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
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