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Just as insects have colonized almost every conceivable terrestrial habitat, crustaceans have colonized nearly every niche in the aquatic environment; freshwater, saltwater, inland brine, the ocean floor, the water column. Crustaceans, carrying the crust-like exoskeleton that gives them their name, have made their way into an incredible diversity of evolutionary niches. And, in the same way many insects have adapted to aquatic life, some crustaceans have adapted to life on land; desert-dwelling woodlice and tree-climbing crabs, to name a few. Crustaceans include lobsters, crayfish, krill, water fleas, crabs, and a wide variety of lesser-known organisms.
The isopod order of crustaceans contains incredible diversity. Isopods include the familiar terrestrial roly-polys; pill bugs and sow bugs. But among terrestrial and marine isopods there are nearly 10,000 described species, ranging in size from less than a millimeter in length to the 4-pound "giant isopod." Amphipods, another order of crustaceans sometimes referred to as “scuds” or “side-swimmers” because of their laterally flattened bodies, are a very common but often-overlooked clean-up squad. They are generally detrivores or scavengers and are found in nearly all aquatic environments and many terrestrial ones. The endangered Kauai Cave amphipod is one of the preferred prey items of the Kauai Cave wolf spider, also endangered.
There is a great diversity of crustaceans, but they also face a great diversity of threats. Crayfish are the second most endangered group of all plants and animals in North America after freshwater mussels. Some amphipods are particularly threatened by perils within a narrow range, such as the federally Endangered Noel’s amphipod, which occurs only near Roswell, New Mexico. Many species of shrimp and crabs are threatened by overfishing and water pollution. Nearly one in three crustaceans evaluated by the IUCN are threatened. This ancient group of animals, which originated around 600 million years ago, is facing unprecedented difficulties in this modern age as many of their specialized habitats are altered or destroyed by human activities.