Nearly three-quarters of our earth is covered by water. Yet we, living on land, usually know more about land creatures than our aquatic denizens. We have many kinds of watery environments, from the fast, clear, cold streams of the Himalaya to the placid rivers of the plains, and natural as well as man-made lakes and ponds. Moreover, we have a long coastline of some 5,600 kilometres, where live creatures so strange and fantastic that it is difficult to believe that such beings exist.
Aquatic life has had to make various adaptations in order to live successfully under water. Some creatures have made minimal changes; for example, the insect Helobetes, which is found hundreds of kilometres away from the shore walking on the sea surface, looks very much like a land insect and breathes air. In contrast, there are others, like sponges or starfish, which do not have any representatives on land.
One of the first changes necessary for aquatic life is in the manner of breathing. While land animals breathe by means of lungs, aquatic animals normally use gills, though some fishes have developed organs which function like lungs and enable them to breathe air directly from the water surface.