Crabs belong to the most diverse group of marine invertebrates, indeed the most diverse group of marine lifecrustaceans. Crabs, like all crustaceans, have a hard shell and jointed legs. Their body plan is based on variations of the basic leg design. The mouthparts are tiny pincers that break off food and then even smaller appendages waft the floating particles into the mouth. Most crab species are scavengers, although a few are parasites of fish, mollusks and even other crustaceans. Crabs are economically important in other parts of the world but in the Galapagos there are no species considered worth harvesting.
The Sally Lightfoot crabs (Grapsus grapsus) are the commonest and most colorful shore crab in the Galapagosthe adults are bright orange and electric blue. The young are dark brown, well-camouflaged against lava rock. Only mature adults are brightly colored. The significance of this color shift is unclear, but it may be related to breeding, allowing the adults to see each other clearly. The crabs provide food for other animals, especially birds such as lava herons.