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any member of the invertebrate order Amphipoda (class Crustacea) inhabiting all parts of the sea, lakes, rivers, sand beaches, caves, and moist (warm) habitats on many tropical islands. Marine amphipods have been found at depths of more than 9,100 m (30,000 feet). Freshwater and marine beach species are commonly known as scuds; those that occupy sand beaches are called sand hoppers, or sand fleas (see sand flea). About 4,600 species have been described. Extraordinarily abundant in the rocky coastal regions of all seas and often exceeding densities of 10,000 per square m (1,000 per square foot), amphipods are often mistaken for tiny shrimp, which they resemble. They are important food for many fishes, invertebrates, penguins, shore birds, small cetaceans, and pinnipeds. Amphipods are also important as scavengers of carrion