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 is thought to be one of the most abundant vertebrate genera in the world.
Bristlemouth specimen showing jaw length.
Image: Sigmops bathyphilus
The mesopelagic zone, also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at the depth where only 1% of incident light reaches and ends where there is no light; the depths of this zone are between approximately 200 to 1,000 meters below the ocean surface.
Sediment trap sample, Thermaikos Gulf, Greece, 2000. Stereoscopic image of the collected material above 63-μm pore size net. Calcareous shells and skeletons of planktonic organisms can be identified.
Sonar data. The green layer in the water column is the deep scattering layer of diel vertically migrating mesopelagic zooplankton and fish.
Illustration by Charles Frederick Holder of various bioluminescent fish that live in the mesopelagic zone
Helmet jellyfish, Periphylla periphylla
