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The freshwater crayfishes are distributed across all but the Indian and Antarctic continents with centers of diversity in the southeastern Appalachian Mountains in the Northern Hemisphere and in south–east Australia in the Southern Hemisphere. There are currently over 640 described species of freshwater crayfishes with an average of 5–10 species still being described each year. Freshwater crayfishes can serve as keystone species in aquatic habitats, but a few species are also significantly invasive and can cause impressive damage to the fragile freshwater habitat. Crayfishes inhabit caves, burrows, streams, lakes and strong burrowers can even be found in terrestrial habitats where they have burrowed to the water table or where rainfall is sufficiently abundant to provide the needed moisture. The freshwater crayfishes, like the habitats in which they are encountered, are generally endangered to some degree and conservation efforts would do well to focus on them as key elements of the freshwater ecosystem. [ABSTRACT FROM AUTHOR]