It's not easy to sum up vertebrate ecology in one page. Vertebrates are found from the tropics to the polar regions, from the deep sea to high mountains, and even the air -- vertebrates and insects being the only groups of animals to have mastered powered flight. For information about the ecology and life history of land-dwelling vertebrates, or tetrapods, see the page highlighted above.
Vertebrates first evolved in the ocean. The invertebrate ancestors of vertebrates had gill slits, but these were used primarily for filter feeding; these organisms took up most of the oxygen they needed through the skin. As early vertebrates grew larger and developed bony scales or plates between their tissues and the water, they developed gills for taking up oxygen from the water. Gills are complex, highly branched and folded structures; this increases the surface area for taking up oxygen. Because water is heavier and more viscous than air, fish must pump water through their mouths, through their gill chambers, and out the gill slits. When you see an aquarium fish gulping water, or "making a gookie," you will also see the gill cover opening and the gills fluttering, as water is drawn over the gills and the fish breathes. Very active fish increase oxygen uptake by swimming rapidly, forcing water into the mouth and over the gills by a sort of "ramjet" action.
With rare exceptions, vertebrates have two separate sexes. Most lay eggs and are said to be oviparous, but a number of fishes and snakes retain ther eggs in their bodies, and the eggs hatch internally. Such animals are called ovoviviparous. Many vertebrates, such as dogfish sharks and almost all mammals, have further modified the ancestral structures of the egg so that the embryo is not only retained inside the body of the female parent, but actively nourished through a special connection with the mother's body. This is known as being viviparous.