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Getting to Know Fishes


Fossil records show that fishes have been in existence for 438 million years. They were the first vertebrates (VER-teh-brehts), or animal with a backbone. A fish is a cold-blooded vertebrate that lives primarily in water and has a skull, gills, and fins. Cold-blooded means the animal's internal temperature changes with its surroundings.

There are two types of vertebrates: those with jaws and those without jaws. The only jawless vertebrates are fishes: lampreys (LAM-prees) and hagfishes. Among the many vertebrates with jaws, two types are fishes. One type has a skeleton made of cartilage (KAR-teh-lej), a tough but bendable tissue. The other type, 95 percent of fishes, has a skeleton made of bone.

Fossil records show that fishes have been in existence for 438 million years. (Jonathan Blair/Corbis. Reproduced by permission.)

Side view of the skeleton of a yellow perch. Ninety-five percent of fishes have a skeleton made of bone. (Illustration by Emily Damstra. Reproduced by permission.)

Fishes vary in length from about one-eighth inch (a few millimeters) to more than 59 feet (18 meters). Fishes can be ribbon shaped like eels, pointed at the ends like salmons, flat from top to bottom like stingrays, or flat from side to side like flounders.

Most fins consist of spines or rays connected by a thin film. Some fins do not contain rays. Fishes have two sets of paired fins, several fins on the body midline, and a tail fin.

Scales protect the skin and deeper tissues from the environment. In some fishes, scales are bony plates that form armor over the surface of the body but do not overlap. Most fishes, however, have thinner, lighter scales that do overlap. The exposed edge of overlapping scales can be smooth or rough. Some fishes do not have scales at all.

The lateral (LAT-uhr-uhl) line runs along each side of a fish's body from head to tail. This line is a series of pores and tiny tubes that sense low vibrations, or sound waves, that cannot be heard. This structure helps fishes avoid obstacles, or things that block the way, and predators, or animals that kill and eat other animals, and find food. The lateral line canals can be seen in most fishes, and in many species are open grooves.

Fishes are unique in that they can move their upper jaw and their lower jaw, a feature that helps them eat large foods. The Different fishes have developed diifferent shaped fins: a. Sea robin; b. Catfish; c. Dogfish; d. Mosquitofish; e. Anglerfish; f. Lumpfish. (Illustration by Marguette Dongvillo. Reproduced by permission.) teeth of many fish predators are triangular and sawlike for cutting through prey, or the animals hunted and caught for food. Predators that swallow prey whole have long backward-pointing teeth that grasp the prey and prevent it from struggling out of the mouth. Many fishes have teeth adapted for biting and crushing hard material, such as shells and coral.

Gills are the organs that fishes use to breathe in water. The gills are behind the head on both sides of the body. Water flows through the mouth, over the gills, and back to the outside. While water flows over the tiny threads that make up the gills, oxygen in it passes into the blood. Some fishes gulp air at the Tooth morphology and tooth-bearing structures typical of fishes: A. Bowfin; B. Mooneye; C. Sand shark; D. Parrotfish; E. Pike minnow; F. Sea lamprey; G. Tooth form and functions. (Illustration by Bruce Worden. Reproduced by permission.) surface of the water and store it in lunglike organs. Other fishes use the swim bladder for breathing.

The swim bladder is a sac of gases in the body that helps a fish hold its position in the water. The fish rises as gas moves from the bloodstream into the swim bladder and moves down as gas leaves the swim bladder. Some fishes do not have a swim bladder. In these fishes oil in the liver keeps the fish steady. This system works because oil is lighter than water.


Freshwater in lakes and rivers makes up less than 0.01 percent of the total amount of water on Earth, yet 42 percent of all fish species live there. The other 58 percent live in the oceans The swim bladder is a sac of gases in the body that helps a fish hold its position in the water. The fish rises as gas moves from the bloodstream into the swim bladder and moves down as gas leaves the swim bladder. (Illustration by Jacqueline Mahannah. Reproduced by permission.) that make up 97 percent of Earth's water. (The other 2.99 percent of water is in ice, the ground, and the atmosphere.) Most ocean fishes live in the tropical and temperate zones, that is, in water 50°F (10°C) or warmer.


Most freshwater fishes live in streams, rivers, and lakes. Other freshwater habitats are wetlands such as marshes and swamps. Saltwater fishes live in oceans and seas near the shore, in coral reefs, on the deep ocean floor, and in open water.


Many fishes eat plankton, or microscopic (MAI-kro-SKA-pihk) plants and animals drifting in the water. The plankton is too small to be seen with the eye. Some fishes pluck the plankton particles from the water. Others take in plankton with the Fishes eat many different things, from microscopic plankton to sea birds, reptiles, and mammals. Here a wolf eel makes a meal of a sea urchin. (Brandon D. Cole/Corbis. Reproduced by permission.) water that flows over their gills. Comblike structures called gill rakers strain out the plankton before it gets to the gills and divert it to the digestive tract where it is changed into food energy. Some fishes eat bottom-dwelling algae (AL-jee), which are tiny plantlike growths that live in water but have no true roots, stems, or leaves; grasses, or even seeds and fruits from plants on the shore or river bank. Other fishes feed on invertebrates, or animals without a backbone, such as insects, spiders, or earthworms, and on other fishes. Invertebrate prey may be bottom dwelling, such as clams, or be in open water, such as squids. Fish eaters actively hunt, chase, herd, grasp, stun, club, shock, ambush, bite, or engulf other fishes. Large fishes may feed on floating sea birds, reptiles, and mammals in addition to fishes. Scavengers (SKAV-ihn-jerz) feed on dead or dying animals.


Fishes swim by contracting or drawing together muscles at the bases of their fins to move the fins and by contracting their body muscles in a wave from head to tail to move the backbone. Fishes use chemical signals for navigation. For example, salmons use this sense to set a course and direction to find the stream in which they were born and to which they will return to reproduce and die. Some fishes, especially sharks, can sense barely detectable electrical currents discharged by prey. Electric signals are received in pit organs on the surface of the body.


The "shellfish" found in restaurants and fish markets is not fish at all but crustaceans (krus-TAY-shuns), such as crabs, and bivalve mollusks (MAH-lusks), such as clams. Bivalve mollusks are invertebrates with a one-part body, a muscular foot but no legs, and a shell made of calcium carbonate. Crustaceans are invertebrates with jointed legs, a shell made of chitin, and a body that has several parts.

Some fishes form shoals or schools for protection from predators, finding food, migration (MY-gray-shun), and reproduction. Shoals are unorganized, often temporary groups. They may consist of different species with a changing membership and may break up and form again quickly. Schools are organized groups that form permanently or temporarily. They usually are made up of only one species.

Salmons use chemical signals to return to the stream in which they were born. (Ralph A. Clevenger/Corbis. Reproduced by permission.)

Fishes reproduce by spawning, that is, producing or releasing eggs that are then fertilized (FUR-teh-lyezd), or united with the male's sperm, outside the female's body; by laying eggs that have been fertilized inside the female's body; or by giving birth to fully developed young that have grown inside the female after the eggs have been fertilized inside her.

Some fishes emerge from the egg as larvae (LAR-vee) and must undergo many changes before becoming adults. During this phase the larvae eat plankton and are plankton. The larval Some fishes care for their eggs and offspring. Here a male lumpfish is guarding its eggs. (Andrew J. Martinez/Photo Researchers, Inc. Reproduced by permission.) stage ends when the skeleton, organ systems, skin color, arrangement of scales, and fins become fully developed, and the young fishes look like adults. The transition can take minutes to years.

Some fishes care for their eggs and offspring by behaviors such as nest building before spawning; mouth brooding, or keeping the eggs in a parent's mouth until hatching; nest guarding; and fanning, or using the fins to move water over eggs or hatchlings to clean them and give them oxygen.


Fishes have been important in human life since ancient times. Fish themes appear in mythology, religion, literature, and art. Fishes are important to humans for food, employment, and income. Many people enjoy the sport of fishing and the hobby of keeping fishes in aquariums.


Fish populations are threatened by habitat alteration, such as by dam building, water pollution, overfishing, and introduction of competing species. Many people are trying to protect Shark fins, hanging to dry on a fishing vessel, are used for such delicacies as shark's fin soup. Federal regulators are trying to determine the impact of fishing on shark populations. (AP/Wide World Photos. Reproduced by permission.) habitats and develop regulations to reduce overfishing and pollution.



Cushing, Colbert E., and J. David Allan. Streams: Their Ecology and Life. San Diego: Academic Press, 2001.

Niesen, Thomas M. The Marine Biology Coloring Book. 2nd ed.

New York: HarperResource, 2000.

Ricciuti, Edward R. Fish. Woodbridge, CT: Blackbirch, 1993.

Web sites:

"FisFAQ." Northeast Fisheries Science Center. http://www.nefsc.noaa.gov/faq (accessed on September 23, 2004).

"Fish." Science Fair Projects Encyclopedia. http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Fish (accessed on September 20, 2004).

Additional topics

Animal Life ResourceFish and Other Cold-Blooded Vertebrates