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.

[1] Cyclothone is believed to be the most abundant fish genus on Earth, with estimates that there are up to a quadrillion individuals (1015, or one million billion in the short scale).

Cyclothone fishes are found in the aphotic zone and have limited access to light and light-dependent food sources.

[7] All species in the genus Cyclothone live in the midwater range of the deep sea, and are most commonly found in the mesopelagic zone at 300–1500 meters depth (roughly 1,000-5,000 feet).

Light is virtually absent (<1%) in the deep sea, meaning that organisms living there cannot rely on using their eyes to catch prey, avoid predators, or find mates.

[14] Cyclothone falls in the middle of the food web, and its main predators are slightly larger deep-sea fishes, such as dragonfish and fangtooths.

Cyclothone fishes have small bioluminescent spots on their ventral (bottom) side that cause them blend in with the surrounding light, allowing them to remain unseen to predators below.