CENSUS

Photo Credit: Melissa Gutterman

Both the purple sea urchin, Strongylocentrotus purpuratus, and the red sea urchin, Strongylocentrotus franciscanus are major grazers of macroalgae along the coast of California. The ability of changes in urchin abundances and grazing behavior to cause the almost complete loss of kelps and other macroalgae from rocky reef ecosystems means they can be key in dictating the ecosystem state of temperate kelp forest habitats.

In kelp forests sea urchins typically feed passively on detrital drift kelp while remaining in the relative safety of rocky cracks and crevices. Urchins, however, may also switch to an active feeding strategy where they emerge from their shelter and graze down all available live macroalgae, causing an ecosystem shift to what is known as an “urchin barrens”. It is thought that this behavioral shift occurs due to a combination of 1) increased urchin densities limiting available shelter, 2) a shortage of drift kelp food availability, 3) an absence of urchin predators. Large spikes in urchin populations are thought to occur due to exceptionally good years for larval urchin settlement and subsequent survival into adulthood. It is for this reason that we aim to quantify patterns of urchin larval recruitment.