Echinoidea

Sea urchins or urchins are the class Echinoidea within the echinoderms. Approximately 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone which actually is not a fixed number but varies greatly, usually defined by the range between the highest and lowest tide, to deep seas of 5,000 m (16,000 ft). They typically have a globular body covered by spiny protective tests, typically from 3 to 10 cm 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 such as crinoids and sponges. Their predators include sharks, sea otters, starfish, wolf eels, triggerfish, and humans. When unchecked by predators, urchins can create urchin barrens, damaged environments devoid of large algae and the animals associated with them.

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