Fish

A fish is an aquatic animal that breathes through gills. Fish have a tough cranium to protect their brain, but they do not have limbs with digits. Fish can be divided into two main groups: jawless fish and jawed fish. JAWED fish include cartilaginous and bony fish.

Most fish are cold-blooded, meaning their body temperature changes with the water around them. Some fish, like the white shark and tuna, can keep their body warmer than the water. Many fish can make sounds to talk to each other. The study of fish is called ichthyology.

Fish have been important to humans for a very long time, especially as food. People catch fish in the wild or farm them in ponds or special cages in the ocean. Fish are also caught for fun or kept in aquaria and garden ponds for decoration. Fish have been part of human culture for ages, appearing in 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 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, small fishlike animals first appeared. These early fish did not have jaws.

Later, fish with jaws began to appear. This included sharks and bony fish. Many different types of fish evolved, including early sharks and strange armored fish.

Today, fish are a large group. Most scientists think that fish and animals like us, the tetrapods, share a common ancestor from bony fish. Fish come in many shapes and sizes, from tiny fish to the giant whale shark, which can be over 16 meters long. Fish have many different ways of living in the water.

Ecology

Fish live in many different water places. About half of all fish live in freshwater, like rivers and lakes. The other half live in the ocean. Coral reefs in the Indo-Pacific are homes to many sea fish. Big rivers in tropical rainforests, like the Amazon, Congo, and Mekong, have many freshwater fish.

Fish can be found almost everywhere in water, from cold mountain streams to the deepest parts of the ocean. Some fish, like mudskippers, can live mostly on land near water. Fish face threats from parasites and predators. Some small fish, called cleaner fish, help remove parasites from bigger fish. Bigger fish and some animals like dolphins and birds eat fish.

Anatomy and physiology

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

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

Fish have a simple brain compared to birds and mammals, but some, like sharks and certain tropical fish, have 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 move around. Many fish can solve problems and remember things, showing they have some level of intelligence.

Behavior

Shoaling and schooling

Main article: Shoaling and schooling

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

Working as a group can make finding food easier. 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 help them stay safe. 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. They mostly use sounds 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. The Haemulon flavolineatum, or French grunt fish, grunts by grinding its teeth, especially when it’s upset.

Other fish use special muscles to make their swim bladders vibrate, creating noise. The Oyster toadfish makes loud grunts this way. The red drum, or Sciaenops ocellatus, makes drumming sounds by vibrating its swim bladder.

The longsnout seahorse, Hippocampus reidi, makes two types of sounds: ‘clicks’ and ‘growls’. Clicks happen during courtship and feeding. Growls are made when they’re stressed.

Conservation

The IUCN Red List says many fish species are in danger. Some of these fish are the Atlantic cod, Devil's Hole pupfish, coelacanths, and great white sharks. Learning about fish can be hard because they live underwater and we often don’t know much about them. Freshwater fish are especially at risk because they live in small bodies of water.

Overfishing is a big problem for many fish. This means people catch fish faster than they can grow back. For example, the Pacific sardine fishery near California had much less fish to catch over time. Other problems for fish include damage to their homes from dams, water pollution, and new species that can harm the fish that live there.

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. Fish also appear in famous movies like Finding Nemo and books like The Old Man and the Sea.