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Attributes of Fish

Fish are a very diverse group of animals with a broad range of shapes, sizes, and behaviors. Fish live in habitats ranging from the Antarctic and the deep sea to coral reefs and freshwater lakes.

Fish and Fishes

The word fish is used to refer to an individual fish or to a group of fish of the same species. Fishes refers to two or more fish of two or more species. For example, one convict tang is a fish, two convict tangs are fish, and a group of convict tangs is still fish (see Fig. 3.1). However, a convict tang together with another fish becomes fishes (see Fig. 3.2). 
<p><strong>A.&nbsp;</strong><span style="line-height: 1.538em;">a convict tang fish by itself.</span></p> <p><strong>B.&nbsp;</strong><span style="line-height: 1.538em;">two convict tang fish.</span></p> <p><strong>C.&nbsp;</strong><span style="line-height: 1.538em;">a school of convict tang fish.</span></p>
Fig. 3.1. When only one species is present, the word fish is used for one.
<p><strong>Fig. 3.2.</strong> A school of fishes.&nbsp;When more than one species is present, the word fishes is used.&nbsp;</p>


Defining a Fish

<p><strong>Fig. 3.3.</strong> Relative percentage of vertebrates. Approximately 50% of living vertebrates are fishes.</p><br />
Defining a fish is difficult because the term fish includes a very wide range of aquatic animals. In fact, there are more species of fishes than all other vertebrates combined. 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, the rate of discovery of new fishes is higher than that for 
other vertebrates. It is expected that, as scientists make new discoveries, fishes will outnumber the other vertebrates by an even greater margin.
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 in the deepest depths of the oceans and in mountain pools high in the Andes. Fishes live in warm desert pools and under the Arctic pack ice. They range in size from the tiny goby Timmaton nanus (1 centimeter, Fig. 3.3.1 A) to the large whale shark Rhincodon typus (12 meters, Fig. 3.3.1 B) and have varying diets including plankton, algae, other fish, seals, and turtles. Some fish are parasites, some are blind, some are extremely venomous and some can even produce electricity. 

Fig. 3.3.1. (A) The goby Timmaton Nanus (B) the whale shark Rhinodon typus
<p><strong>Fig. 3.4.</strong> A sample of fish skeletons displayed at the Smithsonian Natural History Museum in Washington DC shows the diversity of fish, 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).&nbsp;</p><br />
With all of this amazing diversity, it can be a challenge to describe what exactly a fish is. Fig. 3.4 shows a sample of fish skeletons displayed at the Smithsonian Natural History Museum in Washington DC. The diversity of the skeletons shows a wide range of animals classified as fish–jawed, jawless, cartilage, bone, large and small.
Defining a fish can also be difficult because the word fish is also often used to describe things that are not fish, such as starfish (an echinoderm related to sea urchins and sea cucumbers), jellyfish (a cnidarian related to sea anemones and corals), and crayfish (also called crawfish or crawdads, crustraceans resembling small lobsters, see Fig. 3.5). Even though starfish, jellyfish, and crayfish live in the water, they are not true fish. In Exploring Our Fluid Earth, starfish and jellyfish are referred to as sea stars and jelly medusa to avoid confusion with true fish.

<p><strong>A.</strong></p><br />
<p><strong>B.</strong></p><br />
<p><strong>C.</strong></p><br />

Fig. 3.5. Sea stars (A) are echinoderms, jelly medusas (B) are cnidarians, and crayfish (C) are crustaceans. None of these organisms are fish, but they often incorrectly referred to as fish, for example, starfish and jellyfish. 

Scientifically Defining a Fish

Scientists do not always agree on what a fish is. Early scientists often included whales and dolphins with fish, until they examined these animals more closely and realized they were more like mammals. Even now, not all scientists agree on whether the hagfish should be classified as a fish. This disagreement is not bad, however; science is about change and new ideas, and trying to answer questions.
Because fishes are so diverse and variable, there is almost always an exception to any possible definition, so scientists have had to be careful in their wording when defining a fish. Two scientifically accepted definitions of a fish are:
“a poikilothermic, aquatic chordate with appendages (when present) developed as fins, whose chief respiratory organs are gills and whose body is usually covered with scales” (defined by Berra in 1981) and, 
“an aquatic vertebrate with gills and with limbs in the shape of fins” (defined by Nelson in 1994).
See Table 3.2 for definitions of the key terms in each of these definitions.
Table 3.2. Descriptors and definitions of fish terms




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.


Activity: Draw a Fish

Use your background knowledge to draw a picture of a “typical” fish, showing as many parts of a fish as possible.

Classifying Fish

<p><strong>Fig 3.8.</strong>&nbsp;A phylogenetic tree showing how the major groups of fishes are related.</p><br />
Many fish have strange characteristics that have caused scientists to question their relationship to other fishes. Scientists often use a graphic representation of the hypothesized relationship between organisms called a phylogenetic tree. A phylogenetic tree is made up of branches, lines that represent a particular group, and nodes, points where the branches intersect. Groups that are close to one another on the tree, and that branch from the same node, are supposed to be closer relatives based on their shared similarities. The tree in Fig 3.8 describes fish relationships. The circle node descriptions in Fig. 3.8 represent shared characteristics. For example, all fish have a chambered heart, neurocranium, and liver. But only lobe-finned and ray-finned fish have three earbones. In a phylogentic tree, the most primative organisms, those that evolved first, have the least branching (hagfish in Fig. 3.8), and the most derived or advanced organisms, those that evolved more recently, have the most branching (ray-finned fish in Fig. 3.8).

Jawless Fish–Hagfish and Lampreys

Hagfish and lampreys, classified as Agnathans, are some of the most primitive fishes (see Fig. 3.9). They are both scaleless, lack jaws, and have reduced skeletons; hagfish do not have fins. Hagfish are famous for their abilities to tie themselves in knots to escape predators and to produce lots of slime–a single hagfish can produce twenty liters of slime in minutes. Hagfish are normally found in deep water, where they are scavengers. Lamprey have more complete skulls than hagfish, and they have gill arches. Lamprey are parasitic on other vertebrates.

Fig. 3.9. The jawless hagfish (A) and lamprey (B)

Cartilaginous Fishes–Sharks and Rays

Cartilaginous fishes, classified as Chondrichthyes, have skeletons made of flexible cartilage rather than hard bone. They include the chimeras, which have a hard a gill covering called an operculum, and the elasmobranchs (sharks and rays) with naked gills (see Fig. 3.10). This is an extremely diverse group with almost 1,000 documented living species across many habitats. They have heterocercal tails, which means the tail is uneven (with top lobe generally longer than the bottom lobe), Ampullae of Lorenzini organs that detect electrical fields of their prey, and lateral lines, receptors that detect water motion. Some give live birth, whereas some lay eggs. Contrary to many stereotypes, many fishes in this group have good eyesight. This group also includes a number of colorful fishes.

Fig. 3.10. Cartilaginous fish: a chimera (A) and in B (clockwise) a great white shark, a carcharhinid (a shark in the family Carcharhinidae), a ray and a spotted eagle ray.

Lobe-finned Fishes–Lungfishes and Coelocanths

The lobe-finned fishes, classified as Sarcopterygii, have paired, lobed fins, and include lungfish and coelocanths. Although they are probably not the ancestors of modern reptiles, lungfish and coelocanths represent a very interesting body form that is in-between the walking limbs of land animals and the swimming fins of many fish species (see Fig. 3.11). Coelocanths are able to detect electric fields and lungfish are able to breath air using modified lung structures.

Fig. 3.11. Lobe finned fish: a lungfish (A) and a coelocanth (B).

Primitive Ray Finned Fishes–Sturgeons, Paddlefish and Gars

This group of primitive fish, classified as Actinopterygii and Neopterygii, are very diverse (see Fig. 3.12). The relationship between the fish in this group is not fully understood, but they share some characteristics like scale shape, tail shape (heterocercal), and vertebrae structure. Many Actinopterygii and Neopterygiiactually have cartilaginous skeletons, but the origin is different than in sharks and rays. Fish in this group can also have barbels (feelers below their mouths) and bony scutes (big plates) along their bodies. Some of them, like the paddlefish, can also detect electrical fields and many are long-lived (over 110 years for beluga sturgeon). Sturgeon are also famous as a source of caviar, which is a delicacy of raw fish eggs.

Fig. 3.12. Primitive ray finned fish: (A) a gar, (B) a sturgeon with scutes (bony plates) on its side, and (C) a Mississippi paddlefish.

Modern Bony Fish

The modern bony fishes are classified as Teleostei and often called the teleosts. They are united by an outwardly symmetrical tail fin. As teleosts evolved over time and became more advanced evolutionarily, body-type trends arose including (see Fig. 3.13):
  • shorter, deeper bodies (exceptions are specialized long-bodied fish)
  • more than one dorsal fin or long dorsal fins that begin farther forward
  • spines for fin support in dorsal, anal, and pelvic fins
  • pectoral fins moved forward and higher with more vertical orientation (for locomotion)
  • pelvic fins farther forward (even attached to pectoral girdle or forward of it and vertical)
Other evolutionary trends are harder to observe and include:
  • gas bladder (flotation regulation) moved farther forward and higher for more balance
  • body musculature built up on top of head (may be seen as crest)
  • changes in scale shape
Fig. 3.13. Modern bony fishes, organized by evolutionary relationship, from more primitive to more derived: elephant fish, herring, and milkfish in column A, perch and mullet in column B, and flounder to jacks and porcupine fish in column C.


<p><strong>Fig. 3.14.</strong>&nbsp;A gyotaku print by artist Anne Machado.</p><br />
Scientists measure and describe the external features of fish to identify species, assess age and health, and learn about structure and function. Scientists work with variety of fish specimens, including fresh fish, photographs, scientific drawings, and other kinds of detailed images–even fish fossils. 
One way to document details about a fish is the Japanese fish-printing method called gyotaku (GYOH tah koo). This method can produce an accurate image of a fish (Fig. 3.14). 
Gyotaku is a relatively new art form that developed in Japan, probably in the early to mid-nineteenth century. Gyotaku means ‘fish rubbing.’ Gyotaku is valued from both a scientific and artistic perspective. The detail captured in Gyotaku, especially in historical prints, is an important source of information for scientists who want to know the size of fish in the past. The color and artistic arrangement of gyotaku prints made by skilled artists make them valuable pieces of decoration. The oldest known gyotaku print, made in 1862, is owned by the Homma Museum in Sakata, Japan.

Activity: Fish Printing for Form and Function

Use your observation and investigation skills to investigate fish form and function by experimenting with ways of making gyotaku fish prints.

Fish Anatomy

Anatomy is the study of an organism’s structures and their arrangement in relation to each other. Fishes come in a diverse array of shapes and forms, many with special modifications. The external anatomy of a fish can reveal a great deal about where and how it lives. The shape, size, and structure of body parts permit different fish to live in different environments or in different parts of the same environment. 
<p><strong>Fig. 3.17.</strong>&nbsp;Common anatomy terms applied to three different animals: a billfish, a horse, and a person.</p><br />
When describing the basic anatomy, or body plan, of an organism, it is useful to have some common terms to help with orientation. Just as when reading a map we use north, south, east, or west to help us determine the location, certain orientation words are useful in describing anatomy. Table 3.2.1 defines common anatomy terms, Fig. 3.17 depicts their location on three different animals. 
  • Anterior- the head end
  • Posterior- the tail end
  • Dorsal- the back
  • Ventral- the belly
  • Lateral- the side or flank of an organism
Table 3.2.1. Anatomy position words

Anatomy Word

…of the organism


The head end


The tail end


The back


The front or belly


The side or flank

Body Form

Perches are the most common fishes, so the most commonly observed body form of a fish is often called perch-like (Fig. 3.18 A). A perch-like body plan is fairly robust and streamlined. This body plan is usually referred to as fusiform or torpedo-like. Although the fusiform body is typical of many fish, depending on the environment in which they live, other types of body plans that may be seen are compressiform or laterally flattened (Fig. 3.18 B), depressiform, or dorso-ventrally flattened (Fig. 3.18 C), and anguilliform or eel-like (Fig. 3.18 D). See Table 3.3 for additional descriptions of fish body shapes.

Fig. 3.18. (A) Perch, (B) Angelfish, (C) Flounder, (D) Eel


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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.