Science Main Index
Almost three-fourths of the world's surface is covered in
water. This water is home to over 20,000 different species of
fish. The earliest fossils of fish date back over 400 million
There are a wide variety of fish from the goby which is less than
one half an inch long, to the whale shark which can be over 60 feet long.
Most fish breathe through gills. Gills perform the gas exchange between
the water and the fish's blood. They allow the fish to breathe oxygen
in the water.
Fishes are vertebrates that have a skeleton made of either bone or cartilage.
About 95% of fishes have skeletons made of bone. These bony fishes have
a swim bladder, a gas-filled sac, that they can inflate or deflate allowing
them to float in the water even when not swimming. Fishes with a cartilage
skeleton tend to be heavier than water and sink. They must swim to keep
afloat. Cartilaginous (cartilage) fish include the ray and the shark.
Most fish swim using a tail fin. Muscles in the tail fin move it from
side to side, forcing water backward, and propeling the fish forward.
Other fins help the fish change direction and stop. Pectoral fins on their
side help them swim up and down. Dorsal and anal fins on the top and bottom
keep the fish upright. Pelvic fins on the underside help steer left and
Many fish eat plants, while others such as the shark, eat other fish.
If you are interested in more information fish, check out our fish video collection.
Web Sites about Fish:
for Kids maintained by the Monterey Bay Aquarium
See a QuickTime movie showing the birth of a seahorse at the Birch Aquarium of the Scripps Instituion of Oceanography. This will require
the free QuickTime plug-in which you can download.