Almost three-forths 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 years.
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 ills. 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
right.
Many fish eat plants, while others such as the shark, eat other fish.