Diversity of fish


are very diverse animals and can be categorised in many ways. This article is an overview of some of ways in which fish are categorised. Although most fish species have probably been discovered and described, about 250 new ones are still discovered every year. According to FishBase, 33,100 species of fish had been described as of April 2015. That 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 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.

By taxonomy

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,, that has gills throughout life and has limbs, if any, in the shape of fins. Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa, including jawless, cartilaginous and skeletal types.

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. It has four hearts, two brains, and a paddle-like tail.

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 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. 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.
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

s are the most advanced or "modern" fishes. They are overwhelmingly the dominant class of fishes with nearly 30,000 species, covering about 96 percent 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.
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. 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.

By habitat

HabitatAbyssobrotula galatheaeThe world's deepest living fish, Abyssobrotula galatheae, a species of cusk eel, lives in the Puerto Rico Trench at a depth of. Due to the extreme pressure, this appears to be around the theoretical maximum depth possible for fish.
HabitatStone loachAt the other extreme, the Tibetan stone loach lives at altitudes over in the Himalayas.
HabitatBlue sharkSome marine pelagic fish range over vast areas, such as the blue shark that lives in all oceans.
HabitatBlind cave fishOther fish are confined to single, small living spaces, such as the blind cave fish in North America.
HabitatDeath Valley
pupfish		Equally isolated desert pupfish, like the Death Valley pupfish , live in small desert spring systems in Mexico and the southwest United States.
HabitatThermichthys
hollisi		The bythitid vent fish Thermichthys hollisi lives around thermal vents deep.
HabitatSargassum frogfishThe 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. 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.
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By life span

Some of the shortest-lived species are gobies, which are small coral reef–dwelling fish. Some of the longest-lived are rockfish.

By size

By breeding behavior

BreedingGrouperFemale 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.
BreedingToadfishMale toadfish "sing" at up to 100 decibels with their swim bladders to attract mates.
BreedingAnglerfishFemale Haplophryne mollis anglerfish trailing atrophied males she encountered . 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.
BreedingHammerheadsSome sharks such as hammerheads are able to breed parthogenetically, a type of asexual reproduction where the growth and development of embryos occur without fertilization.
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By brooding behavior

BroodingChain catsharkThe chain catshark is oviparous, laying its eggs to hatch in the water.
BroodingGreat white sharkThe great white shark is ovoviviparous, gestating eggs in the uterus for 11 months before giving birth.
BroodingScalloped hammerheadThe scalloped hammerhead is viviparous, bearing its young after nourishing hatchlings internally.
BroodingCyphotilapia frontosaThe female Cyphotilapia frontosa mouthbroods its fry. The fry can be seen looking out of her mouth.
BroodingSeahorsesSeahorse 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.
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By feeding behaviour

FeedingAnglerfishAnglerfish 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.
FeedingArcherfishArcherfish 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, spiders and butterflies on a branch of an overhanging tree above the water's surface. This is partially due to good eyesight, but also due to their ability to compensate for light refraction when aiming.
FeedingTriggerfishTriggerfish also use jets of water to uncover sand dollars buried in sand or overturn sea urchins.
FeedingSilver arowanaOther 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.
FeedingCookiecutter sharkThe 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. Pictured is a pomfret with bite wounds from a cookiecutter shark.
FeedingStriped bassStriped bass eat smaller fish.
FeedingChinese algae eaterChinese algae eaters are kept in aquaria to control algae.
FeedingEmperor angelfishThe Emperor angelfish feeds on coral sponges.
FeedingHerringSchooling herrings ram feed on copepods.
FeedingMangrove jackThe mangrove jack eats crustaceans.
FeedingPuffer fishMany puffer fish species crush the shells of molluscs.
FeedingBucktoothed tetraThe bucktoothed tetra eats scales off other fishes and molluscs.
FeedingCleaner fishThese two small wrasses are cleaner fish, which eat parasites off other fish.
FeedingCleaning stationA 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.
FeedingDoctor fishDoctor fish nibbling on the diseased skin of patients. Doctor 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.

By vision

VisionFour-eyed fishThe 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. 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.
VisionTwo stripe damselfishThe two stripe damselfish, Dascyllus reticulatus, has ultraviolet-reflecting colouration which they appear to use as an alarm signal to other fish of their species. 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.
VisionBarreleyeBarreleyes 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 ; 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 of the siphonophores from which it is believed the barreleye steals food.
VisionFlashlight fishFlashlight fish use a retroreflector behind the retina and photophores to detect eyeshine in other fish.
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By shape

es 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.

By locomotion

Loco-
motion		Dwarf seahorseThe slowest-moving fishes are the sea horses. The slowest of these, the tiny dwarf seahorse, has a sprint speed of one inch per minute.
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motion		Atlantic bluefin tunaThe Atlantic bluefin tuna is capable of sustained high speed cruising, and maintains high muscle temperatures so it can cruise in relatively cold waters.
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motion		Indo-Pacific sailfish
Among the fastesr sprinters are the Indo-Pacific sailfish ' and the black marlin '. Both have been recorded in a burst at over. For the sailfish, that is equivalent to 12 to 15 times their own length per second.
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motion		Shortfin mako
The shortfin mako shark is fast and agile enough to chase down and kill an adult swordfish. However, sometimes in the struggle the swordfish kills the shark by ramming it in the gills or belly. The shortfin mako's speed has been recorded at, and there are reports that it can achieve bursts of up to. It can jump up to in the air. Due to its speed and agility, this high-leaping fish is sought as game worldwide. This shark is highly migratory. Its exothermic constitution partly accounts for its relatively great speed.
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motion		WahooThe wahoo is perhaps the fastest fish for its size, attaining a speed of 19 lengths per second, reaching.
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motion		Flying fish
Flying fish have unusually large pectoral fins, which enable the fish to take short gliding flights above the surface of the water in order to escape from predators. Their glides are typically around, but they can use updrafts at the leading edge of waves to cover distances of at least. In May 2008, a flying fish was filmed off the coast of Japan. The fish spent 45 seconds aloft, and was able to stay aloft by occasionally beating the surface of the water with its caudal fin. The previous record was 42 seconds.
Loco-
motion		Climbing perchesClimbing perches are a family of fishes which have the ability to climb out of water and "walk" short distances. As labyrinth fishes, they possess a labyrinth organ, a structure in the fish's head which allows it to breathe atmospheric oxygen. Their method of terrestrial locomotion uses the gill plates as supports, and the fish pushes itself using its fins and tail.
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motion		Mudskipper
The mudskipper is another type of walking fish. Walking fish are often amphibious and can travel over land for extended periods of time. These fish may use a number of means of locomotion, including springing, snake-like lateral undulation, and tripod-like walking. The mudskipper is able to spend days moving about out of water and can even climb mangroves, although to only modest heights. There are some species of fish that can "walk" along the sea floor but not on land. One such animal is the flying gurnard.
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motion		Handfish
The handfish walks along the seafloor using its pectoral fins, which look like hands.
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By toxicity

ToxicityPuffer fishThe 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".
ToxicitySpotted trunkfishThe 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.
ToxicityGiant morayThe 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. 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.
ToxicityReef stonefishThe most venomous known fish is the reef stonefish. 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.
ToxicityLionfishHead on view of the lionfish, a venomous coral reef fish '. 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.
ToxicityStargazerThe stargazer, Uranoscopus sulphureus. The stargazer buries itself and can deliver electric shocks as well as venom. It is a delicacy in some cultures, and can be found for sale in some fish markets with the electric organ removed. They have been called "the meanest things in creation".
ToxicityStingrayStingrays can sting with their stinger '. 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.
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By human use

Human
use		Yellowfin tunaYellowfin tuna are now being fished as a replacement for the depleted southern bluefin tuna.
Human
use		AnchovyThese schooling anchovy are forage fish.
Human
use		Atlantic codAtlantic cod fisheries have collapsed.
Human
use		Alaska pollockThe Alaska pollock has been described as "the largest remaining source of palatable fish in the world".
Human
use		KoiKoi have been kept in decorative ponds for centuries in China and Japan.
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By vulnerability