What is a fish?

Fish were the first vertebrates (animals with a backbone) to appear on Earth. Today they are by far the most numerous group of vertebrates with over 30,000 species recorded and more being discovered each year.

They are notable for spending most of their life in the water. Some species such as the mudskipper have special adaptations which allow them to spend short periods on land.

These animals can occur in both salt and freshwater. Most species are restricted to only one type of water but some have the ability to move between both.

The first species of fish to appear on Earth evolved over 500 million years ago.

To propel them through the water most species have developed some form of fin which is a flexible appendage attached to the back, side or end of the body which will move from side to side to provide propulsion.

Red Lionfish (Pterois volitans)

More on Fish

Most species of fish are equipped with gills. These appendages allow them to attract oxygen from the water. Some species such as the lungfish will gulp air at the water's surface.

Unlike amphibians which may have gills while they are a larvae fish will retain them through their entire life.

Another primary characteristic of fish is that they are ectothermic, or cold-blooded. This means that these animals rely on outside factors to generate their body heat. As a result of this most species of fish require a specific water temperature in which to survive and outside of these ranges they cannot survive.

Countershading is commonly seen in fish. This form of camouflage is achieved by having light coloring on the underside which blends with the light coming from above and dark colors on the upperside which blend with the dark colors of the ocean floor of depths.


Record Breaking Fish

Photo Credit - Aquaristikhaus, CC BY-SA 3.0 <>, via Wikimedia Commons

The world's smallest fish

The smallest fish is a hard category judge with three species in the mix. One achieves smaller lengths but on average is larger than the second place.

A third has small males but the females are much larger.

One of the three species in the mix and the most likely winner is Paedocypris progenetica. This species has an average length of 10.3 mm (0.41 in).

Photo Credit - Under License

The world's Largest fish

The largest fish of all is the whale shark. These animals average a length of 12m (39ft) long though some individuals reach lengths of up to 14m (46ft) long.

While their large size makes them intimidating this species is not a threat to humans as they feed almost entirely on plankton.

The world's fastest Fish

The world's fastest species of fish is the sailfish. Over short periods of time they have been clocked at speeds as high as 110 km/h (68 mph).

Previously sailfish have been considered a group of subspecies. Further research has seen them grouped under a single species (Istiophorus platypterus). Some authorities suggest that a second species is present in the Atlantic Ocean.


Examples Of Fish

Below are some examples of the groups of fish which exist. How many do you know?

  • Sharks
  • Gars
  • Sturgeons
  • Rays
  • Coelacanths
  • Lungfish
  • Chiameras
  • Lampreys
  • Hagfish

It is estimated there may be as many as 32,000 species of fish on Earth. This makes them the largest group of vertebrates.


Fish Fact Files

Our first fish fact file is coming on March 8th. Check back daily to meet a new fish species each day from March 8th to 14th.


Florida Museum. 2022. Fish Groups. [online] Available at: <> [Accessed 29 January 2022].

National Geographic. 2022. Fish Pictures & Facts. [online] Available at: <> [Accessed 30 January 2022].

Sciencing. 2022. Five Classes of Chordates. [online] Available at: <> [Accessed 30 January 2022].


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