Pink Urchin EchinoideaClass EchinoideaSea urchins are small, globular, thorny sea creatures that make up the majority of the Echinoidea class. They are found in the oceans around the world. Their shells are round and thorny, usually from 3 to 10 cm. Their usual colors are dominated by black and pastel shades of green, olive, brown, purple and red. They move slowly and mostly eat algae.