All About Fish: Facts on Species, Breathing and Sleep

A fish is a water-dwelling vertebrate that breathes with gills and swims with fins. Fish are the largest group of vertebrates: Wikipedia counts over 33,000 living species, more than all other traditional vertebrate classes combined, and ray-finned fishes alone make up about half of all living vertebrates. They fall into three broad groups: jawless fish, cartilaginous fish such as sharks and rays, and bony fish. Whales and dolphins are mammals, not fish.

• Fish breathe oxygen, not air. Their gills hold a network of fine blood vessels that take oxygen from water (NOAA Fisheries).
• Some can gulp air. Gars have a swim bladder with blood vessels that works as a lung, so they survive in low-oxygen water (Wikipedia).
• They feel water movement. A lateral line system along the body senses motion and vibration nearby (Wikipedia).
• Most float with a swim bladder, a gas-filled organ that controls buoyancy (Wikipedia).
• Most lay eggs. About 97% of fish are egg-layers (Wikipedia); some, such as freshwater stingrays, give birth to live young.
• Most are cold-blooded, though a few species are partly warm-blooded (Wikipedia).
• Speed: tunas, billfish and certain sharks can reach 50 mph in short bursts (NOAA Fisheries).
• Age: scientists read growth rings on scales or ear bones (otoliths), like tree rings (NOAA Fisheries).
• Oldest: the orange roughy lives more than 100 years (NOAA Fisheries).
• Most common: deep-sea bristlemouths, about the size of a small minnow (NOAA Fisheries).
• Heat and cold: desert pupfish tolerate water up to 36 C, while Antarctic icefish live in polar seas (Wikipedia).
• Pain: research shows fish are sentient and can experience pain (Wikipedia), which is why careful handling matters when you release one.

Most fish rest rather than sleep the way we do. They have no eyelids, so their eyes stay open; NOAA Fisheries says some float in place, some wedge themselves into mud or coral, and some build a nest. Scientists recognise fish sleep by behaviour: long periods of stillness, a typical resting posture, a daily rhythm, and being harder to wake (Wikipedia, sleep in fish).

Examples from the research:

• Zebrafish sleep longer after being kept awake, a sign of a real need for sleep.
• Parrotfish wrap themselves in a mucus cocoon at night that screens them from predators and parasites.
• Cavefish: surface-dwelling populations sleep 3 to 6 hours a day, while cave populations of the same species stay almost constantly active.
• Tuna and mackerel swim continuously and show no clear signs of sleep.

• Largest: the whale shark, more than 50 ft (NOAA Fisheries); Wikipedia gives 16 m (52 ft).
• Smallest: sources differ. NOAA Fisheries names a Philippine goby that rarely exceeds half an inch; Wikipedia lists fish as small as 8 mm, such as Paedocypris progenetica.
• Heaviest freshwater fish: a 300 kg giant freshwater stingray caught in Cambodia in 2022; see big freshwater fish.
• Deepest: a cusk-eel recorded at 8,370 m in the Puerto Rico Trench (Wikipedia).
• Longest-lived: the orange roughy, more than 100 years; in the Northwest Atlantic, the Acadian redfish passes 40 (NOAA Fisheries).

Long fact lists often repeat numbers without a source, and some are out of date; the species count alone ranges from NOAA's 20,000 to Wikipedia's 33,000+. Every fact on this page names where it comes from, and where two sources disagree we show both. For facts about one species, use our species pages, such as the clownfish, puffer fish or electric eel.

With more than 33,000 species, naming a fish from memory is hard even for experienced anglers. Photograph the fish on its side with fins spread, in even light. FishLens names the likely species from the photo, and our guides explain which features to check.

Research and drafting for this page were assisted by AI and checked line by line against the sources listed below. Every number names the organisation it comes from, and where sources disagree we say so instead of picking one silently. Prices carry the date we checked them (25 September 2026); they change, so confirm before you rely on them. If you find an error, tell us and we will correct it.