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Fish come in many shapes and sizes. This is a sea dragon (kira), a close relative of the seahorse. They are camouflaged as floating seaweed.[1][2]
Dunkleosteus, a giant armoured arthrodire of the later Devonian. Inland seas of North America, up to 6 metres long.[3]

Fish (plural: fish or fishes) are vertebrates which live in water and breathe using gills.

That is a simple way to define fish, but it is not quite accurate. Some amphibia are not fish, but live in water and have external gills. That is why biologists use a more complicated definition: fish are gill-bearing aquatic vertebrates which lack limbs with digits (fingers & toes).

'Fish' is a paraphyletic term because it lacks a monophyletic group of descendants. In other words, it lacks the land vertebrates or tetrapods, which descended from fish. Fish used to be a class of vertebrates which live in water. Now the term covers five classes of aquatic vertebrates:

Jawless fish
Armoured fish
Cartilaginous fish
Ray-finned fish
Lobe-finned fish

Fish are usually covered with scales, and have two sets of paired fins, and several unpaired fins. They are usually cold-blooded. A fish takes in the oxygen from the water using gills. There are many different kinds of fish. They live in fresh water in lakes and rivers, and in salt water in the ocean. Some fish are less than one centimeter long. The largest fish is the whale shark, which can be almost 15 meters long and weigh 15 tons. Most fish live in the water. A group of fish called the Lungfish have developed lungs, because they live in rivers and pools that dry up, certain parts of the year.

Certain animals that have the word fish in their name are not really fish: Crayfish are crustaceans, and jellyfish are Cnidarians. Hagfish and Lampreys do not have a jawbone. Hagfish are craniata, Lampreys are Hyperoartia. Certain animals look like fish, but are not. Whales and dolphins are mammals, for example.

Fish or fishes?

Though often used interchangeably, these words have different meanings. Fish is used either as singular noun or to describe a group of specimens from a single species. Fishes describes a group of different species.[4]

Types of fish

Fish, the oldest vertebrate group, includes a huge range of types, from the Middle Ordovician, about 490 million years ago, to the present day. These are the main groups:[4][5][6]


The anatomy of Lampanyctodes hectoris
1. operculum (gill cover) 2 & 5. lateral line 3. dorsal fin 4 fat fin 6. caudal fin 7. anal fin 8. photophores 9. pelvic fins paired 10. pectoral fins (paired)

Bony and cartilagenous fish

Most kinds of fish have bones. Some kinds of fish, such as sharks and rays, do not have real bones (their skeletons are made of cartilage) they are known as cartilaginous fish.

Fish scales

All fish are covered with overlapping scales, and each major group of fish has its own special type of scale. Teleosts ('modern' fish) have what are called leptoid scales. These grow in concentric circles and overlap in a head to tail direction like roof tiles. Sharks and other chondrichthyes have placoid scales made of denticles, like small versions of their teeth. These also overlap in a head to tail direction, producing a tough outer layer. Shark skin is available for purchase as shagreen, a leather which as original is smooth in one direction, and rough in the other direction. It may be polished for use, but is always rough in texture and resistant to slipping.

The scales are usually covered with a layer of slime which improves passage through the water, and makes the fish more slippery to a predator.


Fish swim by exerting force against the surrounding water. There are exceptions, but this is usually done by the fish contracting muscles on either side of its body in order to generate waves of flexion that travel the length of the body from nose to tail, generally getting larger as they go along. Most fishes generate thrust using lateral movements of their body & tail fin (caudal fin). However, there are also species which move mainly using their median and paired fins. The latter group profits from the gained manoeuvrability that is needed when living in coral reefs for example. But they can not swim as fast as fish using their bodies & caudal fins.[8]


Fish can swim slowly for many hours using red muscle fibres. They also make short, fast bursts using white muscle. The two types of muscle have a fundamentally different physiology. The red fibres are usually alongside a much greater amount of white fibres.

White fibres

The white fibres get their energy by converting the carbohydrate glycogen to lactate (lactic acid). This is anaerobic metabolism, that is, it does not need oxygen. They are used for fast, short bursts. Once the lactic acid builds up in the muscles, they stop working, and it takes time for the lactate to be removed, and the glycogen replaced.[9] Using their white fibres, fish can reach speeds of 10 lengths per second for short bursts.[9]

Red fibres

Swimming for long periods needs oxygen for the red fibres. The oxygen supply has to be constant because these fibres only operate aerobically. They are red because they have a rich blood supply, and they contain myoglobin. Myoglobin transports the oxygen to the oxidising systems. Red muscle gets its energy by oxidising fat, which weight for weight has twice as much energy as carbohydrate or protein.[9] Using their red fibres, fish can keep up a speed of 3–5 lengths per second for long periods.[9]

Body shape

The shape of the body of a fish is important to its swimming. This is because streamlined body shapes makes the water drag less. Here are some common fish shapes.


A smooth, rocket-shaped shark swims quickly through the water.
The rocket-shape of this shark makes it an efficient swimmer. It is fast over short distances.

The picture on the right shows a shark. This shark's shape is called fusiform, and it is an ovoid shape where both ends of the fish are pointy. This is the best shape for going through water quickly.[10][11] Fishes with fusiform shapes can chase prey and escape predators quickly. Many live in the open ocean and swim constantly, like marlins, swordfish, and tuna. Ichthyosaurs, porpoises, dolphins, killer whales all have similar shapes. This is an example of convergent evolution.


The long, ribbon-like shape of an eel's body shows another shape. This enables them to hide in cracks, springing out quickly to capture prey, then returning quickly to their hiding spot.


a depressed flounder.
An adult flounder has both eyes on the upper side of its body.

Flatfish live on the bottom of the ocean or lake. Most use camouflage: they change colors to match the ocean floor.


Fish with compressed shapes have flat, vertical bodies, with one eye on each side. They swim upright and can be very thin. They usually live in reefs where their flat bodies can slip in and out among the corals, sponges, and rocks, keeping hidden from predators. Angelfish, surgeonfish, and butterflyfish are all compressed fish.

Swimming in groups

Many fish swim in groups a lot of the time. Schools of fish can swim together for long distances, and may be chased by predators which also swim in schools. Casual groups are called 'shoals'.

Fish as food

People eat many kinds of fish. The fish that people eat most include carp, cod, herring, perch, sardines, sturgeon, tilapia, trout, tuna, and many others. A person who buys and sells fish for eating is called a fishmonger.


The word to fish is also used for the activity of catching fish. People catch fish with small nets from the side of the water or from small boats, or with big nets from big boats. People can also catch fish with fishing poles and fishhooks with bait. This is often called fishing. There is also different types of lures that can be used. One is a crank bait. Others are plastic worms and rat-l-traps. These are lots of different ways of catching fish.

Fish as pets

Some people keep fish as pets. Goldfish and Siamese Fighting Fish are popular types of pet fish. Koi and other ornamental fish are often kept by groups of people in public ponds for their beauty and calming nature.


  1. Leafy and Weedy Sea Dragon National Geographic Profile. Retrieved 20 July 2009.
  2. Connolly R (2006). Phycodurus eques. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 20 July 2009.
  3. Long J.A. 1995. The rise of fishes: 500 million years of evolution. Johns Hopkins, Baltimore.
  4. 4.0 4.1 Helfman G, Collette BB, Facey DH and Bowen BW 2009. The diversity of fishes: biology, evolution, and ecology Wiley-Blackwell. ISBN 978-1-4051-2494-2
  5. Moyle P.B. and Cech J.J. 2003. Fishes: an introduction to Ichthyology. 5th ed, Cummings. ISBN 978-0-13-100847-2
  6. Maisey J.G. 1996. Discovering fossil fishes. Holt N.Y.
  7. Janvier, Philippe 2010. MicroRNAs revive old views about jawless vertebrate divergence and evolution. Proceedings of the National Academy of Sciences (USA) 107:19137-19138. [1]
  8. Breder C.M. 2003. The locomotion of fishes. Zoologica, 4, 159-256, (1926); Journal of Experimental Biology 206, 2749-2758 (2003)
  9. 9.0 9.1 9.2 9.3 Alexander, R. McNeill. 1974. Functional design in fishes. London: Hutchinson, p31. ISBN 0-09-104751-X
  10. Nielsen, Knut Schmidt 1984. Scaling: why is animal size so important? Cambridge. Chapter 15
  11. Baily, Jill 1997. How fish swim. Benchmark, N.Y.

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