What Are Fish?

Blue Runners fish

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Fish - that word may conjure up a variety of images, from colorful animals swimming peacefully around a reef to brightly-colored fish in an aquarium to something white and flaky on your dinner plate. What is a fish? Here you can learn more about the characteristics of fish, and what sets them apart from other animals.

Fish come in a wide variety of colors, shapes, and sizes - there's the largest fish, the 60+ foot long whale shark, popular seafood fish such as cod and tuna, and completely different-looking animals such as seahorses, sea dragons, trumpet fish, and pipefish. In all, about 20,000 species of marine fish have been identified.


Fish swim by flexing their bodies, forming waves of contractions along their muscles. These waves push water backward and move the fish forward.

One of the most notable features of fish are their fins - many fish have a dorsal fin and anal fin (near the tail, on the underside of the fish) that provide stability. They may have one, two or even three dorsal fins. They may also have pectoral and pelvic (ventral) fins to help with propulsion and steering. They also have a caudal fin or tail.

Most fish have scales covered with a slimy mucus that helps protect them. They have three main types of scales: Cycloid (roundish, thin and flat), ctenoid (scales that have tiny teeth on their edges), and ganoid (thick scales that are rhomboid in shape). 

Fish have gills for breathing - the fish inhales water through its mouth, which passes over the gills, where hemoglobin in the fish's blood absorbs oxygen.

Fish may also have a lateral line system, which detects movement in the water, and a swim bladder, which the fish uses for buoyancy. 


The fishes are divided into two superclasses: Gnathostomata, or vertebrates with jaws, and the Agnatha, or jawless fishes.

Jawed fishes:

  • Class Elasmobranchii, the elasmobranchs: Sharks and rays, who have a skeleton made of cartilage
  • Class Actinopterygii, the ray-finned fishes: fish with skeletons made of bone, and spines in their fins (e.g., cod, bass, clownfish/anemonefish, seahorses)
  • Class Holocephali, the chimeras
  • Class Sarcopterygii, the lobe-finned fish, the coelacanth and lungfishes.

Jawless fishes:

  • Class Cephalaspidomorphi, the lampreys
  • Class Myxini, the hagfishes


With thousands of species, reproduction in fish can be remarkably different. There's the seahorse - the only species in which the male gives birth. And then there are species like cod, in which females release 3-9 million eggs into the water column. And then there are sharks. Some shark species are oviparous, meaning they lay eggs. Others are viviparous and give birth to live young. Within these live-bearing species, some have placenta-like human babies and others do not.

Habitat and Distribution

Fish are distributed in a wide variety of habitats, both marine, and freshwater, throughout the world. Fish have even been found as deep as 4.8 miles beneath the ocean surface.

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Kennedy, Jennifer. "What Are Fish?" ThoughtCo, Apr. 5, 2023, thoughtco.com/fish-profile-2291579. Kennedy, Jennifer. (2023, April 5). What Are Fish? Retrieved from https://www.thoughtco.com/fish-profile-2291579 Kennedy, Jennifer. "What Are Fish?" ThoughtCo. https://www.thoughtco.com/fish-profile-2291579 (accessed June 9, 2023).