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What is a Fish?


Fish and Fishes

Fish are aquatic vertebrate animals that have gills but lack limbs with digits, like fingers or toes. Recall that vertebrates are animals with internal backbones. Most fish are streamlined in their general body form.


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Fig. 4.2. (A) Fish—one individual, such as a convict tang (Acanthurus triostegus) by itself

Image copyright and source

Image courtesy of Brian Gratwicke, Flickr

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Fig. 4.2. (B) Fish—two individual convict tang fish

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Image courtesy of Paul Asman and Jill Lenoble, Flickr

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Fig. 4.2. (C) Fish—many individuals such as an entire school of convict tang fish

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Image courtesy of Kydd Pollack, US Fish and Wildlife Service (USFWS)

The word fish is the same whether it is singular or plural—you can talk about one fish or ten fish. The word fish is used to refer to an individual fish or to a group of fish of the same species. The word fishes refers to multiple species of fish.



Fig. 4.3. Fishes—when more than one species is present, the word fishes is used.

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

If you have two fish, it means you have two individual fish that are the same species, but if you have two fishes, it means you have two individual fish of different species. For example, 1 convict tang is a fish, 2 convict tangs are 2 fish, and a group of 20 convict tangs are still 20 fish (Fig. 4.2). However, a convict tang together with a moray eel would be 2 fishes (Fig. 4.3).

Defining a Fish

Defining a fish is difficult because the term fish includes a very wide range of aquatic animals. In fact, there are about as many species of fishes than there are of all other vertebrates combined (Fig. 4.4). Of the nearly 50,000 species of animals with backbones, approximately 4,500 are mammals, 9,700 are birds, 6,500 are reptiles, 4,000 are amphibians, and 25,000 are fishes. Although these numbers change as new species are discovered, new fishes are found more often than other new vertebrates are found. As scientists make new discoveries, it is expected that the number of species of fishes will outnumber other vertebrates even more.


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Fig. 4.4. Relative percentage of vertebrates. Approximately 50 percent of living vertebrates are fishes.

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Image by Byron Inouye


With so many animals identified as fish, it is not surprising that they come in all shapes and sizes and can be found in a range of habitats. Fishes live down deep and up high—from the deepest depths of the oceans to mountain pools high in the Andes. Fishes live in warm and cold water—from desert pools to under Arctic pack ice.



Fig. 4.5. (A) The <10 mm cyprinid (Paedocypris progenetica)

Image by Aquaristikhaus, Wikimedia Commons

Fig. 4.5. (B) the 12 m whale shark (Rhincodon typus)

Image Courtesy of Zac Wolf

Fishes also range in size from very small, like the tiny cyprinid in Indonesia, which grows to a maximum length of 10 mm (Fig. 4.5 A) to the large whale shark Rhincodon typus, which reaches 12 meters (Fig. 4.5 B), with thier skeletons showing great variety in size, composition, and structure (Fig. 4.6). Fishes have a wide range of diets, including plankton, algae, fish, seals, and turtles. Some fishes are parasites, some are blind, some are venomous, and some can even produce electricity. With all of this amazing diversity, it is challenging to make a definition that describes all fishes.



Fig. 4.6. A sample of fish skeletons displayed at the Smithsonian Institution National Museum of Natural History in Washington, D.C. shows the diversity of fish skeletons, from a jawless hagfish (upper left), to a bat ray (bottom left), to a shark jaw (middle), to a flounder flat fish (upper right), to a bill fish and other ray finned fishes (lower right).

Photos by Kanesa Duncan Seraphin

Defining a fish can also be difficult because the word fish is often used to describe things that are not fish. For example, starfish are really echinoderms—related to sea urchins and sea cucumbers. Jellyfish are cnidarians—related to sea anemones and corals. Crayfish are really crustaceans—related to lobsters (see Fig. 4.7). Even though starfish, jellyfish, and crayfish live in the water, they do not have backbones, and they are not true fish. In Exploring Our Fluid Earth, starfish are called sea stars and jellyfish are called jelly medusa to avoid confusion with true fish.

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Fig. 4.7. (A) Starfish, or sea stars, are really echinoderms (not fish).

Image copyright and source

Image courtesy of National Oceanic and Atmospheric Administration (NOAA), Estuarine Research Reserve Collection

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Fig. 4.7. (B) Jellyfish, or jelly medusas, are really cnidarians (not fish).

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Image courtesy of Anna Fiolek, National Oceanic and Atmospheric Administration (NOAA)

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Fig. 4.7. (C) Crayfish, or crawdads, are really crustaceans (not fish).

Image copyright and source

Image courtesy of André Karwath, Wikimedia Commons


Scientifically Defining a Fish

Scientists do not always agree on what a fish is. In fact, some early scientists thought that whales and dolphins were fish. Even now, not all scientists agree on whether the hagfish should be classified as a fish.


This type of disagreement is sometimes confusing, but it is not bad. Science is about new ideas and trying to answer questions. Often these questions are difficult, and often it is hard to accurately describe relationships between organisms. During the scientific process, definitions often need to be modified, and organisms are often reclassified when new information is found.

Table 4.2. Terms and definitions of words used to describe fish




An animal whose temperature adjusts with the outside temperature. These kind of animals have historically been called cold-blooded, but that is not completely accurate. If it is cold outside, their body temperature will be cold. If it is warm, their body temperature will be warmer.

Most fish are poikilothermic, but some fish, like tunas and lamind sharks, use special blood vessel networks to keep their body temperature warmer than the surrounding water. This helps them to be better hunters.


Living in the water, either seawater or freshwater. Some fishes like lungfish or mudskippers may be able to spend some time out of the water, but they cannot remain permanently out of the water, and they are confined to wet areas.


Animals with notochords. A notochord is a stiff rod of cartilage that supports the nerve cord. Chordates have some other features in common, like gill slits, and a dorsal nerve cord. Mammals, birds, reptiles, amphibians, and fish are all chordates, a few invertebrates are chordates too.


Organs for gas exchange. Most fishes breathe with gills.

However, some fishes have lungs, some can exchange gas through their skins, and some are able to gulp air and exchange gas through their stomachs.


Thin plates that cover the thin skin of fish. Modifications of scales include hard bony plates or spines. Scales can also be very small or absent in some fishes, like blennies or eels.


If fish have appendages, they are thin, flat moveable fan-like parts. However, some fishes, like eels, do not have any appendages.

Fishes are amazingly diverse and variable. Therefore, scientists have to use careful wording when defining a fish. There are two generally accepted scientific definitions of a fish:

  1. Fishes are “aquatic vertebrate(s) with gills and with limbs in the shape of fins.” (Written by Peter Nelson in 1994.)
  2. Fishes are “poikilothermic, aquatic chordate(s) with appendages (when present) developed as fins, whose chief respiratory organs are gills and whose body is usually covered with scales.” (Written by Tim Berra in 1981.)

Notice that neither of these definitions mention skeletal features. Both definitions include fishes with skeletons made of bone (like tuna) and fishes with skeletons made of cartilage (like sharks).


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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.