A pinecone fish has a plump, deep body, measuring about 5 in (12.7 cm) in length. The body is covered by heavy, plate-like scales that overlap, giving the fish the appearance of a pinecone—hence its name. Under each pinecone fish’s lower jaw are two phosphorescent organs, giving the impression that the fish itself produces light. The light is actually produced by bioluminescent bacteria that have a symbiotic relationship with the fish.
Pinecone fish belong to the order Beryciformes, which includes 15 families and 143 species of fish, all marine. This order is considered a primitive predecessor of perches. Characteristically deep-sea fish, most families within the order are small, including fewer than 12 species. Some other forms of Beryciformes are whalefish, squirrel fish, laterneyes, and slimeheads. Pinecone fish belong to the family Monocentridae; there are two genera within the family, Cleidopus and Monocentris, with a total of four species.
Aside from having unusual scales and light producing organs, the fins of pinecone fish are unusual. The fish has two dorsal fins located on its back. The first one consists of a series of four to seven stout spines that point alternately to the left and to the right. The second dorsal fin has nine to 12 soft rays. The pincone fish’s pelvic fin, the first fin located on the fish’s underside, is composed of a very strong, large spine with two to four small, soft rays.
Pinecone fish inhabit the Indian and Pacific Oceans, as far south as South Africa and as far north as Japan. They move in schools at depths between 100 and 800 ft (30 and 250 m). Pinecone fish tend to be nocturnal, hiding in caves and nooks during the day. The Japanese pinecone fish are known to form predatory schools near the bottom of deep waters.