The purple sea urchin, Strongylocentrotus purpuratus, is found along the coast of California in the lower intertidal zones and kelp forests. They are known for their bright purple color and array of spines, which help them move and protect them from predators. Under their spiny exterior they have a shell-like body known as a test. Urchins belong to the phylum enchinodermata (“spiny skin”) along with sea stars. Both sea stars and urchins move using tube feet, which they expand and contract by pumping water in and out of their bodies.
Urchins play an important role in their habitat. Using their 5-toothed mouth parts, called “Aristotle’s lantern,” they graze constantly on fast-growing kelp. If the urchin population grows too large they can easily overgraze the kelp forest. This very thing happened when sea otters were overhunted on the coast of California. Otters are one of the main predators of urchins. Without otters to keep their population in check, they quickly decimated the giant kelp of Monterey Bay. When otters became protected and began to return to the area, the urchin population decreased to a sustainable level.
Kennedy and Pescadero Middle students recently explored the rocky intertidal shore at Pillar Point in Half Moon Bay and along Pebble Beach in Pescadero. They were able to experience firsthand how urchins are able to burrow into rocks to protect themselves. To see other amazing rocky intertidal organisms that the students were able to explore, click the video below.