Now You See Me, Now You Don’t

This is what it looks like when you take a picture of a flashlight fish without using your flash. Image taken at the California Academy of Sciences.

This little stripe of light you’re looking at is coming from a flashlight fish-a small, nocturnal fish that lives in warm oceans, along steep drop offs and caves. They’re usually only spotted by humans in the shallows on moonless nights. So, what causes that beautiful glow? And how does the fish use it?

The light is coming from what’s called a photophore, which is an organ located under their eyes:

A flashlight fish specimen. The arrow is pointing to the photophore, an organ just below their eyes. Image taken at the California Academy of Sciences.

This organ contains a bio-luminescent bacteria, which casts a greenish glow that can be seen as far away as 100 feet. Some flashlight fish have a photophore cover that can be raised and lowered to expose or hide their special lights. Other flashlight fish can rotate their photophore back into their head to hide their lights. This flashlight fish has a cover:

A flashlight fish specimen, partially covering its photophore. Image taken at the California Academy of Sciences.

This gives the fish the appearance of blinking and the photophores seem like eerie, glowing eyes.

At any rate, this special bacteria sack brings a lot of special advantages to the fish. For instance, small prey, such as plankton, small fish, crabs and shrimp, are attracted to the green glow and the light helps the fish see to catch them. They also use their glow to communicate. These fish will normally “blink” 2-3 times per minute, but when they are in danger, they’ll blink up to 75 times per minute. If there is an intruder in a couple’s territory, the larger females will “turn off” their light, get close to the intruder and then turn their light back on, which is meant to scare them. They’ll also use their light to confuse a potential predator. They will “flash and run,” exposing their photophore and then darting in a zigzag motion. Apparently this works pretty well, because these fish are very rarely found in the stomach contents of larger fish.

Of course, this doesn’t stop humans from catching them. Some fishers will catch these fish and then remove their photophore to use as a fishing lure.