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Seaweed is an informal name for the many kinds of marine algae that grow along seashores. Although they often look like plants, seaweeds are not plants. They belong to three broad groups—green, red, and brown algae—that occupy different branches of the tree of life. Plants themselves evolved from ancient green algal ancestors, eventually spreading across nearly every terrestrial environment. Like plants, seaweeds contain chlorophyll and other pigments that allow them to use sunlight to make...