It keeps bacteria in its jaw and they light it.
A pair of organs on the lower jaw of this fish is packed with luminous bacteria, kept alive and fed by the fish in return for their light. The glow is greenish, faint by daylight standards but obvious in a dark cave, and it points forward and downward. It is thought to work as a small lamp for hunting the crustaceans the fish eats, and possibly as a signal between individuals in the dark.
This kind of arrangement, an animal farming a colony of bacteria for their light rather than making it itself, has evolved repeatedly in the sea: flashlight fish, anglerfish, some squid. The bacteria are not inherited, they are acquired from seawater by each generation, which means every one of these fish has to find and recruit its own colony after hatching.
The rest of it is armor. The body is encased in large plate-like scales with raised keels, locked together into a rigid box that the fish cannot bend, which is why it moves stiffly and slowly and why it is shaped like a pinecone.
It spends the day in caves and under overhangs and comes out at night.
The genus. Monocentris Bloch & Schneider 1801 - mono- (one or single) + centris (any sharp point). ETYFish
- rigid armor of large plate-like scales, each with a keel, like a pinecone
- yellow with black scale margins, giving a net-like pattern
- pair of light organs on the lower jaw, glowing dull green in the dark
- up to 17 cm
