Fish and Sea Creatures

Oceans, rivers, and lakes are home to an extraordinary variety of animals. Fish are vertebrates, which means they have a backbone, just like you. But instead of breathing air with lungs, fish breathe using gills. As water flows over the gills, they pull out the oxygen that is dissolved in it, letting the fish breathe without ever coming to the surface.

Fish are beautifully built for life in water. Most have a smooth, streamlined shape that slips easily through the water, fins to steer and push themselves along, and a covering of overlapping scales to protect their bodies. Many also have a special air-filled bag inside called a swim bladder, which they fill or empty to float at just the right depth without sinking or bobbing up.

Not every creature in the sea is a fish. Octopuses, squid, crabs, jellyfish, and sea urchins are all invertebrates — animals without a backbone. A jellyfish has no bones, brain, or heart, yet it has drifted in the oceans for hundreds of millions of years. Each of these animals has its own remarkable way of surviving underwater.