The "sea urchin crab" Echinoecus pentagonus is a species of crab in the family Pilumnidae found from the Red Sea and East Africa to French Polynesia and the Hawaiian Islands. This crab is a parasite that lives in the rectum of a sea urchin. In Hawaii, it chooses only Echinothrix calamaris, leaving few of these urchins unpopulated. Its curved and pointed carapace reaches only 0.5 inches (1.3 cm) in width.