The Sea Urchin Crab (Echinoecus pentagonus) is a parasite on the Banded Urchin (Echinothrix calamaris). The male crabs are small and hard to see among the spines; a single female lives in the rectum of the urchin (which is found on the top of the urchin), feeding on the urchin’s fecal pellets.

This urchin was photographed in about 40 feet of water off the south Kohala Coast near the Mauna Lani Resort on the Big Island of Hawaii.