Animals that live in the ocean include marine mammals such as dolphins and whales and many kinds of marine fish. Crustaceans such as lobsters, crabs and shrimp also live in the ocean, as do mollusks such as squid and octopuses.
Echinoderms, such as sea stars and sea urchins, live in the ocean. Hagfish and sponges, jellyfish and sea anemones also live in the ocean.
Dolphins and whales are known as cetaceans. Though they have streamlined bodies that allow them to swim easily underwater and many have adapted to dive to great depths, they are mammals.
While marine fish spend their lives in the salt water of the ocean, some fish, such as salmon, live for a while in the ocean and then return to freshwater rivers and streams to spawn. These types of fish are called anadromous fish.
Crustaceans that live in the oceans include everything from the tiny water fleas to lobsters to the Japanese spider crab. These animals usually have two pairs of antennae, stalked compound eyes and an exoskeleton that's at least partially made of calcium carbonate.
There are over 50,000 types of mollusks known to science, and many of them live in the ocean. They also include sea slugs and sea snails, cuttlefish and bivalves such as cockles, mussels and oysters.