How many types of crabs are there?

Crabs, a diverse group of marine and some freshwater crustaceans, belong to the infraorder Brachyura and make up an overwhelming majority of nearly 4,500 species of decapod crustaceans. With such staggering richness in their ranks, crabs boast an extraordinary assortment of shapes, sizes, habitats, and ecological roles throughout the world of life under the sea.