Sea urchins or urchins (/ˈɜrtʃɪnz/), archaically called sea hedgehogs, are small, spiny, globular animals that, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. About 950 species of echinoids inhabit all oceans from the intertidal to 5000 m deep.ommon colors include black and dull shades of green, olive, brown, purple, blue, and red. Sea urchins move slowly, and feed on mostly algae. Sea otters, starfish, wolf eels, triggerfish, and other predators hunt and feed on sea urchins.