Summary
Aquatic, gill-breathing vertebrates, including jawless, cartilaginous and bony fishes.
In depth
Jawless (lampreys, hagfish), cartilaginous (sharks, rays) and bony fishes (ray-finned, lobe-finned). Features: gills with countercurrent exchange, lateral line, swim bladder in bony fish, single-circuit circulation.
Fish have fins, scales and a lateral line. Bony fish dominate. Reproduction methods vary from egg-laying to live birth.
Remember: fish are not one clade; tetrapods evolved from lobe-finned fish.
Fish make up about half of all vertebrate species, with over 35,000 species. Features include gills with a large surface and countercurrent flow, a lateral line that detects vibrations, a swim bladder for buoyancy in bony fish (cartilaginous fish rely on oily liver and swimming), scales, fins and a two-chambered heart with single circulation. Ray-finned fish (Actinopterygii) are the most diverse; lobe-finned fish (coelacanth, lungfish) are closely related to tetrapods.
Fish are ectotherms, though tuna and some sharks keep muscles warm. Reproduction varies: external fertilisation in most, and internal in sharks, some live-bearers (guppies) and seahorses where males brood. Overfishing has depleted many stocks, and aquaculture now supplies about half of fish for human consumption.
Example: salmon migrate from sea to natal streams, using smell, to spawn.
Frequently asked questions
What is Fish?
Aquatic, gill-breathing vertebrates, including jawless, cartilaginous and bony fishes.
Which area of biology covers Fish, and how advanced is it?
Fish is one of the 53 topics in Zoology & Animal Diversity. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Reptiles, Amphibians, Mammals.
Can you give an example of Fish?
Salmon migrate from sea to freshwater to spawn.
What should I learn before Fish?
Fish is a Beginner topic, so no earlier topic is required. Other Beginner topics in Zoology & Animal Diversity that pair well with it include Animal Kingdom, Vertebrates and Invertebrates, Sponges.
What topics are related to Fish?
The closest topics to Fish are Reptiles, Amphibians, Mammals, Vertebrates and Invertebrates. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Fish?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Fish. To practise the whole of Zoology & Animal Diversity, use the Zoology & Animal Diversity quiz.
How is Fish different from Reptiles?
Fish: Aquatic, gill-breathing vertebrates, including jawless, cartilaginous and bony fishes. Reptiles: Scaly, ectothermic vertebrates that lay shelled eggs on land, including snakes, lizards and turtles. Read both side by side and note what each one explains.
How should I study Fish step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Reptiles and explain the difference aloud. Finally, take the Zoology & Animal Diversity quiz and come back to this page for any question you miss.