Cyclothone
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, though some can be larger. They 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.[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). [2][3] Their abundance is so large that they are also believed to be the most abundant genus of vertebrate on earth.[4]
Cyclothone are found mostly in the open ocean at tropical to temperate latitudes.[5][6] Within the water column, they reside in the mesopelagic zone (also sometimes called the Ocean Twilight Zone). Cyclothone fishes are found in the aphotic zone and 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 diel vertical migration pattern. Other species, such as Cyclothone acclinadens, are believed to remain at depth for their entire lives.[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).[7] The deep sea is an extreme habitat, and life in the deep ocean has specialized adaptations to survive. 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. The deep ocean is also very cold due to the lack of light and the fact that deep waters originate (downwell) in polar regions; below 200 meters, the average temperature of the ocean is 4°C [39F].[8][9] Organisms in the deep sea are also subject to immense pressure, with pressure increasing by 1 atmosphere (equivalent to the pressure we feel on land at sea level) for approximately every 10 meters depth. At 1,000 meters, the pressure of the ocean is equivalent to 100 times that of pressure experienced at sea level.[10]
In order to survive in such extreme conditions, organisms must be highly specialized to match their physiological tolerances to the physical conditions of the deep sea. For example, deep sea organisms do not possess gaseous structures such as lungs or air-pocket swim bladders, which would change size with changes in depth.[10] Specialized adaptations to deep-sea conditions have been part of the reason why Cyclothone have been wildly successful in regards to biomass, but also make them difficult to study: Cyclothone fishes cannot survive when brought to the surface, and therefore cannot be observed alive in a laboratory setting. [7][11]
In general, Cyclothone fishes appear to be opportunistic feeders and tend to feed on whatever species they can encounter in their extreme environment.[13] Some species have been thought to migrate towards the surface to feed (though not in a diel vertical migration pattern), but some remain at depth their entire lives.[7] They have been commonly known to eat copepods and chaetognaths,[14] but they have also been known to eat euphasiids, mysid shrimp, ostracods, detritus, and even fecal pellets.[13]
