What Divers Should Know About Sea Urchin

Spotting a sea urchin underwater often involves observing a spiny, globular form nestled on the seabed. These fascinating echinoderms inhabit all oceans, thriving from the intertidal zone down to immense depths exceeding 6,850 meters. Typically, they present as a rigid, spherical body covered in moveable spines, usually ranging from 3 to 10 cm across, though some species can reach an impressive 36 cm. They move slowly, primarily using their tube feet or occasionally pushing with their spines across various benthic habitats like rocky shores, cracks, or sandy bottoms.

Urchins feed predominantly on algae, but also consume slow-moving or sessile animals such as crinoids and sponges. Unchecked populations can significantly impact marine ecosystems, leading to "urchin barrens"—environments stripped of large algae and their associated life. During colder months or in areas with strong wave action, these creatures often seek protection by tucking into cracks and around larger underwater structures. Certain irregular forms, like sand dollars, have evolved a flattened body to efficiently burrow through soft sediments.