Most crustaceans live in the ocean or very close to it. Some crustaceans, however, live in fresh water or terrestrial environments and can be found in almost every major habitat on earth.
Crustacean habitats can range from the deepest parts of the ocean near hydrothermal vents to high lakes found in mountains. Some species, such as the clam shrimp and tadpole shrimp, live in temporary pools that remain dry for much of the year. Freshwater species include crayfish and the numerous types of freshwater crabs. The most well-known crustaceans, however, live in or near the ocean or sea. Lobsters, crabs, shrimp, barnacles and krill are just a few of the types of marine crustaceans. Scientists have identified more 70,000 species of crustaceans, which is more than twice the combined amount of identified mammals, birds, reptiles and amphibians, making crustaceans the most diverse type of animal found on the earth in terms of form.
Crustaceans are arthropods, whose bodies are characterized by an external skeletal support system, jointed legs and other appendages. They differ from other classes of arthropods because they have two pairs of antennas instead of one pair or have no antennas at all. Crustaceans also differ from other arthropods, such as insects and spiders, because of the many differences in appearance.