A fish is an aquatic vertebrate with a backbone, gills for breathing in water, and fins for movement, usually spending its entire life in water. These shared traits are what make a fish a fish, even though there are many odd exceptions in the fish world.

Core fish features

  • Lives in water : Fish are adapted to spend their lives in aquatic environments, from shallow streams to the deep ocean. Even species that gulp air or move briefly on land still rely on water as their main habitat.
  • Has a backbone : Fish are vertebrates, meaning they have a spinal column that supports the body and protects the spinal cord. This separates them from invertebrates like octopuses or jellyfish that also live in water but are not fish.
  • Uses gills to breathe : Gills extract oxygen dissolved in water and release carbon dioxide, letting fish “breathe” without air. Some species have additional lung‑like organs, but gills are the defining structure.
  • Has fins for movement : Fins act like built‑in paddles and rudders, helping fish move, steer, brake, and balance in the water. Different fins handle different jobs, like the tail fin for propulsion and dorsal fins for stability.

Typical (but not universal) traits

  • Usually cold‑blooded : Most fish are ectothermic, meaning their body temperature tracks the surrounding water, unlike warm‑blooded mammals or birds. A few fast‑swimming species are partial exceptions and can keep some body parts warmer.
  • Often covered in scales : Many fish have overlapping scales with a slimy mucus layer that reduces friction and protects against infection. Some fish, like some catfish and pufferfish, have reduced scales or none at all, but are still considered fish because they keep the key internal traits.
  • Usually lay eggs : Most fish reproduce by laying eggs in water, though a number of species give birth to live young. The egg‑laying pattern is common but not required for an animal to count as a fish.

How scientists define “fish”

Biologists focus on anatomy and evolution rather than just “lives in water.”

  • Brain and head: Fish have a brain protected by a braincase and a distinct head with eyes and other sensory organs.
  • Limbs as fins: Their paired limbs are modified into fins instead of arms and legs, used mainly for swimming.
  • Evolutionary group: Modern “fish” includes jawless fish, cartilaginous fish (like sharks and rays), and bony fish, which together represent most vertebrate diversity on Earth.

Because of this, animals like whales, dolphins, and turtles are not fish even though they live in water, since they breathe air with lungs and have limbs like flippers rather than true fins, and they belong to mammal or reptile groups instead.

Information gathered from public forums or data available on the internet and portrayed here.