Coral reefs provide habitat and nurseries for as much as a quarter of all ocean species, but they face many natural predators, from sea snails to starfish. Fortunately for the Pocillopora corals in the Indo-Pacific, coral guard crabs come to the rescue. As small as 4 millimeters wide, the crustaceans make their homes in the corals (shown in the above photo) and fiercely defend their hosts in exchange for food and shelter. To study how coral guard crabs of various species and sizes differ in their defense abilities, researchers removed the largest species, Trapezia flavopunctata, from 45 coral colonies surrounding the Moorea Island in French Polynesia for 2 weeks in 2008, when a booming population of crown-of-thorns starfish as large as trashcan lids threatened to wipe out the corals. Coral colonies with T. flavopunctata remained intact, whereas those without were eaten by the starfish overnight, even when other crab species inhabited them, the team reports online today in PeerJ. Other experiments found that the smallest crabs were experts in defending the corals against sea snails, while medium-sized crabs protect the colonies from the cushion stars.