Echinoecus is a genus of crabs that live in association with sea urchins. Formerly considered monotypic, the genus is now thought to contain three species:Echinoecus nipponicus Miyake, 1939 – Japan, on Heliocidaris crassispina (Echinometridae) and Pseudocentrotus depressus (Strongylocentrotidae)Echinoecus pentagonus (A. Milne-Edwards, 1879) – East Africa and Red Sea to Hawaii and French Polynesia, on Diadema savignyi, D. setosum, Echinothrix calamaris and E.