Image by/from Wendy Rathey
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.
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 Cantao State Park, can contain more freshwater fish species than occur in all of Europe.
lampreys
hagfish
sharks
rays
chimaera
lungfish
coelacanths
chondrosteans
holosteans
teleosts
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. 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.
Lampreys attached to a lake trout
Mouth of a sea lamprey
Pacific hagfish resting on bottom at 280 m
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.
Tiger shark
Whale shark
Stingray
This elephant fish is a 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 ("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.
Lungfish can breathe in air as well as water
Model of a coelacanth, thought until 1938 to be extinct. They are deep blue.
This Atlantic sturgeon is a chondrostean
This bowfin is a holostean
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.
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.
Swordfish are teleosts
Rose fish are also teleosts
Eels are teleosts too
So are seahorses
There is 10,000 times as much saltwater in the oceans as there is freshwater in the lakes and rivers. However, only 58 per cent of extant fish species live in saltwater. A disproportionate 41 per cent are freshwater fish (the remaining one per cent are anadromous). This diversity in freshwater species is perhaps not surprising, since the thousands of separate lake habitats promote speciation.
Fish can also be demersal or pelagic. Demersal fish live on or near the bottom of oceans and lakes, while pelagic fish inhabit the water column away from the bottom. Habitats can also be vertically stratified. Epipelagic fish occupy sunlit waters down to 200 metres (110 fathoms), mesopelagic fish occupying deeper twilight waters down to 1,000 meters (3,300 ft), and bathypelagic fish inhabiting the cold and pitch black depths below.
Most oceanic species (78 per cent, or 44 per cent of all fish species), live near the shoreline. These coastal fish live on or above the relatively shallow continental shelf. Only 13 per cent of all fish species live in the open ocean, off the shelf. Of these, 1 per cent are epipelagic, 5 per cent are pelagic, and 7 per cent are deep water.
Fish are found in nearly all natural aquatic environments. Most fish, whether by species count or abundance, live in warmer environments with relatively stable temperatures. However, some species survive temperatures up to 44.6 °C (112.3 °F), while others cope with colder waters; there are over 200 finfish species south of the Antarctic Convergence. Some fish species tolerate salinities over 10 per cent.
Some of the shortest-lived species are gobies, which are small coral reef-dwelling fish. Some of the longest-lived are rockfish.
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.
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 7.9 mm (0.31 in) at maturity. 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. The slender Indonesian fish may still be the smallest vertebrate by weight.
In very deep waters, it is not easy for a fish to find a mate. There is no light, so some species depend on bioluminescence. Others are hermaphrodites, which doubles their chances of producing both eggs and sperm when an encounter does occur.
Fish adopt a variety of strategies for nurturing their brood. Sharks, for example, variously follow three protocols with their brood. Most sharks, including lamniformes, are ovoviviparous, bearing their young after the brood nourish themselves, both after hatching and before birth, by consuming the remnants of the yolk and other available nutrients. Some, such as hammerheads, are viviparous, bearing their young after nourishing hatchlings internally, analogously to mammalian gestation. Catsharks and others are oviparous, laying their eggs to hatch in the water.
Some animals, predominantly fish such as cardinalfish, practice mouthbrooding, caring for their offspring by holding them in the mouth of a parent for extended periods of time. Mouthbrooding has evolved independently in several different families of fish.
There are three basic methods by which food is gathered into the mouths of fish: by suction feeding, by ram feeding, and by manipulation or biting. Nearly all fish species use one of these styles, and most use two.
Early fish lineages had inflexible jaws limited to little more than opening and closing. Modern teleosts have evolved protusible jaws that can reach out to engulf prey. An extreme example is the protusible jaw of the slingjaw wrasse. Its mouth extends into a tube half as long as its body, which creates a strong suction to catch prey. The extended mouth tucks away under its body when not in use.
In practice, feeding modes lie on a spectrum, with suction and ram feeding at the extremes. Many fish capture their prey using both suction pressure combined with a forward motion of the body or jaw.
Most fish are food opportunists, or generalists. They eat whatever is most easily available. For example, the blue shark feeds on dead whales and nearly everything else that wriggles: other fish, cephalopods, gastropods, ascidians, or crustaceans. Ocean sunfish prefer jellyfish.
Many species of fish can see the ultraviolet end of the spectrum, beyond the violet wavelength of visible light.
Mesopelagic fishes live in the deeper waters of the twilight zone, down to depths of 1000 metres, where the amount of sunlight available is not sufficient to support photosynthesis. These fish are adapted for an active life under low light conditions.
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.
The humpback turretfish is a boxfish with an armoured triangular shaped body
The leafy sea dragon is camouflaged to look like floating seaweed
A number of species jump while swimming near the surface, skimming the water. Other species walk along the bottom on their fins.
Among the fastesr sprinters are the Indo-Pacific sailfish (left) and the black marlin (right). Both have been recorded in a burst at over 110 kilometres per hour (68 mph). For the sailfish, that is equivalent to 12 to 15 times their own length per second.
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 50 kilometres per hour (31 mph), and there are reports that it can achieve bursts of up to 74 kilometres per hour (46 mph). It can jump up to 9 meters (30 ft) 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.
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 50 meters (160 ft), but they can use updrafts at the leading edge of waves to cover distances of at least 400 meters (1,300 ft). In May 2008, a flying fish was filmed off the coast of Japan (see video). The fish spent 45 seconds aloft, and was able to stay aloft by occasionally beating the surface of the water with its caudal (tail) fin. The previous record was 42 seconds.
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.
The handfish walks along the seafloor using its pectoral fins, which look like hands.
Deepsea tripod fishes use their very elongate pelvic fins and caudal fin, which act like "stilts", to perch and walk on the seafloor.
A 2006 study found that there are at least 1200 species of venomous fish. There are more venomous fish than venomous snakes. In fact, there are more venomous fish than the combined total of all other venomous vertebrates. Venomous fish are found in almost all habitats around the world, but mostly in tropical waters. They wound over 50,000 people every year.
Venomous fish carry their venom in venom glands and use various delivery systems, such as spines, sharp fins, barbs, spikes or fangs. Venomous fish tend to be either very visible, using flamboyant colors to warn enemies, or skilfully camouflaged and may be buried in the sand. Apart from the defense or hunting value, venom helps bottom-dwelling fish by killing the bacteria that tries to invade their skin. Few of these venoms have been studied. They are a yet-to-be-tapped resource for bioprospecting to find drugs with medical uses.
Treatment for venom stings usually includes the application of heat, using water at temperatures of about 45 °C (113 °F), since heat breaks down most complex venom proteins.
Fish are sought after by humans for their value as commercial food fish, recreational sport fish, decorative aquarium fish and for tourism, as they attract snorkelers and scuba divers.
Throughout human history, important fisheries have been based on forage fish. Forage fish are small fish which are eaten by larger predators. They usually school together for protection. Typical ocean forage fish feed near the bottom of the food chain on plankton, often by filter feeding. They include the family Clupeidae (herrings, sardines, menhaden, hilsa, shad and sprats), as well as anchovies, capelin and halfbeaks. Important herring fisheries have existed for centuries in the North Atlantic and the North Sea. Likewise, important traditional for anchovy and sardine fisheries have operated in the Pacific, the Mediterranean, and the southeast Atlantic. The world annual catch of forage fish in recent years has been around 25 million tonnes, or one quarter of the world's total catch.
Higher in the food chain, Gadidae (cod, pollock, haddock, saithe, hake and whiting) also support important fisheries. Concentrated initially in the North Sea, Atlantic cod was one of Europe's oldest fisheries, later extending to the Grand Banks. Declining numbers led to international "cod wars" and eventually the virtual abandonment of these fisheries. In modern times, the Alaska pollock supports an important fishery in the Bering Sea and the north Pacific, yielding about 6 million tonnes, while cod amounts to about 9 million tonnes.
Recreational and sport fishing is big business U.S. saltwater fishers spend about $30 billion annually and support 350,000 jobs. Some of the more popular recreational and sport fish include bass, marlin, porgie, shad, mahi-mahi, smelt whiting, swordfish, and walleye.
Fishkeeping is another popular pastime, and there is a large international trade for aquarium fish.
Snorkeling and scuba diving attracts millions of people to beaches, coral reefs, lakes, and other bodies of water to view fish and other marine life.