AT A GLANCE
- Common Name: Splitfin Flashlightfish
- Scientific Name: Anomalops katoptron
- Category: Fish
- Subcategory: Bony Fishes
- Range: Central and western Pacific Ocean, from the Philippines and Indonesia east through Micronesia and Melanesia to the Tuamotu Archipelago and the Great Barrier Reef
- Habitat Type: Steep outer coral reef drop-offs, submarine caves, vertical sea walls, and deep surge channels between 5 and 400 meters depth
- IUCN Red List Status: Least Concern
- Lifespan: 6 to 10 years
- Diet: Carnivorous/Planktivorous (nocturnal hunting for zooplankton, mysid shrimps, copepods, and pelagic larvae attracted to its light)
- Average Size/Weight: Total length 20 to 28 cm; weight 150 to 300 g
- Key Defining Feature: Nocturnal, jet-black reef fish featuring a large, bean-shaped bioluminescent organ beneath each eye that glows with blinding blue-green light and can rotate downward to turn off
Overview
The Splitfin Flashlightfish (Anomalops katoptron), also known as the Two-fin Lanternfish, is one of the most astonishing bioluminescent wonders of the marine world. Belonging to the flashlightfish family Anomalopidae, this nocturnal reef inhabitant lives in the pitch-black caves and deep oceanic drop-offs of the Pacific. Underneath each large eye sits an immense, bean-shaped headlight packed with billions of symbiotic luminescent bacteria, which the fish uses like an underwater flashlight to navigate, communicate, and blind predators in total darkness.
Biology and Physical Characteristics
Anomalops katoptron has an elongated, compressed, aerodynamic body covered in tiny, rough, dark scales. Its coloration is a uniform, velvety pitch-black to purplish-charcoal, which makes it virtually invisible in the night ocean. Its crowning anatomical feature is the subocular photophore: an enlarged, kidney-shaped organ mounted on a rotational cartilaginous stalk directly under the eye socket. Inside this organ, billions of symbiotic bioluminescent bacteria (Candidatus Photodesmus) continuously produce an intense, glowing blue-green light via enzymatic luciferase reactions. Because the bacteria never stop glowing, the fish ‘blinks’ its light by physically rotating the organ downward into a dark pocket of pigmented skin, turning the beam on and off at will.
Behavior and Ecology
Strictly nocturnal, Splitfin Flashlightfish spend daylight hours hiding in deep, lightless submarine caves and vertical reef drop-offs hundreds of feet down. On dark, moonless nights, schools of hundreds rise toward shallow reef slopes to feed. They use their glowing headlights to illuminate the water ahead, spotting microscopic plankton that are drawn irresistibly toward the light. By rapidly rotating their photophores in synchronized patterns—blinking up to 90 times per minute—schools communicate, maintain shoal cohesion, confuse predators with sudden phantom flashes, and find mates in the midnight ocean.
Conservation and Human Impact
The Splitfin Flashlightfish is evaluated as Least Concern by the IUCN Red List. It has a broad Pacific distribution and occupies deep reef drop-offs that are insulated from coastal development and commercial fishing. It is occasionally observed by night divers and collected for public aquariums.
Interesting Factoid
The Splitfin Flashlightfish has biological headlights under its eyes that glow so brightly with living bacteria that it can turn them on and off by literally rotating them upside down into built-in pockets in its skull.