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. 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. Science Main Index Fishes are vertebrates that have a skeleton made of either bone or cartilage. 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.