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Simple: because that’s how their paw joints bend. When flexed, the paws on one side of the crab’s body pull it laterally and those on the other side of the body help, pushing the animal in the same direction. “The movement of their legs is more efficient laterally, but crabs are also capable of moving forward, but more slowly”, says biologist Sérgio Luiz de Siqueira Bueno, from the University of São Paulo (USP). Crabs belong to the phylum of arthropods – which includes animals with jointed legs – and to the class of crustaceans, which also includes shrimp. They have five pairs of legs, the front two of which have evolved into sharp claws that are used for hunting and fighting with members of the same species. Crabs are considered a kind of sea vulture, as they love carrion and all kinds of debris. Another curiosity is that some of them, like the aratu, despite not being able to swim, can climb trees.