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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 octopuses and moray eels. Over half the world's fish species live in coral reefs, many hiding away by day and emerging after dark to feed.

On reefs and rocky shores, many animals are sessile, meaning that they spend their entire adult lives fixed in one place. These species, which include sponges, barnacles, and mollusks, as well as reef-building corals themselves, typically spend the early part of their lives as drifting larvae, before settling on a solid surface and changing shape. Sessile animals are common in aquatic habitats because it is relatively easy for them to collect food, which typically is pushed in the animal’s direction by water currents. By contrast, very few sessile animals have evolved on land.

In open water, depth has a marked influence on animal lifestyles. The surface layers of the open sea teem with small and submicroscopic animals, which feed either on algae and other plantlike organisms or on each other. These animals form part of the plankton, a complex community of living things that drifts passively with the currents. Many planktonic animals can adjust the depth at which they float, but larger animals such as fish, squid, and marine mammals, are strong enough to commute between the surface and the depths far below.

Even in the clearest water, light quickly fades with increasing depth. Deeper than about 150 m (500 ft), not enough light penetrates for photosynthesis to occur, so algae are unable to survive. With increasing depth, water pressure rises and temperature falls, ultimately coming close to the freezing point on the ocean floor. Despite these extreme conditions, animal life is found in the ocean's greatest depths, fueled by the constant rain of organic debris that drifts down from far above. In a habitat where prey is widely scattered, many deep-sea fish can swallow animals larger than themselves, an adaptation that allows them to go weeks or months between meals.