The Shadowless Hunter
In the shallow coastal waters of the Hawaiian Islands lives a small cephalopod with a unique form of camouflage. The Hawaiian bobtail squid, Euprymna scolopes, is a nocturnal predator that grows to only about 30 millimeters in length. During the day, it remains buried in the sand, sometimes gluing sand grains to its body for extra cover. When night falls, it emerges to hunt for small crustaceans. This is when its biological stealth technology activates.
The squid possesses a specialized light organ in its mantle that houses a colony of bioluminescent bacteria called Vibrio fischeri. This is a highly specific relationship; only V. fischeri is able to colonize the light organ. The bacteria produce a blue-green glow, and in return, the squid provides them with a sugar and amino acid solution. This partnership is symbiosis. The squid uses the bacterial light for a behavior called counter-illumination. As it swims, it projects light downwards from its body, adjusting the brightness to match the moonlight and starlight filtering through the water from above. This effectively erases the squid's shadow, making it invisible to predators like the Hawaiian monk seal looking up from the seafloor. The light organ is complex, containing tissues that act as a lens and a reflector to direct the light, while the squid's ink sac absorbs any light from being emitted upwards.