.One thousand meters under the sea, where the red wavelength of sunlight doesn't penetrate, red organisms are effectively invisible. But things are trickier in the middle ocean depths, between 600 and 1000 meters, where sunlight can still reveal the silhouettes of colorful sea creatures. So a couple of mid-ocean-dwelling cephalopods—the animal class including octopuses and squids—have come up with a flexible strategy: Start out transparent, but change colors whencertain predators come around.   
 Researchers witnessed this behavior in the octopus Japetella heathi (shown) and the squid Onychoteuthis banksii. They exposed the creatures to a beam of directed blue light, as might come from certain bioluminescent predators, as well as to other stimuli such as passing shadows. When the blue light hit them, the cephalopods contracted muscles that stretched their pigment-containing cells, turning their skin red. In the wild, this quick blush hides the cephalopods from their predators, as red objects are imperceptible under blue light. Thus, J. heathi and O. banksii stay invisible, even though they're no longer transparent.