Fish
Pineconefish
Monocentris japonica · also called pineconefish, knightfish, pinecone fish
The pineconefish looks exactly like its name — a small, stiff, armour-plated fish that could pass for a swimming pinecone. Two glowing organs on its lower jaw, lit by colonies of bacteria, work as living headlights it uses to hunt shrimp in the dark.
Overview
Pineconefish live on rocky and coral reefs of the western Pacific and Indian Ocean, sheltering in caves by day. About 12–17 cm, they are encased in large, ridged plate-like yellow scales arranged like a pinecone, emerging at night to hunt small shrimp and crustaceans.
Living headlights
A pair of jaw organs house bioluminescent bacteria that glow blue-green, shifting to red at night, thought to attract and illuminate the fish’s prey in the dark.
Armour over speed
The heavy, ridged scales form a rigid protective shell, so the fish swims slowly and stiffly and relies on its armour rather than escape.
Status
The pineconefish is Least Concern, widespread across its Indo-Pacific range with no known population concerns.
Pineconefish: common questions
Why does the pineconefish look like a pinecone?
Its body is covered in unusually large, hard, plate-like scales, each with a prominent ridge or keel and a dark outline, that overlap in a regular pattern strikingly similar to the woody scales of a pinecone, which is where both its common name and its alternative name of knightfish, referring to a knight's armour, come from. These heavy scales fuse into an effectively rigid protective shell around most of the body, trading away the flexibility and speed most fish rely on in exchange for a strong physical defence against being bitten or crushed by a predator, which suits the pineconefish's slow, deliberate lifestyle in and around reef crevices.
How do the pineconefish's light organs work?
The two light organs, one on each side of the lower jaw, are pockets that house dense colonies of bioluminescent bacteria, which continuously produce a blue-green glow through a chemical reaction as long as the fish keeps them supplied with oxygen and nutrients through a rich blood supply. The fish cannot switch the bacterial light itself on and off, but it can control how much light escapes by adjusting the position of its jaw and surrounding tissue, and the light appears blue-green by day and shifts toward red at night, an unusual change thought to relate to how the fish uses the organs when actively hunting after dark.
What are the light organs used for?
The leading explanation is that the glowing jaw organs function as a kind of built-in hunting lamp. When the pineconefish emerges from its daytime shelter to feed after dark, the light is thought to attract or illuminate the small shrimp and other crustaceans it preys on, drawing them within range or making them visible to the fish in the darkness so it can pick them off. The steady glow may also help members of a group stay together or recognise one another at night, and there is some suggestion it could play a role in deterring or confusing predators, though the feeding function is considered the primary one.
Is the pineconefish related to the flashlight fish?
They are related in the broad sense that both belong to a group of fish that have independently or through shared ancestry evolved partnerships with bioluminescent bacteria housed in dedicated light organs, and the pineconefish and the flashlight fishes are often discussed together as classic examples of bacterial bioluminescence in fish. They differ in the details, with flashlight fish having a much larger light organ beneath the eye that they can rapidly shutter on and off, and living in deeper or darker water, whereas the pineconefish has smaller jaw organs with a more constant glow and a heavily armoured body, but both illustrate the same fundamental strategy of an animal farming glowing microbes to produce light it could not make on its own.