Unrestricted Urchins

When their primary predators disappear, sea urchins can turn entire kelp forests into underwater deserts.

Endowed with five bony teeth and a slew of prickly spikes, purple sea urchins have the unusual ability to gradually burrow into a rock, creating a customized shelter from predators like sea stars and sea otters. When predators are prevalent, the urchins are confined to these holes, so they feed by waiting for passing pieces of algae to collide with them. However, when a primary predator like the California sea otter suffers from severe population declines, sea urchins gain more freedom to roam the ocean floor. While they don't move quickly, urchins can use their tube feet to carry them toward an anchored food source, like kelp. Once they find a choice piece of kelp, they use their teeth to scrape away at its base, cutting the entire strand loose from the ocean floor.

Without otters to balance out the ecosystem, California's famous kelp forests get hit with a double dose of pressure from purple sea urchins - first because the urchins can graze more freely, and then because their population numbers soar. In some otter-free areas, urchins have become so numerous that they have virtually wiped out the bottom cover of edible algae. These former kelp forests are now known as "urchin barrens," since little remains of their biological and structural diversity.

By scraping away at a rock with their teeth and spikes, purple sea urchins can eventually hollow out a hole in which to hide from predators, like this sea star.
Photo: Robert Potts, CAS Special Collections/Manzanita
Sea otters keep California's kelp forests healthy by snacking on sea urchins, which can clear-cut entire underwater forests if their predators disappear.
Photo: US Fish and Wildlife Service

Purple sea urchins (Strongylocentrotus purpuratus) are common tidepool inhabitants along the California coast, where they graze on kelp and other algae.
Photo: Sherry Ballard, CAS Special Collections/Manzanita