Which shark is widely described as the largest known luminous vertebrate?

The kitefin shark is widely described as the largest known luminous vertebrate.

The species is Dalatias licha, a deep-sea shark also called the seal shark or black shark. It can reach about 1.8 metres, or nearly 6 feet, making it much larger than better-known luminous sharks such as lanternsharks and cookiecutter sharks. Researchers documented its bioluminescence in detail in a 2021 study of sharks collected near New Zealand.

Its light comes from photophores, specialized organs containing light-producing cells. The glow is strongest on the underside and can also occur on the fins. One proposed function is counter-illumination: matching the faint light from above so the shark’s silhouette is harder to see from below. Another possibility is that the downward-facing light helps illuminate prey or the seafloor.

“Luminous” here means naturally producing light, generally called bioluminescence. It does not mean merely reflecting light or displaying biofluorescent markings. The kitefin is a slow-moving, solitary predator that eats fishes, rays, cephalopods, crustaceans, and sometimes pieces bitten from larger animals.