Western Indo-Pacific Ocean to southern Japan and south to Australia’s Great Barrier Reef

Their preferred habitat is the seaward sides of tropical reefs that have caves and are near steep drop-offs at depths of 1-400 m (3.3-1312 ft).

Giant or splitfin flashlightfish have a very short blunt snout, a large upturned mouth, a deeply forked slender tail, and a light-emitting organ called a photophore under each eye. They are a dark purplish-gray in color. The second dorsal and anal fins are black, with a whitish zone at the base and a blue outer edge. The caudal fin is also black with a blue outer edge.

The size of this flashlightfish species appears to depend on the depth at which they live. In shallow water, individuals are small, measuring up to 15 cm (6 in) in length. In deeper water, they are much larger, measuring to 30-35 cm (12-13.8 in) in length. Females are larger than males.

They are nocturnal, staying well-hidden during the day and emerging at night to feed on zooplankton, which are attracted to the light from the photophores. They also eat the smaller fishes that come to share the zooplankton. Normally, flashlightfish feed in currents just out from the reef wall. On dark moonless nights they feed near the surface of the water.

Little is known about the reproduction of these fish except that sexes are separate, pelagic (open ocean) spawning takes place, and eggs are fertilized externally.

Giant flashlightfish have a large bean-shaped light organ, called a photophore, below each eye. While the light is produced constantly, the fish turns the light “off” by rotating the organ in its socket so it is no longer visible. To turn the light back “on”, the photophore is rotated again. The fish’s own eyes are protected against the light’s glare by the photophore’s black lining. While they usually flash their light 2-3 times per minute, when threatened they can cause the light to flash up to 50-70 times per minute. The blinking of these light organs is used for communication with other giant flashlightfish, to assist in schooling and mating, and to attract prey.

Predators are confused by the use of a “blink and run” strategy in which the fish rapidly swim in one direction with their lights “on” then, zigzagging, switch directions and swim with the lights ”off”.

The bioluminescent photophores contain billions of symbiotic light-emitting bacteria, which biochemically produce a bright lime-green light by generating chemicals similar to those used in commercially produced light-sticks. This is an example of a mutualistic relationship—the fish benefits by getting light from the bacteria and the bacteria benefit by getting nutrients and oxygen from the fishes’ blood, as well as a safe home.

Little is known about the population densities of these fish.

The bright light of schooling flashlight fish can be seen at night up to 30.5 m (100 ft away). This light is said to be the brightest glow produced by any known living organism.