Fish: Types, History, Habitats, Biodiversity, Definitions

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Actinopterygii (Ray-finned fish) Wikipedia

Fish are aquatic vertebrates (animals with backbones). Nearly all are cold-blooded. Most have a long body covered with scales, breath oxygen from the water with gills and have paired fins rather than arms and legs. Amphibians such as frogs, toads and salamanders can have backbones and can breath with gills in the early stages of their life. They can be distinguished from fish by the fact they have arms and legs and fish have fins.

Fish are the most diverse group among the vertebrates, with over 33,000 different types of fish species. “Fish” is a term that describes ray-finned fishes, sharks, lampreys, and coelacanths, but is not a taxonomic group. Fish are the most diverse group among the vertebrates, There are around 81,000 known fish and lower chordates species. Of these there about half are ray-finned fish. There are around 20,000 species of fish in the oceans. Scientists believe there may be around 20,000 more undiscovered fish species. New species of fish are being discovered at a rate of several hundred a year.

The United Nations Convention on International Trade on Endangered Species (CITES), the United Nations wildlife trade body, covers fish and ocean creatures as well as land animals. Among the species that are listed are great white sharks, whale sharks and seahorses.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium ; MarineBio

Types of Fish

The three taxonomic groups of which fish belong are 1) bony fish (Osteichthyes), 2) jawless fish (Agnatha), such as lampreys, and 3) cartilaginous fish, which have cartilage instead of bones; (Chondrichthyes, mostly sharks and rays). There are two kinds of bony fish: A) ray-finned fish (actinopterygians); and B) lobe-finned fish (Sarcopterygii). Nearly all bony fish are ray-finned fish. Coelacanths are a lobe-fin fish..[Source: Nicholas White, Animal Diversity Web (ADW) /=]

The name Sarcopterygii is derived from Greek “sárx” (flesh) and ptérux (wing, fins). The group Tetrapoda, a mostly terrestrial superclass including amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (mammals) evolved from certain sarcopterygians. Some scientists group tetrapods as a subgroup within Sarcopterygii. The only known living non-tetrapod sarcopterygians are two species of coelacanths and six species of lungfishes. /=\

The largest fish is the whale shark (See Sharks) The smallest fish is the quarter-inch-long south infantfish discovered the Great barrier Reef in 1979. The spiny-headed devilfish is the world’s shortest fish. An adult male is 0.24 inches long. The south infantfish is the world’s second shortest fish at.27 inches and the world’s lightest fish, weighing 1/500,000th of a pound. There are 700 species of poisonous fish.

First Fish — from the Pre-Dinosaur Era


Isaac Skromne of the University of Richmond wrote n The Conversation; The oldest fossils of animals resembling a fish date back between 518 million and 530 million years ago. Discovered in China and called Haikouichthys, these animals were about an inch long (2.5 centimeters) and had a head with seven to eight slits at its base that looked like gills. They also had a distinct spine surrounded by muscles. [Source: Isaac Skromne, Assistant Professor of Biology, University of Richmond, The Conversation, July 18, 2022]

But there are ways Haikouichthys did not resemble any modern fish. For example, they didn’t have a jaw. Instead, their mouth was a cone-like opening similar to the ones seen in modern hagfish and lampreys. They also appear not to have had side fins.

Whether the Haikouichthys was the first fish or not remains controversial. There are very few other fishlike fossils from the same time period. But paleontologists keep digging. Who knows, maybe in a few years they will discover an even older fishlike animal that will dethrone Haikouichthys as the oldest fishlike creature.

Ray-finned fish (by far the most common of fish today) appeared in the fossil record during the Devonian period, between 400-350 million years ago but it wasn’t until the Carboniferous period 360 million years ago that they had become dominant in freshwaters and started to invade the seas.

Early History of Fish

The ancestors of fish were sea squirts and lamprey. The later evolved around 550 million years ago and are still around today. It has a primitive sort of backbone, two eyes, a single nostril and a tongue with sharp spines instead of a jaw. It latches itself onto the flanks of fish and literally eats them alive.

Early proto fish that appeared around 500 millions ago were the size of large minnows. They were heavily armored, often completely covered in bony plates. They had two eyes and a single nostril and no jaw like the lamprey and moved around by beating a tail that emerged from the armor. They had no fins and fed themselves by nuzzling along the ocean floor, sucking up mud and sand and filtering out edible parts. Slits on the sides of their throat were used to filter mud and acted as gills.

Over time proto fish grew in size and increased in variety. They sprouted dorsal fins which gave them stability but no paired lateral fins that made it possible to maneuver around with ease. For a hundred million years fish existed in this form.

Later History of Fish

Around 400 million years ago, proto-fish began to develop jaws from filtering slits and primitive gills. The bony plats which covered them developed into teeth. This freed them from nuzzling in the mud and transformed them to creatures that could bite in the open sea. Over time flaps of skin grew out of the sides of their bodies and these developed into lateral fins.

cetacean vertebra bitten
by Megalodon
Over more time the first true fish shed their bony plates and developed an internal skeleton with a vertebrae column running through the body. Most had two pairs of lateral fins: the pectorals behind the throat and the pelvis fins near the anus. Over yet more time the bony plates on the bodies evolved into scales and the fish developed swim-bladders.

Also around 400 million years ago fish began to evolve into two separate families: 1) bony fish; and 2) cartilaginous fish like sharks and rays who lost much of their internal skeletons and instead supported their bodies with cartilage. A fairly complete fossil of a 30-centimeter-long bony fish that lived 418 million years was found in southern China in the mid 2000s. It has a jaw and fat, fleshy lobe fins as opposed to thin ray fins found on most modern fish.

The seas 400 million years ago were inhabited by all kinds of wild creatures, including sea scorpions that were twice the size of a man and had claws the size of a human head. A fossil of one such creature was found in 2007 in 390-million-year-old rocks in Germany. The creature, which had a 2.5-meter-long body, was sort of like a giant water bug — the largest known bug that ever lived — and is thought to be ancestor of modern scorpions, and perhaps spiders too. Ocean Dinosaurs and Sea Creatures Move to Land

Tiktaalik Rosae — the Link Between Sea Creatures and Land Animals

Tiktaalik Rosae is regarded as a transition fossil between sea creatures and land animals. Dated to 375 million years ago and discovered on Canada’s Ellsmere Island, it has fins like an ancestral fish but its pectoral fin contains arm bones like those of land-dwelling animals. With a bendable shoulder and elbow, plus a proto-wrist, this fin could support the body and propel it on land. Its ribs and limb bones resemble those of later four-legged amphibians and other terrestrial species.

The announcement of the discovery of Tiktaalik Rosae was made in an April 2006 article published in Nature by team of researchers lead by Neil Shubin of the University of Chicago. The scaly creature was 1.2 to 2.7 meter long and is seen as a link between scaley, armored fish like Eusthenopteron, which lived 385 million years, and Icthyostega, a land creature that lived around the same time and looked like a cross between a Komodo dragon and a frog.

Between 230 and 90 million years ago, fish-like predatory dinosaurs called ichthyosaurs cruised the seas, Although they were reptiles their body and fin shapes were very much like that of modern predatory fish and sea mammals such as bluefin tuna, great white sharks and dolphins. A detailed fossil imprint of ichthyosaur skin reveal their skin contained multiple layers of fiber bundles like those found on dolphins, sharks and tuna.

Studying Fish

20120521-tiger shark caught by scientists.jpg
tiger shark caught by scientists
Large fish can be fitted with devises that measure water and body temperature, depth and ambient sunlight. Tiny memory chips store data. The problem with devises is that have been implanted in a target fish they have to be retrieved. Though retrieval rates for some species like bluefin tuna are as high as 30 percent the rates for other species are often quite low.

Pop up tags record data and release from the animal after a set amount time, at which time they float to the surface of the sea and send data using satellites and the global positioning system. The retrieval rate of these tags is as high 90 percent but glitches often prevent scientist from getting more than 80 percent of the expected data.

Some tags give scientist insights into when animals eat and when they mate. Ambient sunlight allows researchers to fix longitude. Latitude can be determined by light and surface temperature readings. With bluefin tuna scientists can tell when they eat because the food they eat is colder than themselves, registering a drop in body temperature that is flowed by warming as the fish digests the food.

New magnetic sensors tell scientist what fish have eaten by measuring how wide the fish open their mouths, an indicator of prey size. Real time tags have been placed on the dorsal fins of fish that spend a lot of time at the surface such as salmon, blue and mako sharks, with the exposed tags transmitting data when ever the tags are above the surface.

See Studying Sharks

Pelagic Fish

Pelagic fish inhabit coastal waters, open oceans, and lakes. They do not live near the bottom or the shore. Pelagic fish can be categorized as coastal and oceanic fish, based on the depth of the water they inhabit. Coastal pelagic fish inhabit sunlit waters up to about 655 feet deep, typically above the continental shelf. Examples of species include forage fish such as anchovies, sardines, shad, and menhaden and the predatory fish that feed on them. Oceanic pelagic fish typically inhabit waters below the continental shelf. Examples include larger fish such as swordfish, tuna, mackerel, and even sharks. [Source: NOAA]

There is no distinct boundary from coastal to ocean waters so some oceanic fish become partial residents of coastal waters, often during different stages of their lifecycle. However, true oceanic species spend their entire life in the open ocean. Oceanic pelagic fish, such as tuna, have agile bodies made for long distance migration. Many oceanic pelagic fish travel in schools while some are solitary that drift with ocean currents. [Source: NOAA]

Pelagic fish get their name from the area that they inhabit called the pelagic zone. The pelagic zone is the largest habitat on earth with a volume of 330 million cubic miles. Different species of pelagic fish are found throughout this zone. Numbers and distributions vary regionally and vertically, depending on availability of light, nutrients, dissolved oxygen, temperature, salinity, and pressure.

Bony Fish

Bony fish make up nine out of 10 fish species. Of the four classes of fishes they evolved most recently and are regarded as the most advanced. They vary a great deal in size and shape but most tend to be small. All have light but strong internal skeletons, which supports their body and gives them precise control over their body movements. Most also have a gas-filled swim bladder that allows them to adjust their buoyancy within narrow limits. With a buoyancy bladder fish don’t have use their fins to maintain buoyancy like cartilaginous fish do.

The skeleton of a bony fish has three main parts: the skull, backbone and fin skeleton, The gills are paired and located behind the head. The gill flaps are covered by a bony flap and the lower gill chamber has bony supports, allowing the fish to gulp water and pump it through its gills. In this way the bony fish can respire while sitting stationary and doesn’t need to move to breath like sharks and cartilaginous fish. The bony supports in the gill chamber are used in gulping and seizing food.

Parts of a bony fish (Anoplogaster cornuta)

Most bony fish are covered in scales and they are covered by a thin layer of skin that secrets mucous that helps fight off parasites and disease-causing organisms. Even fish that don’t have scales still have the mucous secreting skin.

Bony fish tend to have keen senses of hearing and sight, which play an important roles in fish communication and schooling. The eyes are generally on the sides of the head, giving the fish a wide field of vision.

Bony fish are very precise and dexterous swimmers. A separate set of muscles controls each fin. The symmetrical tail is often what is used to propel the fish forward. Teeth may be found on the jaws, throat, roof of the mouth or tongue.

Cartilaginous Fishes

Sharks, rays and a group of deep-water fish called chimaeras and their relatives are cartilaginous fish, or elasmobranch, as opposed to bony fish, the classification most fish fall into. All cartilaginous fish possess skeletons made of cartilage rather than bone and specialized teeth that can be replaced throughout their lives. Some have cartilage strengthened by mineral deposits and bonelike dorsal spines.

Cartilaginous skeletons are much lighter and more flexible than bony skeletons. On the land they would be unable to support the weight of large animals but in water they are effective for animals up to 40 feet in length. Most have skin covered by thousands, even millions, of interlocking scales called dermal denticles. They have a similar composition to teeth and give skin a sandpaper-like texture, increase durability and reduce drag.

Cartilaginous fish have five to seven pairs of gills. When water enters the mouth the gill slits are closed. When waters passes through the open gills the mouth is closed. These fish also lack the air bladder that give bony fish their buoyancy and instead have an oil-rich liver that adds to their buoyancy. Even so many are negatively buoyant and need to swim to stay afloat.

All cartilaginous fish have a sensory system of pores called ampullae of Lorenzini, named after an Italian biologist who discovered them in 1678, that send out electrical signals that can be used to locate prey and avoid predators. Most also have an effective lateral line system running from their tail to their snout that helps them detect small vibrations.

Parts of a shark (cartilaginous fish)

All cartilaginous fish are carnivorous but for some this means they feed primarily on zooplankton. Most feed on live prey but will feed on carrion if it is available. Few feed exclusively on carrion. Reproduction take place internally when the male passes sperm into the female’s cloaca with a modified pelvic fin. Some species release embryos in leathery egg cases. Other species give birth to live young that hatched from eggs that broke open inside the female. In yet others, embryos develop in placenta-like structures. In all cases the young do not go through a larval stage like many bony fish; instead they are born as miniature adults.

Ray-Finned Fishes

Ray-finned fish (scientific name: Actinopterygii) are the largest and most successful group of fishes and make up half of all living vertebrates. At present, approximately 42 orders, 431 families, and nearly 24,000 species are recognized within this class but there are still many undiscovered species and there are still debates among scholars about taxonomy. But it also important to keep in mind that in some areas species are becoming extinct faster than they are being discovered such as in the Amazon and Congo Basins. [Source: R. Jamil Jonna,Animal Diversity Web (ADW) /=]

Ray-finned fish fossils first appeared in deposits from the late Silurian (425 to 405 million years ago) or early Devonian (405 to 345 million years ago) periods. They began to dominate the fish fauna at the beginning of the Carboniferous period, 360 million years ago. The most derived forms (i.e. teleosts) became more numerous in the Late Cretaceous Period (100 million to 66 million years ago). It was at this time that major diversification began and has continued to this day. /=\

The earliest ray-finned fish are grouped in the subclass Chondrostei, of which only sturgeons , bichirs and paddlefishes survive today. The rest of the ray-finned fish, which includes the vast majority of species, are in the subclass Neopterygii, meaning ‘new fins’. The large majority of neopterygians are placed in the group Teleostei (infraclass). The bowfin is the only surviving species of the halecomorphs, the largest group outside of the teleosts and gars, with seven species, are the only other surviving non-teleosts. /=\

Habitats of Ray-Finned Fishes

Ray-finned fishes inhabit a variety of environments, including some that are quite extreme such as high altitude lakes and streams, desert springs, subterranean caves, ephemeral pools, polar seas, and the depths of the ocean. Small pink snailfish have been found in the Marianas Trench as a depth of 8,200 meters (27,000 fee). It is the world’s deepest living fish so far discovered. Across these habitats water temperatures may range from -1.8̊C to nearly 40̊C, pH levels from 4 to 10+, dissolved oxygen levels from zero to saturation, salinities from zero to 90 parts per million. Some fish, such as muflat-dwelling mudskippers, even spend considerable time outside of water:

According to Animal Diversity Web: Disparate localities may have similar geographic conditions, yet fish species composition varies widely across similar regions. In other words, patterns of fish distribution are not simply related to how well a fish is adapted to a particular type of environment, which is why invasive species can be so devastating

In marine areas, species concentrations are highest around coral reefs, where butterflyfishes and angelfishes , wrasses , parrotfishes and triggerfishes are common. In the arctic seas five notothenoid families dominate: thornfishes , plunderfishes, Antarctic dragonfishes , and notothens.

Researchers have long divided freshwater and saltwater habitats. However, habitat boundaries are often crossed by migratory species, some of which are diadromous — meaning they migrate between fresh water and the sea. Depending on the type of migration, they can be anadromous (migrate up rivers to spawn), with a pattern of freshwater-ocean-freshwater (typical of salmon and lampreys), or catadromous (migrate from freshwater to the sea to spawn), which is characteristic of freshwater eels.

Ray-Finned Fish Diversity

According to Animal Diversity Web (ADW): The truly spectacular array of body forms within this class can only be appreciated by familiarizing oneself with some of the more than 25,000 species of ray-finned fish. Consider the fact that ray-finned fish may fly, walk, or remain immobile (in addition to 'swimming'), exist in virtually all types of habitats except constantly dry land (though some can walk over land), feed on nearly every type of organic matter, utilize several types of sensory systems (including chemoreception, electroreception, magnetic reception and a “distance-touch” sensation, and some even produce their own light or electricity. In addition, color diversity in ray-finned fishes is “essentially unlimited, ranging from uniformly dark black or red in many deepsea forms, to silvery in pelagic (living in the open ocean, far from land) and water-column fishes, to countershaded in nearshore fishes of most littoral [near-shore] communities, to the strikingly contrasted colors of tropical freshwater and marine fishes”.

Of course, extravagant coloration is not helpful for fish at risk of being eaten, yet bright coloration is environment-specific and bright colors at one depth are cryptic at others due to light attenuation. Further, color change is common in brightly colored (as well as many other) fishes and occurs under a variety of circumstances. Pigments are responsible for a many types of color change, but there are also structural colors, resulting from light reflecting off of crystalline molecules housed in special chromatophores (cells located mainly in the outer layer of skin). The silvery sheen displayed by many pelagic (living in the open ocean, far from land) fishes is an example of structural color. Numerous ray-finned fish are also sexually dimorphic (males and females look different), and body form changes drastically during development, so there is significant diversity within, as well as among, species. /=\

Among the largest ray-finned fish are the pirarucu (also known as giant arapaima , up to 2.5 meters in length) in freshwater and the black marlin (up to 900 kilograms) in saltwater; the longest is the oarfish , Lampris guttatus, which averages between five and 8 meters in length; and the smallest, a variety of diminutive gobies in saltwater and minnows, catfishes and characins in freshwater. /=\

Ray-finned fish may have ganoid, cycloid, or ctenoid scales, or no scales at all in many groups. With the exception of Polypteriformes, the pectoral radials are attached to the scapulo-coracoid, a region of the pectoral girdle skeleton. (The pectoral radials are one of a series of endochondral — growing or developing within cartilage — bones in the pectoral and pelvic girdle on which the fin rays insert). Most have an interopercle and branchiostegal rays and the nostrils are positioned relatively high on the head. Finally, the spiracle (respiratory opening between the eye and the first gill slit — connects with the gill cavity) and gular plate (behind the chin and between the sides of the lower jaw) are usually absent, and internal nostrils are absent.

They are heterothermic (having a body temperature that fluctuates with the surrounding environment), have bilateral symmetry (both sides of the animal are the same) and are polymorphic (“many forms”, species in which individuals can be divided into easily recognized groups, based on structure, color, or other similar characteristics), poisonous venomous

Sexual Dimorphism (differences between males and females) depends of : Both sexes are roughly equal in size and look similar. female larger male larger Sexes Tyrannosaurus colored or patterned differently with the female being more colorful. male more colorful Males and females have different shapes. Ornamentation is different. /=\

Fish Diversity Wikipedia

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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