It sounds like something that’s advertised in the back of a comic book: a flashlight that only you and your family can see, allowing you to send messages and identify friend and foe in the dark!
Small but fierce looking dragonfish. Credit: National Oceanic and Atmospheric AdministrationBut this flashlight is found on the face of one of the scariest-looking creatures in the deep ocean: a black dragonfish. It’s not very big, but it has a wide mouth filled with long, sharp teeth.
There are several species of dragonfish, and like most other deep-sea organisms, they produce their own light to help them find prey, avoid predators, and attract mates.
Their bodies are lined with small glowing patches called photophores. These organs produce blue-green light -- the wavelengths that travel farthest in the ocean.
But the black dragonfish also has photophores just below its eyes that produce red light. The light actually starts out as blue-green. But those wavelengths are absorbed by a pigment that re-emits the light at reddish-orange wavelengths. And before it enters the water, the light passes through a filter that makes it even redder. In fact, some of the light is infrared, which is invisible to the human eye.
The long wavelengths are also invisible to the other organisms in the deep sea -- their eyes are tuned only to the blue-green. So the black dragonfish can illuminate potential prey without the prey ever seeing the light, and without attracting predators. It can also communicate with other members of its species -- using a flashlight that only another dragonfish can see.
