Strongylocentrotus franciscanus is the name for the red sea urchin, the most common mobile invertebrate we encounter. Urchins are well known from temperate and tropical habitats around the world to be important herbivores. In tropical oceans like the Bahamas, urchins help herbivorous fishes keep macroalgae (seaweed) at bay, allowing corals to grow. In temperate oceans like California, Alaska, New Zealand and the Mediterranean, they are known to form foraging herds that can mow kelp forests down and replace them with “urchin barrens,” regions of coralline algae pavement and encrusting invertebrates.
The urchins of the San Juan Islands violate our preconceptions about urchins as important herbivores. First, they don’t just eat algae, and second, even when they do, they don’t bother going to search for it.
Robin Elahi, a recent Ph.D. graduate from the Sebens Lab, studied the impact of red sea urchins on rock walls, and found them to consume lots of invertebrates, such as social ascidians. Robin found that urchins are responsible for creating clearings on the walls by eating away space occupying species, and then chitons maintain these open patches by bulldozing or consuming new recruits.
When the urchins aren’t eating invertebrates, they’re waiting for algae to come to them. The narrow channels between the San Juan Islands create strong tidal currents, which carrie drift algae back and forth through the channels. Urchins sit and wait for drift algae to catch in their spines, rather than waste energy foraging for live kelp. The result of this behavior is that urchins in the San Juans do not form massive herds that wipe out kelp beds. The transport of this drift algae from the shallow productive zone allows urchins and other herbivores to live in much deeper habitats than they might otherwise be able to. Several FHL researchers have studied various components of this relationship, including Sarah Carter, Kevin Britton-Simmons, and Aaron Galloway, a Ph.D. student in the Spatial Subsidy Lab at FHL.