Fish

Fish are cold-blooded vertebrates that live in water and breathe with gills. There are around 34,000 known species of fish, more than all the other vertebrate groups put together. Fish were the first vertebrates to evolve, around 530 million years ago, and they still rule the oceans, rivers and lakes today. They range from the tiny stout infantfish (less than 1 cm long) to the giant whale shark (up to 18 metres long, the biggest fish in the world).

• Known speciesapprox. 34,000More than all other vertebrates combined
• SmallestStout infantfish7 mm long, in coral reefs
• LargestWhale sharkUp to 18 m, harmless plankton-eater
• FastestBlack marlinUp to 130 km/h
• DeepestMariana snailfishCaught at 8,178 m deep
• Three main groupsJawless, cartilaginous, bonyBony fish are by far the largest

What makes a fish?

• Live in water: all fish are aquatic.
• Cold-blooded: a few (like tuna) can warm themselves slightly, but most are pure ectotherms.
• Breathe with gills: extract oxygen directly from water.
• Have fins: typically dorsal, pectoral, pelvic, anal and caudal (tail).
• Vertebrates: have a backbone.
• Mostly covered in scales: overlapping plates that protect their skin.
• Lay eggs: most fish lay huge numbers of small eggs; a few give birth to live young.

The three main groups

• Jawless fish: the oldest and simplest fish. Just two living groups: lampreys (around 40 species) and hagfish (around 80 species). Both look eel-like and lack proper jaws. They were once a huge group; today they are a tiny remnant.
• Cartilaginous fish: fish with skeletons made of cartilage instead of bone. Includes all sharks (around 500 species), rays, skates and chimaeras. Some of the most ancient fish lineages, hardly changed in hundreds of millions of years.
• Bony fish: the biggest group by far, with over 32,000 species. Have skeletons made of bone. Includes almost every fish you would recognise: salmon, trout, tuna, herring, goldfish, koi, sea horses, swordfish, eels.

How fish breathe

Fish extract oxygen from water using gills: filaments of thin tissue inside the head, packed with tiny blood vessels. Water flows in through the mouth, over the gills (which absorb the oxygen), and out through gill slits on the sides of the head.

Water contains far less oxygen than air (only about 1 to 10 mg of oxygen per litre, compared with 200 mg of oxygen per litre of air). So gills have to be much more efficient than lungs to extract enough. A typical fish gill removes about 80% of the available oxygen from each gulp of water, which is why even small fish can be so active.

Famous fish

• Great white shark: the largest predatory fish, up to 6 m long.
• Whale shark: the largest fish overall, up to 18 m. Harmless filter-feeder, eats plankton.
• Anglerfish: deep-sea predators with a bioluminescent lure dangling from their head.
• Sea horses: the only fish with a horse-like head. The males carry the babies in a pouch.
• Electric eel: produces electric shocks of up to 860 volts to stun prey (despite the name, it is not actually a true eel).
• Coelacanth: a "living fossil" thought to have gone extinct 66 million years ago, until a live one was caught off South Africa in 1938.
• Mudskipper: a fish that crawls out of the water and breathes air for hours at a time, the closest modern fish gets to the fish-to-amphibian transition.

Fact The world's smallest vertebrate is a fish: Paedocypris progenetica, a tiny carp from the swamp forests of Sumatra. Adults are around 7.9 mm long, smaller than the width of a pencil. They live in highly acidic peat water where almost no other vertebrate can survive.

Life in the deep sea

The deep ocean is the largest habitat on Earth, and fish dominate it. Below about 200 m there is no sunlight, the pressure is enormous (one atmosphere extra for every 10 m down), and food is scarce. Deep-sea fish have evolved spectacular adaptations:

• Bioluminescence: glowing organs to attract prey, find mates or distract predators.
• Huge eyes (or no eyes at all): to catch tiny amounts of light, or save energy in total darkness.
• Slow metabolism: deep-sea fish move and eat very slowly to save energy.
• Soft, jelly-like bodies: to cope with the pressure.
• Huge mouths and stomachs: so they can eat whatever rare food comes along.

The deepest known fish, the Mariana snailfish, lives at over 8,000 metres down in the Mariana Trench. Below about 8,400 m, the pressure breaks the chemistry of fish proteins and no fish can survive. That makes 8,400 m the deepest depth in the ocean at which a fish has ever been recorded.

Did you know? A few fish are actually warm-blooded. Tuna, swordfish, marlin and several shark species can raise their muscle temperature above the surrounding water by up to 15 °C, allowing them to swim faster and hunt more effectively. This partial warm-bloodedness evolved separately in fish at least 5 times.

Deeper dive: the coelacanth, a fish from the time of the dinosaurs

One of the most extraordinary biological discoveries of the 20th century happened in December 1938, off the coast of South Africa. A young museum curator named Marjorie Courtenay-Latimer was inspecting the catch of a local fishing trawler when she noticed an unusual fish among the catch: a metre-long blue-grey fish with strange lobed fins and an odd extra tail-spike. She sent a sketch to a Cape Town biologist, Professor J.L.B. Smith. He immediately recognised it as a coelacanth, a fish previously thought to have gone extinct around 66 million years ago.

Coelacanths had been known to science from fossils for over 100 years. They were a major branch of the lobe-finned fish, the same group that gave rise to the first amphibians and so to all four-legged land animals. After the End-Cretaceous extinction, coelacanths disappeared from the fossil record. They were assumed to be long gone.

The 1938 discovery showed that at least one coelacanth species had survived in the deep waters off the African coast all along. A second species was discovered in 1997 in Indonesia. Coelacanths have hardly changed from their fossil relatives in 360 million years. They live at depths of around 150 to 700 metres, give birth to live young (rather than laying eggs like most fish), and use their lobed fins in a swimming gait that resembles the walking motion of land animals.

The coelacanth is one of the best examples of a "living fossil": an ancient species that has remained essentially unchanged for hundreds of millions of years while everything around it evolved. It is a direct biological link to the time when the ancestors of mammals first crawled onto land.

For the descendants of fish that crawled onto land, see amphibians. For other vertebrate groups, see mammals and reptiles.