World Builders™
World Builders™
Session Six  --  Water Animals
Session Six  --  Water Animals 



Fishes are vertebrates, with internal skeletons that grow inside the animal.

Fishes have a body pattern that has proved to be very successful.   The backbone provides flexibility of movement, and a base from which ribs can grow to protect the internal organs. Limb bones can be organized around the spine.

In fishes the skeleton has ossified, which means that the bones are hard and made of calcium carbonate.  This gives the internal skeleton strength.  However, fish do not need their bones to be strong enough to support their bodies: the water supports them.

  • the head contains the brain, eyes, internal ears, nasal sacs, and mouth
  • breathing begins in the mouth, and oxygen is extracted by the gills
  • the skeleton has a bony head that protects the brain, a backbone, and bones that support the body
  • the digestive process begins in the mouth and goes through stomach and intestines to end near the back of the body
  • blue fishthe circulatory system has a two chambered heart and blood vessels that feed the body's cells, bring them oxygen, and carrying away wastes
  • the blood uses red blood cells containing hemoglobin to transport oxygen
  • there are two sexes: females produce eggs, males produce sperm. (However, in some species, males later turn into females or females turn into males.) Most fishes lay their eggs in the water, but a few, such as guppies, are live bearers.
  • the pectoral (front) fins stabilize the body and help it to turn: later they become front legs, arms, or wings in other kinds of animals
  • the pelvic (back) fins also stabilized the body: they are the forebears of back legs
  • the tail provides propulsive power and helps with balance
  • extensions of the spine support the dorsal fin (the fin on the back) which provides balance. (Similar projections show up in the dinosaurs and mammals such as the buffalo, although their function has changed.)
  • Some fish have swim bladders to help them with buoyancy.

Fishes and Buoyancy

Fishes have two problems to solve: one is physical, the problem of moving in the water: the other is chemical, the problem of keeping their body fluids at the right balance of salinity (saltiness).

Most fishes have body shapes that move easily in water. The fins help to keep the body right side up. Fishes need to be buoyant: if their bodies were too heavy they would lie on the bottom of the lake or sea and would have to work hard to swim. Fish need to weigh the same as, or a little less than, the water that would fill the space occupied by their bodies. They do not have massive (heavy) bones, and some have a swim bladder, an organ that is filled with a gas and so makes the body mass lighter in relationship to the water around it.

Fishes and Salinity

Life began in the early oceans, and the cells developed to work best with a certain amount of salt in their protoplasm. However, as time went on, salt continued to build up in the oceans, and so ocean fishes have less salt in their tissues than the water that surrounds them. They drink a lot of water, and have developed ways to get rid of the extra salt in their bodies.

Some fishes live in rivers and lakes. Their body tissues are more salty than the water around them. They drink only a little of the water to conserve their salt.

Fishes and Temperaturefish 7

Fishes are cold blooded, which means that their bodies are at the same temperature as the surrounding water. Each species of fish has a temperature range in which it functions well. If the water gets too cold, the fish almost stops eating and just waits. This adaptation allows it to survive over winters unless the temperature gets too low or remains too low too long, in which case it dies. If the temperature gets too high, the fishes die.

Fish are able to sense temperatures which are appropriate for them. They seek out these temperatures: for instance, in a lake they may seek out shallow water which has been warmed by the sun or deep water in the middle of the lake, which is cooler.

Life processes are chemical, and chemical reactions take place more rapidly as temperature rises. Would you expect fish to grow more rapidly in warm or cold water?

Fishes and Adaptation

Fishes have been on earth since Devonian times, and are among the most successful of earth's multicellular life forms. They have had hundreds of millions of years to adapt to living in aquatic communities. They have developed into many species. Fishes of many different sizes, body shapes, and colors inhabit the waters of our planet.

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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .