Coelacanths are an ancient group of lobe-finned fish (Sarcopterygii) in the class Actinistia. As sarcopterygians, they are more closely related to lungfish and tetrapods than to ray-finned fish.
Deep-sea fish are fish that live in the darkness below the sunlit surface waters, that is below the epipelagic or photic zone of the sea. The lanternfish is, by far, the most common deep-sea fish. Other deep-sea fishes include the flashlight fish, cookiecutter shark, bristlemouths, anglerfish, viperfish, and some species of eelpout.
Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some bioluminescent bacteria, and terrestrial arthropods such as fireflies. In some animals, the light is bacteriogenic, produced by symbiotic bacteria such as those from the genus Vibrio; in others, it is autogenic, produced by the animals themselves.
The Gymnotiformes are an order of teleost bony fishes commonly known as Neotropical knifefish or South American knifefish. They have long bodies and swim using undulations of their elongated anal fin. Found almost exclusively in fresh water, these mostly nocturnal fish are capable of producing electric fields to detect prey, for navigation, communication, and, in the case of the electric eel, attack and defense. A few species are familiar to the aquarium trade, such as the black ghost knifefish, the glass knifefish, and the banded knifefish.
Anomalopidae are a family of fish distinguished by bioluminescent organs located underneath their eyes, for which they are named. These light organs contain luminous bacteria and can be "shut off" by the fish using either a dark lid or by being drawn into a pouch. They are used to communicate, attract prey, and evade predators.
The Beryciformes are a poorly-understood order of carnivorous ray-finned fishes consisting of 7 families, 30 genera, and 161 species. They feed on small fish and invertebrates. Beyond this, little is known about the biology of most member species because of their nocturnal habits and deepwater habitats. All beryciform species are marine and most live in tropical to temperate, deepwater environments. Most live on the continental shelf and continental slope, with some species being found as deep as 2,000 m (6,600 ft). Some species move closer to the surface at night, while others live entirely in shallow water and are nocturnal, hiding in rock crevices and caves during the day. Several species are mesopelagic and bathypelagic. Beryciformes' bodies are deep and mildly compressed, typically with large eyes that help them see in darker waters. Colors range from red to yellow and brown to black, and sizes range from 8–61 cm (3.1–24.0 in). Member genera include the alfonsinos, squirrelfishes, flashlight fishes, fangtooth fishes, spinyfins, pineconefishes, redfishes, roughies, and slimeheads. A number of member species are caught commercially, including the alfonsino, the splendid alfonsino, and the orange roughy, the latter being much more economically important. Some species have bioluminescent bacteria contained in pockets of skin or in light organs near the eyes, including the anomalopids and monocentrids.
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.
Pelagic fish live in the pelagic zone of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with demersal fish that live on or near the bottom, and reef fish that are associated with coral reefs.
A viperfish is any species of marine fish in the genus Chauliodus. Viperfishes are mostly found in the mesopelagic zone and are characterized by long, needle-like teeth and hinged lower jaws. A typical viperfish grows to lengths of 30 cm (12 in). Viperfishes undergo diel vertical migration and are found all around the world in tropical and temperate oceans. Viperfishes are capable of bioluminescence and possess photophores along the ventral side of their body, likely used to camouflage them by blending in with the less than 1% of light that reaches to below 200 meters depth.
Fish are very diverse animals and can be categorised in many ways. Although most fish species have probably been discovered and described, about 250 new ones are still discovered every year. According to FishBase about 34,800 species of fish had been described as of February 2022, which is more than the combined total of all other vertebrate species: mammals, amphibians, reptiles and birds.
Counter-illumination is a method of active camouflage seen in marine animals such as firefly squid and midshipman fish, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength.
Phosphorosaurus is an extinct genus of marine lizard belonging to the mosasaur family. Phosphorosaurus is classified within the Halisaurinae subfamily alongside the genera Pluridens, Eonatator, and Halisaurus.
Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Birds and mammals normally adjust focus by changing the shape of their lens, but fish normally adjust focus by moving the lens closer to or further from the retina. Fish retinas generally have both rod cells and cone cells, and most species have colour vision. Some fish can see ultraviolet and some are sensitive to polarised light.
Opisthoproctus soleatus is a species of fish in the family Opisthoproctidae. It was first described in 1888 by Léon Vaillant. The species lives in most tropical seas, but is more common in the eastern Atlantic, from western Ireland to Mauritania and from Sierra Leone to Angola, and also in the South China Sea. O. soleatus can grow to a standard length of 10.5 centimetres (4.1 in) and usually live from about 500 to 700 metres deep.
Photoblepharon palpebratum, the eyelight fish or one-fin flashlightfish, is a species of saltwater anomalopid fish of the order Beryciformes. It is native to the western and central Pacific Ocean where it is found along seaward reefs close to the ocean floor, usually near rocks and corals it can use as cover. At only 12.0 cm (4.7 in) in length, it is a small fish, and more stout than other members of its family. Its body is mostly black, with a line of reflective scales running the length of its body and a distinguishing white spot at the upper corner of its preopercle. Its most notable features are its subocular bioluminescent organs which it likely uses to attract and find prey, confuse predators, and communicate with other fish. These organs are blinked on and off by the fish using a dark lid that slides up to cover them.
The splitfin flashlightfish or two-fin flashlightfish is a species of fish in the family Anomalopidae. It is found in warm waters in the central and western Pacific Ocean near shallow reefs 200–400 m (660–1,300 ft) in depth. It can grow to a length of 35 cm (14 in) TL. It is the only known member of the genus Anomalops.
Kryptophanaron alfredi, the Atlantic flashlightfish, is a species of flashlight fish native to the western Atlantic Ocean. During the day, it is found in waters as deep as 200 metres (660 ft). On moonless nights, it ascends to shallow waters around 25 metres (82 ft) in depth where it feeds on small shrimp and copepods as it swims over the ocean floor. This species grows to a length of 12.5 centimetres (4.9 in) TL. This species is the only known member of the genus Kryptophanaron and can be found in the aquarium trade.
The flashlight fish is a species of anomalopid fish of the order Beryciformes. It is native to the western Indian Ocean and the Red Sea where it is found near coral reefs. It is a nocturnal predator, hiding in crevices and caves by day and emerging to feed at night. At 11.0 cm (4.3 in) in length, it is small and relatively stout. Its body is mostly solid black, with the exception of white discoloration around its pectoral fins and a lateral line of reflective scales. It is most notable for the bioluminescent organs located underneath its eyes, which it uses to find prey, evade predators, and communicate with other members of its species. Like other Beryciformes, its reproduction is oviparous. It has no commercial value, and this combined with its timid, nocturnal nature makes population sizes and trends unknown. It is listed as Least Concern by the International Union for Conservation of Nature (IUCN) due to a wide distribution and lack of known threats.
Protoblepharon rosenblatti, the Cook Islands flashlightfish, is a species of flashlight fish found in the waters surrounding the Cook Islands. It can reach lengths of up to 22.0 cm (9 in) and can be found as deep as 274 m (900 ft).
The electric eels are a genus, Electrophorus, of neotropical freshwater fish from South America in the family Gymnotidae. They are known for their ability to stun their prey by generating electricity, delivering shocks at up to 860 volts. Their electrical capabilities were first studied in 1775, contributing to the invention in 1800 of the electric battery.
