Ocean Habitats: Nearly three quarters of the earth is covered by water, and more than 93 percent of this water is in the oceans. Within the oceans, there are distinct areas, each with different types of marine life and habitats.  Shallow inshore waters are home to the greatest variety of habitats because they contain the greatest water movement, variations in temperature, and salinity (saltiness). In some locations, twice daily tides temperately expose the area between high and low tides to the air.

The surface layers of the open ocean are well lit and teem with life. Tiny floating plants use energy from the sun to grow and reproduce. They are eaten by plankton, which in turn is eaten by fish and whales. Since light cannot penetrate below about 650 feet, there is far less in the ocean depths. Below 3,300 feet, the ocean environment is characterized by low temperature, great pressure, and total darkness.

Territorial Fish: Damselfish are very territorial fish that live on tropical coral reefs. They create seaweed gardens by killing a patch of coral and keeping other seaweed eating animals away. Soon a patch of seaweed grows and attracts tiny shrimp and crabs, which the damselfish eats.  Many damselfish defend their territories from plant eating fish that are larger than them selves. But they often ignore noncompetitive species that feed on plankton. The threadfin butterfly fish uses selective territorial behavior. Only if disturbed at night does it chase away by threatening them with its spiny fins.

Marking a Territory: Aquatic animals use sound and color instead of scent to mark territorial boundaries. Fish, sea lions, shrimp, and crabs use sound to help maintain their territories. Sea lions bark (even below water) and shrimp and crabs rub their legs together to produce a sound that warns trespassers away. Some fish produce sounds by using bony structures in their heads or even in their swim bladders.

Other fish, such as the marine angelfish and the butterfly fish, have bright colors that warn off similarly marked competitors of the same or related species. The bright colors can be seen from great distances in the clear waters of a coral reef.  Juvenile reef fish may have completely different color patterns from adults of the same species. In this way they avoid conflict with their elders until they reach maturity. Marking that differ from the adult's protect young semicircle angelfish.

Defending a Home, Space & Food: Many ocean animals defend a home, the space around them, or a food supply. An aggressive display may actually reduce fighting. It usually consists of posturing that warns other animals away. The mantis shrimp is a well armored crustacean that lives in small burrows or crevices on coral reefs. If another shrimp or a fish ventures into its home, the mantis shrimp drives the intruder away by snapping its pincers shut. The loud noise that results intimidates the potential competitors into fleeing.

Stuck to a rock with tentacles waving, the sea anemone does not seem very aggressive. But it must complete with other anemones for a space where plankton and small fish are borne by the current. If the poison tipped tentacles of two anemones come into contact, a battle may follow. the fight continues until one combatant concedes by slowly creeping out of its opponents range. The mantis shrimp uses both a visual threat and a warning sound to deter intruders.

Defending a Territory for Breeding: The defense of a food supply may be linked to breeding, as illustrated by the dwarf angelfish, an habitant of coral reefs. A group of four to seven fish set up a territory in an area with abundant seaweed.  The male defends seaweed.  The male defends this area, while his harem of females feeds and produces young.

A strong male defends a territory large enough to support a group of females and their offspring. Weaker males, less effective at defense, attract fewer females. This pattern may be important as a way of controlling populations. When numbers are high, weaker fish are forced into less suitable rates, where they are more susceptible to disease and predators and less successful at breeding. The stronger a male angelfish is, the better the territory he establishes.

Sea Lion Territories: During the breeding season from May to August, male california sea lions establish rookeries (breeding territories) on isolated beaches. Female sea lions come ashore to the rookery to give birth. The newborn pups benefit from the protection of the male, who also protects his harem of females from other males.  Shortly after they give birth, the females are ready to breed again an mate with the male whose territory they occupy.

The male sea lion patrols the edge of his rookery both in and out of the water, threatening are usually for show, with much barking, head shaking, staring, and lunging. But occasionally males are seriously injured and are left scarred from fights.  The male is so concerned with defending his territory with defending his territory that he usually stops feeding for several weeks, relying on his stored up fat deposits. Outside the breeding season, the sea lion is sociable and much less aggressive. The combat of two males sea lions may look and sound dangerous, but it rarely is.

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