Cyclothone is a genus that at least contains 13 extant species of bioluminescent fish, known as 'bristlemouths' or 'bristlefishes' due to their shared characteristic of sharp, bristle-like teeth. These fishes typically grow to around 1 to 3 inches (2.5 to 7.6 cm).

They are most commonly found in the mesopelagic zone of the ocean, and are mostly found at depths of over 300-1500 meters (1,000-5,000 feet).

The Cyclothone is believed to be the most abundant genus of fish on Earth, with estimates that there are up to a quadrillion individuals.

Their abundance is so large that they are also believed to be the most abundant genus of vertebrates on earth.

Cyclothone are found mostly in the open ocean at tropical to temperate latitudes. Within the water column, they reside in the mesopelagic zone.

Cyclothone have limited access to light (and light dependent food sources). Some species of this genus, such as Cyclothone signata, are believed to migrate towards the surface, although they do not appear to do so in a vertical pattern.

Specialized adaptations to deep sea conditions have been the biggest part of the reason why Cyclothone have been wildly successful in regards to biomass, but also make them very difficult to study.

Cyclothone fishes cannot survive when brought to the surface, and therefore cannot be observed alive in a laboratory setting.

More Info: en.m.wikipedia.org