Mesopelagic zone in the context of "Bathypelagic"

The Gonostomatidae are a family of mesopelagic marine fish, commonly named bristlemouths, lightfishes, or anglemouths. It is a relatively small family, containing only eight known genera and 32 species. However, bristlemouths make up for their lack of diversity with relative abundance, numbering in the hundreds of trillions to quadrillions. The genus Cyclothone (with 13 species) is thought to be one of the most abundant vertebrate genera in the world.

The fossil record of this family dates back to the Eocene epoch. Living bristlemouths were discovered by William Beebe in the early 1930s and described by L. S. Berg in 1958. The fish are mostly found in the Atlantic, Indian, and Pacific Oceans, although the species Cyclothone microdon may be found in Arctic waters. They have elongated bodies from 2 to 30 cm (0.79 to 11.81 in) in length. They have a number of green or red light-producing photophores aligned along the undersides of their heads or bodies. Their chief common name, bristlemouth, comes from their odd, equally sized, and bristle-like teeth. They are typically black in color which provides camouflage from predators in deep, dark waters. They mainly feed on zooplankton and small crustaceans due to their small size.

Cyclothone is a genus containing 13 extant species of bioluminescent fish, commonly known as 'bristlemouths' or 'bristlefishes' due to their shared characteristic of sharp, bristle-like teeth. These fishes typically grow to around 1–3 inches (25–76 mm), though some can be larger. These deep-sea fish are most commonly found in the mesopelagic zone of the ocean, mostly at depths of over 300 meters (1,000 feet), and many species have bioluminescence.

Cyclothone is believed to be the most abundant fish genus on Earth, with estimates that there are up to a quadrillion individuals (10, or one million billion in the short scale) within the current global population. They are so abundant that they are also believed to be the most abundant genus of vertebrate on earth.