The pineapplefish, Cleidopus gloriamaris, is a species of fish in the family Monocentridae, and the sole member of its genus. It is also known as the knightfish or the coat-of-mail fish, due to the armor-like scales covering its body, and the port-and-starboard light fish, as it has a pair of bioluminescent organs that are reminiscent of navigation lights on ships. Its specific epithet is from the Latin gloria and maris, meaning "glory of the sea".
The pineapplefish grows up to 22 cm (8.7 in) long. It has a plump, rounded body almost completely covered in large, rough scales with sharp, backward-pointing spines. The head is large, with mucous pits bordered by rough ridges, and is armored with heavy bone. The snout is blunt and overhangs the wide mouth. The teeth are tiny and thin, present on the jaws, palatine, and vomer. There are two pits containing bioluminescent bacteria on the lower jaw near the corners of the mouth, which are concealed when the mouth is closed. This photophore is green in young fish and becomes more red as it ages. The first dorsal fin consists of 5-7 strong spines; the spines are free of a membrane and point alternatingly left and right. The second dorsal fin contains 12 soft rays. Each pelvic fin contains an enormous spine, nearly as long as the head, and 3-4 rudimentary rays. The pelvic spine can be locked erect at a right angle to the body. The anal fin contains 11-12 rays and the pectoral fins 14-15 rays.
The scales of the pineapplefish are yellow to whitish with black rear margins, forming the striking pattern that gives this fish its name. The lips, chin, and parts of the jaw are black. There is a red stripe on the lower jaw running to the photophore. This species is very similar to the pinecone fishes of the genus Monocentris, and is placed within that genus by some authors. Cleidopus differs from Monocentris in having a narrow preorbital bone and in the position of its light organs, which are near the tip of the lower jaw in Monocentris. This species also differs from the Japanese pineapplefish (Monocentris japonica) in having a more rounded snout.