Gonostomatidae

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. These 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. 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.

Description

Bristlemouths are extremely small, measuring on average 7.5 centimetres (3.0 in). Bristlemouths have elongated bodies, small eyes, short snouts, large mouths, and large jaws. The position of the dorsal fin begins in line with the anal fin. The difference between bristlemouths species is found in the intensity of their pigmentation and photophore size. For the majority of the species, the morphology remains the same.

Bristlemouths are mostly dark in pigmentation but at times can display translucently. Bristlemouths contain a pineal organ which functions to detect slow changing ambient light. This allows the bristlemouth to have control over its circadian clock and seasonal behavior.

Bristlemouths are light emitting fish. Bristlemouths rely on their bioluminescence for different outcomes. Some rely on it to find prey while others use it to avoid predation. However, the most common way that their bioluminescence is used is as a mating signal between these fish, in the same way humans may "dance or wear bright colors at the nightclub." (To attract a partner).

Bristlemouths have large jaws that are capable of catching prey larger than themselves. The length of the Scopeloides glarisianus's (an extinct species of Bristlemouth) lower jaw is equal to 70% of the length of their head. The lower jaw of the Bristlemouths is not functional in terms of masticating their prey. It is therefore hypothesized that they swallow their prey tail first.