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,[1] which is more than the combined total of all other vertebrate species: mammals, amphibians, reptiles and birds.
Fish species diversity is roughly divided equally between marine (oceanic) and freshwater ecosystems. Coral reefs in the Indo-Pacific constitute the centre of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests, especially the Amazon, Congo, and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth. Exceptionally rich sites in the Amazon basin, such as Cantão State Park, can contain more freshwater fish species than occur in all of Europe.[2]
Fish systematics is the formal description and organisation of fish taxa into systems. It is complex and still evolving. Controversies over "arcane, but important, details of classification are still quietly raging".
The term "fish" describes any non-tetrapod chordate, (i.e., an animal with a backbone), that has gills throughout life and has limbs, if any, in the shape of fins.[3] Unlike groupings such as birds or mammals, fish are paraphyletic, since the tetrapod clade is within the clade of lobe-finned fishes.[4]
Jawless fish were the earliest fish to evolve. There is current debate over whether these are really fish at all. They have no jaw, no scales, no paired fins, and no bony skeleton. Their skin is smooth and soft to the touch, and they are very flexible. Instead of a jaw, they possess an oral sucker. They use this to fasten onto other fish, and then use their rasp-like teeth to grind through their host's skin into the viscera. Jawless fish inhabit both fresh and salt water environments. Some are anadromous, moving between both fresh and salt water habitats.
Extant jawless fish are either lamprey or hagfish. Juvenile lamprey feed by sucking up mud containing micro-organisms and organic debris. The lamprey has well-developed eyes, while the hagfish has only primitive eyespots. The hagfish coats itself and carcasses it finds with noxious slime to deter predators, and periodically ties itself into a knot to scrape the slime off. It is the only invertebrate fish and the only animal which has a skull but no vertebral column.[5]
Cartilaginous fish have a cartilaginous skeleton. However, their ancestors were bony animals, and were the first fish to develop paired fins. Cartilaginous fish don't have swim bladders. Their skin is covered in placoid scales (dermal denticles) that are as rough as sandpaper. Because cartilaginous fish do not have bone marrow, the spleen and special tissue around the gonads produces red blood cells. Their tails can be asymmetric, with the upper lobe longer than the lower lobe. Some cartilaginous fishes possess an organ called a Leydig's organ which also produces red blood cells.
There are over 980 species of cartilaginous fish. They include sharks, rays and chimaera.
Bony fish include the lobe-finned fish and the ray finned fish. The lobe-finned fish is the class of fleshy finned fishes, consisting of lungfish and coelacanths. They are bony fish with fleshy, lobed paired fins, which are joined to the body by a single bone.[6] These fins evolved into the legs of the first tetrapod land vertebrates, amphibians. Ray finned fishes are so-called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays").
There are three types of ray finned fishes: the chondrosteans, holosteans, and teleosts. The chondrosteans and holosteans are among the earlier fish to evolve, and share characteristics with both teleosts and sharks. In comparison with the other chondrosteans, the holosteans are closer to the teleosts and further from sharks.
Teleosts are the most advanced or "modern" fishes. They are overwhelmingly the dominant class of fishes (or for that matter, vertebrates) with nearly 30,000 species, covering about 96 per cent of all extant fish species. They are ubiquitous throughout fresh water and marine environments from the deep sea to the highest mountain streams. Included are nearly all the important commercial and recreational fishes.[7]
Teleosts have a movable maxilla and premaxilla and corresponding modifications in the jaw musculature. These modifications make it possible for teleosts to protrude their jaws outwards from the mouth.[8] [9] The caudal fin is homocercal, meaning the upper and lower lobes are about equal in size. The spine ends at the caudal peduncle, distinguishing this group from those in which the spine extends into the upper lobe of the caudal fin.
| Habitat | Abyssobrotula galatheae | The world's deepest living fish, Abyssobrotula galatheae, a species of cusk eel, lives in the Puerto Rico Trench at a depth of 8372m (27,467feet). Due to the extreme pressure, this appears to be around the theoretical maximum depth possible for fish.[17] [18] | ||
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| Stone loach | At the other extreme, the Tibetan stone loach lives at altitudes over 5200m (17,100feet) in the Himalayas. | |||
| Blue shark | Some marine pelagic fish range over vast areas, such as the blue shark that lives in all oceans. | |||
| Blind cave fish | Other fish are confined to single, small living spaces, such as the blind cave fish in North America. | |||
| Death Valley pupfish | Equally isolated desert pupfish, like the Death Valley pupfish (pictured), live in small desert spring systems in Mexico and the southwest United States. | |||
| Thermichthys hollisi | height=50px | The bythitid vent fish Thermichthys hollisi lives around thermal vents 2400m (7,900feet) deep. | ||
| Sargassum frogfish | height=50px | The highly camouflaged sargassum frogfish lives in drifting sargassum seaweed. It has adapted fins which can grab strands of sargassum, enabling it to climb through the seaweed.[19] It avoids threats from larger predator fish by climbing out of water onto the surface of a seaweed mat, where it can survive for some time.[20] |
Some of the shortest-lived species are gobies, which are small coral reef–dwelling fish. Some of the longest-lived are rockfish.
| Life span | Seven-figure pygmy goby | Gobies, a type of small coral reef-dwelling fish (pictured), are some of the shortest lived fishes. The seven-figure pygmy goby is the shortest lived of all fish species. It lives at most for 59 days, which is the shortest lifespan for any vertebrate.[21] | ||
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| Ram cichlid | Short lived fish have particular value in genetic studies on aging. In particular, the ram cichlid is used in laboratory studies because of its ease of breeding and predictable aging pattern.[22] | |||
| Rougheye rockfish | Some of the longest living fishes are rockfish. The longest lived fish is the 205 years reported for the rougheye rockfish, Sebastes aleutianus (pictured). This fish is found offshore in the North Pacific at 25m-900mm (82feet-3,000feetm) and exhibits negligible senescence.[23] [24] | |||
| Orange roughy | The orange roughy may be the longest lived commercial fish, with a maximum reported age of 149 years. | |||
| Koi | There are stories about Japanese koi goldfish passed from generation to generation for 300 years. Scientists are sceptical. Counting growth lines on the scales of fish confined to ponds or bowls is unreliable, since they lay down extra lines.[25] The maximum reliably reported age for a goldfish is 41 years. | |||
| Atlantic tarpon | height=70px | One of the longest living sport fish is the Atlantic tarpon, with a reported age of 55 years.[26] | ||
| Green sturgeon | height=70px | Some of the longest living fish are living fossils, such as the green sturgeon. This species is among the longest-living species found in freshwater, with a reported age of 60 years. They are also among the largest fish species found in freshwater, with a maximum reported length of 2.5m (08.2feet) and a maximum reported weight of 159kg (351lb).[27] [28] [29] | ||
| Australian lungfish | height=70px | Another living fossil is the Australian lungfish. One individual has lived in an aquarium for at least 75 years, and is the oldest fish in captivity. According to fossil records, the Australian lungfish has hardly changed for 380 million years.[30] [31] | ||
| Greenland shark | height=70px | The Greenland shark has a lifespan of 392 ± 120 years. This is the longest known lifespan of all vertebrate species.[32] | ||
| Size | Paedocypris progenetica | align=center | Paedocypris progenetica, a type of minnow, is the smallest of all fish species. It lives in the dark-colored peat swamps of the Indonesian island of Sumatra. The females of this species have a standard length of 7.9mm at maturity.[33] [34] Until recently, this was the smallest of all known vertebrates. However, in 2012 a minute Papua New Guinea frog, Paedophryne amauensis, with a standard length of 7.7 mm (0.30 in) was discovered.[35] The slender Indonesian fish may still be the smallest vertebrate by weight. | |
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| Photocorynus spiniceps | Male individuals of the anglerfish species Photocorynus spiniceps are long at maturity, and thus could be claimed as an even smaller species. However, these males do not survive on their own merit, only by sexual parasitism on the larger female.[36] [37] [38] [39] | |||
| Stout infantfish | height=50px | The stout infantfish, a type of goby, is the second smallest known fish. Females grow to a length of 8.4mm and males are mature at . | ||
| Sinarapan | According to the Guinness Book of World Records, the sinarapan, also a goby, is the world's smallest commercially harvested fish.[40] Found in the Philippines, they have an average length of 12.5mm, and are threatened by overfishing. | |||
| Whale shark | The largest fish is the whale shark. It is a slow-moving, filter-feeding shark with a maximum published length of 20m (70feet) and a maximum weight of 34t. Whale sharks can live up to 70 years and are a vulnerable fish. | |||
| Ocean sunfish | align=center | The ocean sunfish is the heaviest bony fish. It can weigh up to 2300kg (5,100lb). It is found in all warm and temperate oceans. | ||
| King of herrings | align=center height=70px | The king of herrings is the longest bony fish. Its total length can reach 11m (36feet), and it can weigh up to 272kg (600lb). It is a rarely seen oarfish, found in all the world's oceans at depths of between 20m (70feet) and 1000m (3,000feet). | ||
| Mekong giant catfish | align=center height=70px | The largest recorded freshwater fish is a Mekong giant catfish caught in 2010, weighing .[41] [42] The Mekong giant catfish is critically endangered. |
| Breeding | Grouper | align=center | Female groupers change their sex to male if no male is available. Grouper are protogynous hermaphrodites, who school in harems of three to fifteen females. When no male is available, the most aggressive and largest females change their sex to male. | |
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| Toadfish | align=center | Male toadfish "sing" at up to 100 decibels with their swim bladders to attract mates. | ||
| Anglerfish | align=center | Female Haplophryne mollis anglerfish trailing atrophied males she encountered (pictured).[44] The female anglerfish releases pheromones to attract tiny males. When a male finds her, he bites on to her and holds on. When a male of the anglerfish species Haplophryne mollis bites into the skin of a female, he releases an enzyme that digests the skin of his mouth and her body, fusing the pair to the point where the two circulatory systems join up. The male then atrophies into nothing more than a pair of gonads. This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available.[45] | ||
| Hammerheads | align=center | Some sharks such as hammerheads are able to breed parthenogenetically, a type of asexual reproduction where the growth and development of embryos occur without fertilization. |
| Brooding | Chain catshark | align=center | The chain catshark is oviparous, laying its eggs to hatch in the water. | |
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| Great white shark | align=center | The great white shark is ovoviviparous, gestating eggs in the uterus for 11 months before giving birth. | ||
| Scalloped hammerhead | align=center | The scalloped hammerhead is viviparous, bearing its young after nourishing hatchlings internally. | ||
| Cyphotilapia frontosa | align=center | The female Cyphotilapia frontosa mouthbroods its fry. The fry can be seen looking out of her mouth. | ||
| Seahorses | align=center | Seahorse males practice pouch-brooding similar to kangaroos. When seahorses mate, the female deposits her eggs into a special pouch on the male's belly. The pouch seals shut while he nurtures the developing eggs. Once the eggs hatch, the pouch opens and the male goes into labour. |
| Feeding | Anglerfish | Anglerfish are lie-in-wait ambush predators. The first spine of their dorsal fin has been modified so it can be used like a fishing line with a lure at the end. Most anglerfish, like the one pictured, live in the darkness of the deep sea and have a bioluminescent lure.[51] | |
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| Archerfish | Archerfish prey on land-based insects and other small animals by shooting them down with water droplets from their specialized mouths. Archerfish are remarkably accurate; adults almost always hit the target on the first shot. They can bring down arthropods such as grasshoppers,[52] spiders and butterflies on a branch of an overhanging tree[53] above the water's surface.[54] This is partially due to good eyesight, but also due to their ability to compensate for light refraction when aiming.[55] | ||
| Triggerfish | Triggerfish also use jets of water to uncover sand dollars buried in sand or overturn sea urchins. | ||
| Silver arowana | Other fish have developed extreme specializations. Silver arowana, also called monkey fish, can leap two meters out of the water to capture prey. They usually swim near the surface of the water waiting for potential prey. Their main diet consists of crustaceans, insects, smaller fishes and other animals that float on the water surface, for which its draw-bridge-like mouth is exclusively adapted for feeding. The remains of small birds, bats, and snakes have also been found in their stomachs. | ||
| Cookiecutter shark | The cookiecutter shark is a small dogfish which derives its name from the way it removes small circular plugs, looking as though cut with a cookie cutter, from the flesh and skin of cetaceans and larger fish, including other sharks. The cookiecutter attaches to its larger prey with its suctorial lips, and then protrudes its teeth to remove a symmetrical scoop of flesh.[56] Pictured is a pomfret with bite wounds from a cookiecutter shark. | ||
| Striped bass | Striped bass eat smaller fish. | ||
| Chinese algae eater | Chinese algae eaters are kept in aquaria to control algae. | ||
| Emperor angelfish | The Emperor angelfish feeds on coral sponges. | ||
| Herring | Schooling herrings ram feed on copepods. | ||
| Mangrove jack | The mangrove jack eats crustaceans. | ||
| Puffer fish | Many puffer fish species crush the shells of molluscs. | ||
| Bucktoothed tetra | The bucktoothed tetra eats scales off other fishes (lepidophagy) and molluscs. | ||
| Cleaner fish | These two small wrasses are cleaner fish, which eat parasites off other fish. | ||
| Cleaning station | A reef manta ray at a cleaning station, maintaining a near stationary position atop a coral patch for several minutes while being cleaned by cleaner fishes.[57] | ||
| Doctor fish | Doctor fish nibbling on the diseased skin of patients. Doctor fish (nibble fish) live and breed in the outdoor pools of some Turkish spas, where they feed on the skin of patients with psoriasis. The fish are like cleaner fish in that they only consume the affected and dead areas of the skin, leaving the healthy skin to recover. |
| Vision | Four-eyed fish | The four-eyed fish feeds at the surface of the water with eyes that allow it to see above and below the surface at the same time. Four-eyed fish have two specially-adapted eyes which are raised above the top of their head. The eyes are divided in two different parts, and the fish floats at the water surface with only the lower half of each eye underwater. The two halves are divided by a band of tissue and the eye has two pupils, connected by part of the iris. The upper half of the eye is adapted for vision in air, while the lower half is adapted for vision in water.[59] The lens of the eye also changes in thickness top to bottom to account for the difference in the refractive indices of air versus water. Their diet mostly consists of the terrestrial insects which are available at the surface, where they spend most of their time. | |
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| Two stripe damselfish | The two stripe damselfish, Dascyllus reticulatus, has ultraviolet-reflecting colouration which they appear to use as an alarm signal to other fish of their species.[60] Predatory species cannot see this if their vision is not sensitive to ultraviolet. There is further evidence for this view that some fish use ultraviolet as a "high-fidelity secret communication channel hidden from predators", while yet other species use ultraviolet to make social or sexual signals.[61] [62] | ||
| Barreleye | Barreleyes are a family of small, unusual-looking mesopelagic fishes, named for their barrel-shaped, tubular, telescopic eyes which are generally directed upwards to detect the silhouettes of available prey. The eyes, which dominate and protrude from the skull, may be swivelled forwards in some species. Their eyes have a large lens and a retina with an exceptional number of rod cells and a high density of rhodopsin (the "visual purple" pigment); there are no cone cells. The barreleye species Macropinna microstoma has a transparent protective dome over the top of its head, somewhat like the dome over an airplane cockpit, through which the lenses of its eyes can be seen. The dome is tough and flexible, and presumably protects the eyes from the nematocysts (stinging cells) of the siphonophores from which it is believed the barreleye steals food.[63] [64] | ||
| Flashlight fish | Flashlight fish use a retroreflector behind the retina and photophores to detect eyeshine in other fish.[65] [66] [67] | ||
Boxfishes have heavily armoured plate-like scales fused into a solid, triangular, boxlike carapace, from which the fins, tail, eyes and mouth protrude. Because of this heavy armour, boxfish move slowly, but few other fish are able to eat the adults.[68]
| Toxicity | Puffer fish | The puffer fish is the most poisonous fish in the world. It is the second most poisonous vertebrate after the golden dart frog. It paralyzes the diaphragm muscles of human victims, who can die from suffocation. In Japan, skilled chefs use parts of a closely related species, the blowfish, to create a delicacy called "fugu", including just enough toxin for that "special flavour". | |
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| Spotted trunkfish | align=center | The spotted trunkfish, a reef fish, secretes a colourless ciguatera toxin from glands on its skin when touched. The toxin is only dangerous when ingested, so the fish poses no immediate risk to human divers. However, predators as large as nurse sharks can die from eating a trunkfish. | |
| Giant moray | align=center | The giant moray is a reef fish at the top of the food chain. Like many other apex reef fish, it is likely to cause ciguatera poisoning if eaten.[79] Outbreaks of ciguatera poisoning in the 11th to 15th centuries from large, carnivorous reef fish, caused by harmful algal blooms, could be a reason why Polynesians migrated to Easter Island, New Zealand, and possibly Hawaii.[80] [81] | |
| Reef stonefish | align=center | The most venomous known fish is the reef stonefish.[82] It has a remarkable ability to camouflage itself amongst rocks. It is an ambush predator that sits on the bottom waiting for prey to approach. Instead of swimming away if disturbed, it erects the 13 venomous spines along its back. For defense, it can shoot venom from each or all of these spines. Each spine is like a hypodermic needle, delivering the venom from two sacs attached to the spine. The stonefish has control over whether to shoot its venom, and does so when provoked or frightened. The venom results in severe pain, paralysis and tissue death, and can be fatal if not treated. Despite its formidable defenses, stonefish have predators. Some bottom feeding rays and sharks with crushing teeth feed on them, as does the Stokes's sea snake.[83] | |
| Lionfish | Head on view of the lionfish, a venomous coral reef fish (pictured). Unlike stonefish, a lionfish can release venom only if something strikes its spines. Although not native to the U.S. coast, lionfish have appeared around Florida and have spread up the coast to New York. They are attractive aquarium fish, sometimes used to stock ponds, and may have been washed into the sea during a hurricane. Lionfish can aggressively dart at scuba divers and attempt to puncture their facemask with their venomous spines. | ||
| Stargazer | The stargazer, Uranoscopus sulphureus. The stargazer buries itself and can deliver electric shocks as well as venom. It is a delicacy in some cultures (cooking destroys the venom), and can be found for sale in some fish markets with the electric organ removed. They have been called "the meanest things in creation". | ||
| Stingray | Stingrays can sting with their stinger (pictured). Such envenomations can occur to people who wade in shallow water and tread on them. This can be avoided by shuffling through the sand or stamping on the bottom, as the rays detect this and swim away. The stinger usually breaks off in the wound. It is barbed, so it can easily penetrate but cannot be easily removed. The stinger causes local trauma from the cut itself, pain and swelling from the venom, and possible later infection from bacteria. Occasionally, severed arteries or death can result.[84] |
| Human use | Yellowfin tuna | Yellowfin tuna are now being fished as a replacement for the depleted southern bluefin tuna. | ||
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| Anchovy | align=center | These schooling anchovy are forage fish. | ||
| Atlantic cod | align=center | Atlantic cod fisheries have collapsed. | ||
| Alaska pollock | align=center | The Alaska pollock has been described as "the largest remaining source of palatable fish in the world".[88] | ||
| Koi | align=center | Koi (and goldfish) have been kept in decorative ponds for centuries in China and Japan. |
| Other | Bony-eared assfish | align=center | Fish hold the records for the relative brain weights of vertebrates. Most vertebrate species have similar brain-to-body weight ratios. The deep sea bathypelagic bony-eared assfish has the smallest ratio of all known vertebrates.[89] | |
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| Elephantnose fish | align=center | At the other extreme, the elephantnose fish, an African freshwater fish, has an exceptionally large brain-to-body weight ratio. These fish have the largest brain-to-body oxygen consumption ratio of all known vertebrates.[90] | ||
| Hallucinogenic fish | align=center | The hallucinogenic dream fish, Sarpa salpa, a species of bream recognizable by the golden stripes running the length of its body, can induce LSD-like hallucinations if it is eaten. These widely distributed coastal fish became a recreational drug during the Roman Empire, and are called "the fish that make dreams" in Arabic. Other hallucinogenic fish are Siganus spinus, called "the fish that inebriates" in Reunion Island, and Mulloidichthys samoensis, called "the chief of ghosts" in Hawaii.[91] | ||
| Nopoli rockclimbing goby | align=center | The Nopoli rockclimbing goby uses its mouth to climb waterfalls by inching up rocks like a caterpillar, using its mouth as a sucker together with another sucker on its stomach. When the fish is young, it undergoes a radical transformation when it moves from saltwater to a freshwater stream. The mouth migrates over a period of two days from the front of its head to its chin. This allows the fish to feed by scraping algae from rocks. Pictured is the goby before and after the transformation.[92] [93] | ||
| Vampire fish | Smaller species of vampire fish, native to the Amazon River, have an alleged tendency to burrow into and parasitise the human urethra. However, despite ethnological reports dating back to the late 19th century, the first documented case of the removal of a vampire fish from a human urethra did not occur until 1997, and even that incident has remained a matter of controversy.[94] |