Diversity of 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 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 paraphyletic, since the tetrapod clade is within the clade of lobe-finned fishes.

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.

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 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.

By habitat

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. Due to the extreme pressure, this appears to be around the theoretical maximum depth possible for fish.
Habitat	Stone loach		At the other extreme, the Tibetan stone loach lives at altitudes over in the Himalayas.
Habitat	Blue shark		Some marine pelagic fish range over vast areas, such as the blue shark that lives in all oceans.
Habitat	Blind cave fish		Other fish are confined to single, small living spaces, such as the blind cave fish in North America.
Habitat	Death Valley pupfish		Equally isolated desert pupfish, like the Death Valley pupfish , live in small desert spring systems in Mexico and the southwest United States.
Habitat	Thermichthys hollisi		The bythitid vent fish Thermichthys hollisi lives around thermal vents deep.
Habitat	Sargassum frogfish		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. 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

Breeding	Grouper		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.
Breeding	Toadfish		Male toadfish "sing" at up to 100 decibels with their swim bladders to attract mates.
Breeding	Anglerfish		Female 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.
Breeding	Hammerheads		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.
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By brooding behavior

Brooding	Chain catshark		The chain catshark is oviparous, laying its eggs to hatch in the water.
Brooding	Great white shark		The great white shark is ovoviviparous, gestating eggs in the uterus for 11 months before giving birth.
Brooding	Scalloped hammerhead		The scalloped hammerhead is viviparous, bearing its young after nourishing hatchlings internally.
Brooding	Cyphotilapia frontosa		The female Cyphotilapia frontosa mouthbroods its fry. The fry can be seen looking out of her mouth.
Brooding	Seahorses		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.
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