What Is a Fish? Definition, Types & Examples

Fish are aquatic vertebrates with gills and fins — but 'fish' is not one neat clade. Bony fish, cartilaginous fish, and jawless fish explained simply.

Global Animal Guide · July 10, 2026

Great white shark, a cartilaginous fish — Photo: Pterantula (Terry Goss) at en.wikipedia · CC BY 2.5 · source · credits

Quick answer

In everyday language, fish are aquatic vertebrates that breathe with gills and swim with fins. Biologically, 'fish' is a grade, not a single clade that excludes tetrapods — because land vertebrates evolved from fish ancestors. Living groups include jawless fish (lampreys, hagfish), cartilaginous fish (sharks, rays), and bony fish (the vast majority).

Last updated: July 2026.

Fish are aquatic, gill-breathing vertebrates with fins. Living types: jawless, cartilaginous, and bony fish — over 30,000 species.

Three big living groups

Jawless fish — Lampreys and hagfish
Cartilaginous fish — Sharks, rays, skates, chimaeras
Bony fish — Ray-finned fish (tuna, goldfish, seahorses) and lobe-finned relatives

Frequently asked questions

Are sharks fish?

Yes — cartilaginous fish (class Chondrichthyes).

Are whales fish?

No — whales are mammals that returned to the sea.

How many fish species are there?

Over 30,000 described species — more than all other vertebrates combined.

Do all fish lay eggs?

No — many sharks and some bony fish give live birth.