Science, Tech, Math › Animals & Nature 10 Essential Facts About Fish Share Flipboard Email Print Animals & Nature Marine Life Marine Life Profiles Marine Habitat Profiles Sharks Key Terms Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Forestry Dinosaurs Evolution View More By Bob Strauss Science Writer B.S., Cornell University Bob Strauss is a science writer and the author of several books, including "The Big Book of What, How and Why" and "A Field Guide to the Dinosaurs of North America." our editorial process Bob Strauss Updated August 09, 2019 One of the six main groups of animals—along with invertebrates, amphibians, reptiles, birds, and mammals—fish are so abundant in the world's oceans, lakes, and rivers that new species are constantly being discovered. 01 of 10 There Are Three Main Fish Groups Life On White/Getty Images Fish are broadly divided into three classes. The Osteichthyes, or bony fish, include both ray-finned and lobe-finned fish, accounting for over 30,000 species in all, ranging from familiar food fish like salmon and tuna to more exotic lungfish and electric eels. The Chondrichthyes, or cartilaginous fish, include sharks, rays and skates, and the Agnatha, or jawless fish, include hagfish and lampreys. (A fourth class, Placoderms, or armored fish, has long since gone extinct, and most experts lump Acanthodes, or spiny sharks, under the Osteichthyes umbrella.) 02 of 10 All Fish Are Equipped With Gills LuismiX/Getty Images Like all animals, fish need oxygen in order to fuel their metabolism: the difference is that terrestrial vertebrates breathe air, while fish rely on oxygen dissolved in water. To this end, fish have evolved gills, complex, efficient, multi-layered organs that absorb oxygen from the water and excrete carbon dioxide. Gills only work when oxygenated water is constantly streaming through them, which is why fish and sharks are always moving—and why they expire so quickly when they're plucked from the water by human fishermen. (Some fish, like lungfish and catfish, possess rudimentary lungs in addition to their gills, and can breathe air when circumstances demand.) 03 of 10 Fish Were the World's First Vertebrate Animals BSIP/UIG/Getty Images Before there were vertebrates, there were chordates—small marine animals possessing bilateral symmetry heads distinct from their tails, and nerve cords running down the length of their bodies. A little over 500 million years ago, during the Cambrian period, a population of chordates evolved into the first true vertebrates, which then went on to spawn all the reptiles, birds, amphibians and mammals we know and love today. (A sixth animal group, invertebrates, never subscribed to this backbone trend, yet today they account for a whopping 97 percent of all animal species!) 04 of 10 Most Fish Are Cold-Blooded Dave Fleetham / Design Pics/Getty Images Like the amphibians and reptiles to which they're distantly related, the vast majority of fish are ectothermic, or cold-blooded: they rely on the ambient temperature of the water to fuel their internal metabolisms. Surprisingly, though, barracudas, tunas, mackerels, and swordfish—which belong to the fish suborder Scombroidei—all have warm-blooded metabolisms, albeit using a system quite different from that of mammals and birds; a tuna can maintain an internal body temperature of 90 degrees Fahrenheit even when swimming in 45-degree water! Mako sharks are also endothermic, an adaptation that endows them with added energy when pursuing prey. 05 of 10 Fish Are Oviparous Rather Than Viviparous Daniela Dirscherl/Getty Images Oviparous vertebrates lay eggs; viviparous vertebrates gestate their young (for at least a brief period of time) in the mother's womb. Unlike other vertebrates, most fish species fertilize their eggs externally: the female expels hundreds or thousands of small, unfertilized eggs, at which point the male releases its sperm into the water, at least some of which find their mark. (A few fish engage in internal fertilization, males using a penis-like organ to impregnate the female.) There are some exceptions that prove the rule, though: in "ovoviviparous" fish, the eggs hatch while still in the mother's body, and there are even a few viviparous fish like lemon sharks, the females of which have organs very similar to mammalian placentas. 06 of 10 Many Fish Are Equipped With Swim Bladders Dorling Kindersley/Getty Images Fish live in stratified ecosystems: the food chain is very different 20 feet below the surface than it is one or two miles deep. For this reason, it's in a fish's best interests to maintain a constant depth, which many species accomplish with the aid of a swim bladder: a gas-filled organ inside their bodies that maintains the fish's buoyancy and removes the need to swim at maximum speed. It's widely believed, though not yet proven, that the primitive lungs of the first tetrapods ("fish out of water") evolved from swim bladders, which were "co-opted" for this secondary purpose to allow vertebrate animals to colonize the land. 07 of 10 Fish May (or May Not) Be Able to Feel Pain John Kuczala/Getty Images Even people who advocate more humane treatment of "higher" vertebrates like cows and chickens don't have much of an opinion when it comes to fish. But there are a handful of (somewhat controversial) studies showing that fish are capable of feeling pain, even though these vertebrates lack the brain structure, called the neocortex, that's associated with pain in mammals. In England, the Royal Society for the Protection of Animals has adopted a stance against cruelty to fish, which presumably applies more to gruesomely disfiguring fish hooks than to industrial fish farms. 08 of 10 Fish Are Incapable of Blinking Image Source RF/Justin Lewis/Getty Images One of the traits that make fish seem so alien is their lack of eyelids, and hence their inability to blink: a mackerel will maintain the same glassy stare whether it's relaxed or alarmed, or, for that matter, whether it's alive or dead. This raises the related question of how, or even whether, fish sleep. Their wide-open eyes notwithstanding, there is some evidence that fish do sleep, or at least engage in restorative behavior similar to human sleep: some fish float slowly in place or wedge themselves into rocks or corals, which may indicate a reduced amount of metabolic activity. (Even when a fish appears motionless, ocean currents still keep its gills supplied with oxygen.) 09 of 10 Fish Sense Activity With "Lateral Lines" VCG Wilson/Corbis/Getty Images Although many fish have excellent vision, they don't quite measure up when it comes to hearing and smell. However, these marine vertebrates are equipped with a sense that terrestrial vertebrates completely lack: a "lateral line" across the length of their bodies that senses the motion of water, or even, in some species, electrical currents. A fish's lateral line is especially important for maintaining its place in the food chain: predators use this "sixth sense" to home in on prey, and prey uses it to avoid predators. Fish also use their lateral lines to congregate in schools and to choose the right direction for their periodic migrations. 10 of 10 There Are Only So Many Fish in the Sea piazzagabriella/Getty Images The world's oceans are so huge and deep, and the fish that inhabit them are so populous and prolific, that you can excuse many people for believing that tuna, salmon, and the like are inexhaustible food sources. Nothing could be further from the truth: overfishing can easily render a fish population extinct, as humans harvest a species for their dinner tables faster than it can reproduce and replenish its own stock. Unfortunately, despite the proven risk of species collapse, commercial fishing of certain fish species continues unabated; if the trend persists, some of our favorite food fish may vanish from the world's oceans within 50 years.