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Sea urchins are a group of spiny globular echinoderms which form the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres (16,000 feet; 2,700 fathoms). Their tests (hard shells) are round and spiny, typically from 3 to 10 centimetres (1 to 4 inches) across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, and triggerfish. This photograph, taken off the northern coast of Haiti near Cap-Haïtien, shows two species of sea urchin: a West Indian sea egg (top) and a reef urchin (bottom).Photograph credit: Nick Hobgood, edited by Lycaon