible as they can be. Their eyes and guts, which are impossible to make clear, are instead reflective but when hit with a flash of bluish light like that produced by headlight fish, they turn on skin pigments, called chromatophores, to become red in the blink of an eye.