Aquatic Habitats
Animal life first arose in water. Millions of years later, marine and freshwater habitats continue to support a large proportion of the animal life on earth. Aquatic habitats—particularly in the seas and oceans–rarely experience abrupt changes in conditions, which is a major advantage for living things. In the seas and oceans, the greatest diversity of animal life is found in habitats close to shores. The richest of all these habitats are coral reefs , underwater ridges that form in clear water where the minimum temperature is 20° C (68° F) or above. Coral reefs are composed of an accumulation of the remains of coral —invertebrates with stony skeletons—calcareous red algae, and mollusks. One of the reasons for the great diversity of animal life in reefs is that living coral creates a complex three-dimensional landscape, with many different microhabitats. The smallest crevices provide hiding places for scavengers such as crabs and shrimps , while larger ones conceal predators such as o