Fish, explained
What is a fish?
A fish is an animal that lives in water, breathes with gills throughout its life, and does not have fingers or toes. Most fish have a backbone and swim with fins.
What people usually mean by “fish”
In everyday speech, fish are water-living animals such as salmon, tuna, eels, seahorses, sharks, and rays. Their bodies can look very different, but they breathe with gills and do not have limbs (arms, legs, wings, or similar body parts; from the Old English word lim, meaning a branch or body part) with fingers or toes. Whales, dolphins, seals, turtles, and penguins are not fish because they belong to other animal groups and breathe air with lungs.
What scientists mean
Scientists use “fish” as a practical name for several ancient branches of animals, not as the name of one single branch. Living fish are usually discussed in three big groups:
- Jawless fish, including lampreys and hagfish.
- Cartilaginous fish (fish whose skeleton is made mostly of cartilage, the tough and bendy material in your nose and ears; “cartilage” comes from the Latin cartilago, meaning gristle), including sharks, rays, skates, and chimaeras.
- Bony fish, including almost every other fish, from minnows and seahorses to tuna and ocean sunfish.
There is a scientific twist: the first four-legged animals evolved from lobe-finned fish (fish with thick, fleshy fin bases; a “lobe” is a rounded part). That means the fish family tree also gave rise to amphibians (animals such as frogs that usually begin life in water; from Greek words meaning “both lives”), reptiles (animals such as snakes and turtles; from the Latin repere, “to crawl”), birds, and mammals (animals whose mothers make milk; from the Latin mamma, “breast”). So “fish” is useful and familiar, but it does not form one complete branch if we leave all those land animals out.
What about shellfish and other water animals?
“Shellfish” is a food and fishing term. It includes crustaceans (hard-shelled animals such as crabs, shrimp, and lobsters; from the Latin crusta, “shell”) and mollusks (soft-bodied animals such as clams, snails, squid, and octopuses; from the Latin mollis, “soft”). Jellyfish are cnidarians (animals with stinging cells; from the Greek knidē, “nettle”), and starfish are echinoderms (animals with spiny skin; from Greek words meaning “hedgehog skin”). None of these animals are fish.
How this site sorts them
The main database covers animals commonly recognized as fish: jawless fish, cartilaginous fish, and bony fish. Other water animals appear separately so their names do not blur their real relationships.
How many fish are there?
Scientists have described more than 35,000 living fish species, and they continue to identify and rename species as knowledge grows. This site is a focused introduction, not a list of every scientifically named species. It covers about 2,000 widely known, ecologically important, or otherwise remarkable fish, plus a smaller group of aquatic animals that are not fish.