FREEMAN, A.S.: Grazing behavior of sea urchins in response to waterborne cues from predators

Modification of prey behavior due to predation threats can have cascading impacts on other community members. This study explored how a threat of predation altered the grazing behavior of two sea urchins (Strongylocentrotus droebachiensis and S. franciscanus). Large and small sea urchins were allowed to graze on Laminaria sp. in the presence and absence of waterborne cues from the predator, Pycnopodia helianthoides. Results indicated that sea urchins with test diameters of less than 8 cm (i.e. small S. franciscanus and adult S. droebachiensis) significantly reduced grazing in the presence of waterborne cues from Pycnopodia. In contrast, adult S. franciscanus (>12 cm test diameter) did not alter their grazing behavior in the presence of the predator. These results are discussed in terms of the relative threat of predation to different size urchins, size refuges from predation, and optimal foraging.