# Fish

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 years.

There are many varieties of fish - from the goby, which is less than half an inch long, to the whale shark, which can be over 60 feet in length. Most fish breathe through gills, which enable the fish to breathe the oxygen in water. The gills perform a gas exchange between the water that passes through the them and the fish's blood.

Fish are vertebrates that have a skeleton made of either bone or cartilage. About ninety-five percent of fish have skeletons made of bone. These bony fish 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. Fish with a skeleton made of cartilage tend to be heavier than water and sink, and therefore must swim to keep afloat. Cartilaginous fish include the ray and the shark.

Most fish use their tail fins as a means of propulsion. Muscles in the tail fin move it from side to side, forcing water backward and propelling the fish forward. Other fins help the fish change direction and stop. Pectoral fins on the sides help fish 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.