The oceans swarm with life and are the largest animal habitat on Earth. More life forms are found there than in tropical rain forests. However, most species live along the continental margins. Some of those enter mid-ocean regions when they migrate to different areas in search of food or for breeding purposes. The numbers that live in mid-ocean permanently are fewer. The Gulf Stream, for example, supports so few species as to be almost desert-like.

Living organisms must maintain a balance in body fluids and salt levels in their blood. Animals that live in the sea have developed ways to cope with its high salinity. Naked animals or those with thin "skins" or shells usually have high levels of salt in their blood and do not need to expel any excess. Others, such as most fish, have special organs that remove the extra salt from their...