Fish
About 530 million years ago during the Cambrian explosion, fishlike animals with a notochord and eyes at the front of the body appeared in the fossil record. Haikouichthys represents one of these earliest forms that lacked true jaws but possessed a distinct head structure. During the late Cambrian, other jawless forms such as conodonts emerged to fill new ecological roles. Jawed vertebrates first appeared in the Silurian period, marking a major shift in evolutionary history. Giant armoured placoderms like Dunkleosteus roamed the seas around 400 million years ago. These creatures developed heavy bony plates that served as protective exoskeletons against invertebrate predators. The Devonian era earned the epithet "the age of fishes" due to the massive diversification of species. Placoderms, lobe-finned fishes, and early sharks all flourished during this time before many went extinct.
Modern phylogenetics views fish as a paraphyletic group that includes all vertebrates except tetrapods. This classification breaks from the long tradition of grouping all fish into a single class known as Pisces. As of 2016, there are over 32,000 described species of bony fish alone. Cartilaginous fish account for over 1,100 species while hagfish and lampreys number more than 100. Teleosts make up 96% of all living fish species today. About half of all living vertebrates belong to the ray-finned fishes class Actinopterygii. Three families dominate the diversity with Cyprinidae being the largest followed by Gobiidae and Cichlidae. Some species like Paedocypris progenetica measure only a few millimeters in length. Others such as the whale shark reach enormous sizes that dwarf most other aquatic animals.
Fish exchange gases using gills located on either side of the pharynx. Each filament contains a capillary network providing a large surface area for exchanging oxygen and carbon dioxide. Capillary blood flows in the opposite direction to water resulting in efficient countercurrent exchange. Sharks usually have five pairs of gill openings sometimes six or seven depending on the species. Bony fish possess a single gill opening on each side hidden beneath a protective bony cover called an operculum. The lateral line system detects gentle currents and vibrations through sensors embedded in the skin. Blind cave fish navigate almost entirely through sensations from this lateral line system. Catfish and sharks utilize ampullae of Lorenzini electroreceptors to detect weak electric currents around them. Rainbow trout and cichlids possess very large optic lobes suited for hunting by sight. Mormyrids process their electrical sense through a massive cerebellum relative to body size.
Coral reefs in the Indo-Pacific constitute the center of diversity for marine fishes globally. Continental freshwater fishes are most diverse in large river basins of tropical rainforests like the Amazon Congo and Mekong. Neotropical freshwaters alone host more than 5,600 fish species representing about 10% of all vertebrate species on Earth. The deepest living fish found so far is a cusk-eel named Abyssobrotula galatheae recorded at the bottom of the Puerto Rico Trench. Jonah's icefish live in cold waters of the Southern Ocean under the Filchner, Ronne Ice Shelf at latitude 79°S. Desert pupfish survive in desert springs with water temperatures as high as 36 degrees Celsius. Mudskippers feed and interact with one another on mudflats before retreating underwater to hide in burrows. A single undescribed species of Phreatobius lives strictly among waterlogged leaf litter acting as a true land fish. Cavefish of multiple families inhabit underground lakes rivers or aquifers beneath the surface.
A shoal forms when each fish swims independently but adjusts its behavior to remain close to others. Schooling creates a much tighter group where all members move at the same speed and direction simultaneously. Capelin migrate annually in large schools between feeding areas and spawning grounds for reproduction. French grunt fish produce grunting noises by grinding their teeth together especially during distress. These sounds occur at frequencies around 700 Hz lasting approximately 47 milliseconds. Longsnout seahorse Hippocampus reidi produces clicks and growls by rubbing coronet bone across grooved sections of the neurocranium. Oyster toadfish emit loud grunts by contracting sonic muscles along the sides of the swim bladder. Male toadfishes generate boat whistle calls that are longer in duration and lower in frequency to attract mates. Red drum Sciaenops ocellatus vibrates its swimbladder to create repeated sounds ranging from 100 to over 200 Hz.
The Food and Agriculture Organization reported that in 2017, 34 percent of marine fisheries were classified as overfished. The Atlantic northwest cod fishery reduced populations to just 1% of historical levels by 1992. Pacific sardine catches declined steadily from an 800,000 tonne peak in 1937 to an economically inviable 24,000 tonnes in 1968. River dams like Kariba Dam on the Zambezi river caused large reductions in fish catch volumes. Industrial bottom trawling damaged seabed habitats extensively on Georges Bank in the North Atlantic. The opening of the Suez Canal in 1869 facilitated Lessepsian migration bringing hundreds of Indo-Pacific species into the Mediterranean Sea. The predatory Nile perch deliberately introduced to Lake Victoria in the 1960s collapsed haplochromine cichlid populations. Only three species remain dominant now including the silver cyprinid and Nile tilapia alongside the introduced predator. Freshwater fish seem particularly threatened because they often live in relatively small water bodies.
Historically and today most fish harvested for human consumption comes by means of catching wild fish. Fish farming has been practiced since about 3,500 BCE in ancient China becoming increasingly important globally. In 2007 about one-sixth of the world's protein was estimated to be provided by fish. Over 65 million tonnes of marine fish were captured in 2020 while some 50 million tonnes were farmed mainly freshwater. Anchoveta represented 4.9 million tonnes of the total catch followed by Alaska pollock at 3.5 million tonnes. Skipjack tuna accounted for 2.8 million tonnes with Atlantic herring and yellowfin tuna each reaching 1.6 million tonnes. Ancient Mesopotamians made fish offerings to gods from the very earliest times. Cylinder seals from Old Babylonian periods featured filling motifs of fish frequently. Early Christians used the ichthys symbol to represent Jesus Christ. The constellation Pisces associates with a legend from Ancient Rome where Venus and Cupid were rescued by two fishes.
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Common questions
When did fishlike animals first appear in the fossil record?
Fishlike animals with a notochord and eyes at the front of the body appeared about 530 million years ago during the Cambrian explosion. Haikouichthys represents one of these earliest forms that lacked true jaws but possessed a distinct head structure.
How many described species of bony fish exist as of 2016?
As of 2016, there are over 32,000 described species of bony fish alone. Teleosts make up 96% of all living fish species today while cartilaginous fish account for over 1,100 species.
Where do coral reefs constitute the center of diversity for marine fishes globally?
Coral reefs in the Indo-Pacific constitute the center of diversity for marine fishes globally. Continental freshwater fishes are most diverse in large river basins of tropical rainforests like the Amazon Congo and Mekong.
What percentage of marine fisheries were classified as overfished by the Food and Agriculture Organization in 2017?
The Food and Agriculture Organization reported that in 2017, 34 percent of marine fisheries were classified as overfished. The Atlantic northwest cod fishery reduced populations to just 1% of historical levels by 1992.
When did fish farming begin being practiced in ancient China?
Fish farming has been practiced since about 3,500 BCE in ancient China becoming increasingly important globally. Over 65 million tonnes of marine fish were captured in 2020 while some 50 million tonnes were farmed mainly freshwater.