Crabs are small, orange critters that eat detritus, and thus can keep water clean. They originated on Albia in the Creatures 2 game, but have been brought to the Capillata/Ark with AquaShee's C2 Crabs for C3/DS.
Crabs in Creatures 2 are by default only found in the deepest section of the Eastern Ocean. There are typically only two of them, they do not breed, and are inedible to creatures. Jay Dino made an edible, breedable, variety of crabs, known as Soft Shell Crabs, which come with a steamer pack that allows for them and the original crabs (along with several other critters) to be cooked and eaten by creatures.