fish

Introduction & Top Questions
General features
- Structural diversity
- Distribution and abundance
Natural history
- Life history
- Behaviour
- Locomotion
- Reproduction
Form and function
- Body plan
- The skin
- The muscle system
- The digestive system
- The respiratory system
- The circulatory system
- Excretory organs
- Endocrine glands
- The nervous system and sensory organs
  - Olfaction
  - Taste
  - Sight
  - Hearing
  - Other senses (touch, pain, and special senses)
Evolution and paleontology
- Agnatha: early jawless fishes
- Acanthodii: early jawed fishes
- Placodermi: plate-skin fishes
- Chondrichthyes: sharks and rays
- Holocephali
- Sarcopterygii: fleshy-finned fishes
- Actinopterygii: ray-finned fishes
Classification
- Distinguishing taxonomic features
- Annotated classification
- Critical appraisal

References & Edit History Related Topics

Images & Videos

A tiger shark, Galeocerdo cuvier, swims in clear, shallow water above a sandy and rocky sea floor.

Illustration of an opah (Lampris guttatus), also known as a moonfish, showing its round, laterally compressed body with silvery scales covered in white spots and reddish fins, alongside a scale bar indicating its large size.

How do fish survive under icy lakes in winter?

Watch footage of the deepest fish ever caught on camera

All about ray fins: Seahorses, trunkfish, and more

At a Glance

fish summary

Quizzes

Dromedary camels (Camelus dromedarius). Animals, mammals.

Ultimate Animals Quiz

Sea otter (Enhydra lutris), also called great sea otter, rare, completely marine otter of the northern Pacific, usually found in kelp beds. Floats on back. Looks like sea otter laughing. saltwater otters

Animal Group Names

Deadliest Animals Quiz

Young chimpanzee dressed in a shirt and sweater vest, scratching his head thinking. (primates)

Wild Words from the Animal Kingdom Vocabulary Quiz

Match the Baby Animal to Its Mama Quiz

Related Questions

How are ovoviviparous fishes different from viviparous fishes?
How do anglerfishes use a modified dorsal spine to attract prey?
How do annual killifishes survive when their temporary ponds dry up?
How do body muscles and tail fins propel fishes through water?
How do catadromous eels differ from anadromous salmon?
How do cichlids, sticklebacks, and gouramis make nests for their eggs?
How do clingfishes stay attached to rocks in powerful waves?
How do fishes extract oxygen from water with their gills?
How do fishes survive in hot springs or extremely cold polar waters?
How do fishes use lateral lines to sense pressure and water movement?
How do flounders travel over the seafloor?
How do most bony fishes reproduce through external fertilization?
How do pectoral and pelvic fins help fishes turn and stay stable?
How do rays move along the ocean bottom?
How do salmon migrate between freshwater streams and the ocean during their life cycle?
How do sharks and rays transfer sperm during internal fertilization?
How do some deep-sea fishes produce light in dark ocean waters?
How do wrasses benefit larger fishes by removing parasites?
How does a swim bladder help a bony fish stay at a chosen depth?
How does parental care improve the survival of fish eggs and young?
How does the labyrinth organ help gouramis and Siamese fighting fish breathe air?
How does water move through a fish’s mouth and over its gills?
What environmental cues help Atlantic salmon return to the river where they were born?
What roles do the dorsal and anal fins play in swimming?
Where do pelagic fishes usually release their eggs?
Which aquatic habitats can bony fishes live in?
Why are gill filaments filled with many small blood vessels?
Why can fish species have very different body shapes and fin arrangements?
Why do electric eels regularly rise to the surface for air?
Why do fast-swimming fishes have folded and overlapping muscle segments?
Why do live-bearing fishes usually have fewer and larger young than egg-laying fishes?
Why do many bottom-living deep-sea fishes have small eyes and downturned mouths?
Why do many egg-laying fishes produce thousands or millions of eggs?
Why do mid-water deep-sea fishes often have large eyes and huge mouths?
Why do some deep-sea fishes have reduced swim bladders or no swim bladder?
Why do some fishes form schools while larger predators often hunt alone?
Why do some plankton and fishes migrate upward at night and downward before dawn?
Why do young fishes often live in different habitats from adult fishes?
Why does a streamlined fusiform body shape help a trout swim efficiently?
How do fish sleep?
How do fish hear?
Are white sharks cold-blooded or warm-blooded?
Do white sharks attack humans?
What is a ray?
How do rays differ from sharks?
What makes the tails of many stingrays dangerous?
How do lampreys reproduce?
What distinguishes agnathans from other craniates?
How do hagfishes feed?

Science Birds, Reptiles & Other Vertebrates Fish

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External Websites

Healthline - Eleven Evidence-Based Health Benefits of Eating Fish
Florida Museum of Natural History - Discover Fishes - Learn About Fish
Harvard T.H. Chan School of Public Health - The Nutrition Source - Fish: Friend or Foe?
Better Health Channel - Fish
WebMD - Health benefits of Fish
National Center for Biotechnology Information - PubMed Central - Are Fish Wild?
Animal Corner - Fish
Food and Agriculture Organization of the United Nations - Consumption of Fish and Shellfish and teh Regional Markets
University of Hawaiʻi at Mānoa - Exploring Our Fluid Earth - What is a Fish?
A-Z Animals - Fish

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

fish - Children's Encyclopedia (Ages 8-11)
fish - Student Encyclopedia (Ages 11 and up)

fish

Written by

Stanley H. Weitzman

Research Scientist Emeritus, Division of Fishes, National Museum of Natural History, Smithsonian Institution, Washington, D.C. Author of numerous articles on fishes.

Stanley H. WeitzmanAll

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

Britannica Editors

Last updated

Oct. 26, 2026 •History

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part...

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