Fish

A fish is an aquatic, anamniotic, gill-bearing vertebrate animal with a tough cranium to protect the brain, but lacking limbs with digits. Fish can be grouped into the more basal jawless fish and the more common jawed fish, the latter including all living cartilaginous and bony fish, as well as the extinct placoderms and acanthodians. In English, the plural of "fish" is fish when referring to individuals and fishes when referring to species.

Most fish are cold-blooded, their body temperature varying with the surrounding water, though some large, active swimmers like the white shark and tuna can maintain a higher core temperature. Many fish can communicate acoustically with each other, such as during courtship displays. The study of fish is known as ichthyology.

There are over 33,000 extant species of fish, easily the largest group of vertebrates and more than all species of the other traditional classes, namely amphibians, reptiles, birds, and mammals, combined. Most fish belong to the class Actinopterygii, the ray-finned fishes, which accounts for approximately half of all living vertebrates.

Fish have been an important natural resource for humans since prehistoric times, especially as food. Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or breeding cages in the ocean. Fish are caught for recreation or raised by fishkeepers as ornaments for private and public exhibition in aquaria and garden ponds. Fish have had a role in human culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

Etymology

The word "fish" comes from very old languages. It is related to the German word "Fisch," the Latin word "piscis," and the Old Irish word "íasc." We are not exactly sure where the word originated, but some experts think it may have roots in an ancient language spoken long ago.

In biology, the words "fish" and "fishes" mean different things. "Fish" can be used to talk about one fish or many fish of the same kind. "Fishes" is used when talking about many different kinds or groups of fish.

Evolution

Main article: Evolution of fish

Fish have been around for a very long time. About 530 million years ago, during a time called the Cambrian explosion, small fishlike animals with simple bodies and eyes first appeared in the fossil record. These early fish did not have jaws.

Later, during a period called the Silurian, fish with jaws began to appear. This included both cartilaginous fish, like sharks, and bony fish. The Devonian period saw an explosion of fish diversity, with many different types evolving, including early sharks and strange armored fish.

Today, fish are a large and varied group. Most scientists now think that fish, including all bony fish, are a group that also includes animals like us, the tetrapods, because tetrapods evolved from bony fish. This means that fish, as a group, include many different kinds of animals, from tiny fish only a few millimeters long to the giant whale shark, which can be over 16 meters long. Fish come in many shapes and sizes, from fast-swimming tuna to slow-moving eels, and they have many different ways of living in the water.

Ecology

Fish live in many different water habitats. About half of all fish species live in freshwater, such as rivers and lakes, and the other half live in the ocean. Coral reefs in the Indo-Pacific are home to many marine fish species, while large river basins in tropical rainforests, like the Amazon, Congo, and Mekong, have lots of freshwater fish.

Fish can be found almost everywhere in the water, from cold mountain streams to the deepest parts of the ocean. Some fish, like mudskippers, can even live mostly on land near water. Fish face threats from parasites and predators, but some smaller fish, called cleaner fish, help remove parasites from larger fish. Larger fish and even some animals like dolphins and birds eat fish as part of their diet.

Anatomy and physiology

The body of a typical fish is built for swimming. It moves forward by squeezing muscles on each side of its backbone, creating S-shaped curves. Fins help the fish steer, working like airplane flaps. Many fish have a special organ called a swim bladder that lets them stay at one depth in the water by adjusting the amount of gas inside it.

Fish breathe through gills, which are covered by a bony flap called the operculum. Gills allow fish to take in oxygen from water and release carbon dioxide. Some fish can also breathe air, which helps them survive in water with low oxygen or even out of water for short periods.

Fish have a simple brain compared to birds and mammals, but some, like sharks and certain tropical fish, have relatively larger brains. They use their senses—such as sight, smell, and a special system called the lateral line to feel movements in the water—to find food and navigate their environment. Many fish can solve problems and show memory, indicating they have some level of intelligence.

Behavior

Shoaling and schooling

Main article: Shoaling and schooling

Fish sometimes group together in loose collections called shoals, where they swim and find food independently but stay near each other. In tighter groups called schools, fish move together in perfect harmony, all swimming at the same speed and direction. This teamwork helps protect them from predators by improving watchfulness.

Working as a group can make finding food easier, and fish decide whether to stay in or leave a group based on what benefits them. When they spot a predator, they act together, moving as one. These group actions aren’t just about hiding—they can also include scattering and then coming back together. Fish also form groups to lay eggs. For example, the capelin travels in large schools between places where they eat and where they lay their eggs.

Communication

See also: Acoustic communication in aquatic animals

Fish talk to each other using sounds, mostly to find food, show aggression, or attract mates. These sounds differ among species and depend on what’s happening. Some fish make noise by rubbing their bones together, called stridulatory sounds. The Haemulon flavolineatum, or French grunt fish, grunts by grinding its teeth, especially when it’s upset. These grunts are around 700 Hz and last about 47 milliseconds.

Other fish use special muscles to make their swim bladders vibrate, creating noise. The Oyster toadfish makes loud grunts this way, and males also make longer “boat whistle” calls to attract females. These calls range from 140 Hz to 260 Hz. The red drum, or Sciaenops ocellatus, makes drumming sounds by vibrating its swim bladder, with frequencies from 100 to over 200 Hz. Different sounds are used depending on whether they’re trying to mate or avoid a predator. Female red drum don’t make sounds and don’t have the muscles to produce them.

The longsnout seahorse, Hippocampus reidi, makes two types of sounds: ‘clicks’ and ‘growls’. Clicks happen during courtship and feeding, ranging from 50 Hz to 800 Hz, with higher frequencies when they’re close together to spawn. Growls are made when they’re stressed, and these sounds come with body vibrations.

Conservation

The 2025 IUCN Red List identifies 2,367 fish species as endangered or critically endangered. Some of these include the Atlantic cod, Devil's Hole pupfish, coelacanths, and great white sharks. Studying fish can be challenging because they live underwater, and information about their populations is often limited. Freshwater fish are especially at risk because they usually live in small bodies of water.

Overfishing is a big problem for many fish species. The Food and Agriculture Organization found that in 2017, 34 percent of the world's marine fish stocks were overfished. This means people are catching fish faster than they can reproduce. For example, the Pacific sardine fishery near California saw its catch drop from 800,000 tonnes in 1937 to just 24,000 tonnes in 1968. Other threats to fish include habitat damage from things like dams, water pollution, and the introduction of non-native species that can harm local fish populations.

Importance to humans

Fish have been an important food source for humans for a very long time. They give us protein, and many people around the world catch wild fish or grow fish on farms to eat. Fishing also provides jobs for millions of people.

Fish are also enjoyed for fun and beauty. Many people love to fish for sport, and others keep fish in aquariums to admire. Fish appear in art, stories, and many cultures have special meanings for them. For example, early Christians used a fish symbol to represent Jesus. Fish also appear in famous movies like Finding Nemo and books like The Old Man and the Sea.