Sea urchins are small, globular, spiny sea creatures, composing most of class Echinoidea. They are found in oceans all over the world. Their shell, or “test”, is round and spiny, typically from 3 to 10 cm across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae. Sea otters, wolf eels, and other predators feed on urchins. Sea urchins are harvested and served as a delicacy.
Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms they have fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive “tube feet”. The pentamerous symmetry is not obvious at a casual glance but is easily seen in the dried shell or test of the urchin.
Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is defined by primarily having a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved different shapes. Although many sea cucumbers have branched tentacles surrounding the oral opening, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars (sea star) and brittle stars.