Fish
Flashlight Fish
Anomalops katoptron · also called flashlight fish, lantern-eye fish
The flashlight fish is a small reef fish that comes with its own lights built in. Below each eye it carries a large organ packed with glowing bacteria, and it can switch that glow on and off at will. On a dark, moonless night a school of them turns a patch of reef water into a drifting field of blue points of light.
Overview
Flashlight fish live on coral reefs of the tropical Indo-Pacific, in caves and along steep drop-offs. They are 10–14 cm long, deep-bodied and dark blue-black, with a kidney-shaped light organ set into a pocket below each eye. They eat small planktonic crustaceans at night.
Living headlights
Each light organ is filled with luminous bacteria that the fish farms and cannot make light without — one of the few cases of a shallow-water fish using bacterial bioluminescence. Since the bacteria glow continuously, the fish controls the light by rotating the whole organ down into a dark pocket and back out again, blinking several times a minute.
The blink-and-run trick
A hunted flashlight fish swims in a straight line with its lights on, then abruptly blinks them off and changes direction, so a predator following the light carries on toward empty water. In a school, the lights also help the fish hold formation through a black night.
Status
The flashlight fish is Least Concern, widespread and not targeted by fisheries. By day it stays deep, often below 200 m; it rises into shallow reef water only after dark.
Flashlight Fish: common questions
How does the flashlight fish make light?
It does not make the light itself. Each of its two light organs is a chamber, sitting just under the eye, that is filled with billions of bioluminescent bacteria. The fish supplies the bacteria with oxygen and nutrients through a dense network of blood vessels, and in return the bacteria produce a continuous blue-green glow. This kind of arrangement, where an animal cultures light-making bacteria in a special organ, is common in the deep sea but rare in fish that live on shallow reefs, and the flashlight fish is one of the best-known examples.
If the bacteria glow all the time, how does the fish turn the light off?
By hiding it. In Anomalops katoptron the entire light organ is hinged, and the fish can rotate it downward so that the glowing surface faces into a black-lined pocket and no light escapes, then swing it back up to shine again. A close relative, Photoblepharon, uses a different method, drawing a flap of dark skin up over the organ like an eyelid. Either way the fish gets full control over a light source it cannot switch off at the source, blinking it slowly while cruising and rapidly when hunting or alarmed.
What does the flashlight fish use its light for?
Several things at once. The steady glow helps the fish see and possibly lure the small planktonic animals it eats, and feeding fish blink faster. In a school, the lights let individuals keep track of one another and hold formation through a dark, moonless night when they are most active. And the blinking is a defence — a flashlight fish being chased can swim with its lights on, then blink off and veer away, leaving a predator homing in on a patch of empty water. Some evidence also suggests the fish use blink patterns to communicate.
Can you keep a flashlight fish in an aquarium?
It is kept by advanced marine aquarists but is not an easy fish. It needs a dimly lit or dark tank, since bright lighting stresses it and can suppress the glow, along with cool, very clean water, hiding places, and a steady supply of small live or frozen planktonic food, which it will only take at night. The light organ can also fade or fail if the fish is unhealthy or poorly fed. For most people the flashlight fish is best appreciated on a night dive over an Indo-Pacific reef, where a school passing by is an unforgettable sight.