Red and purple sea urchins (Mesocentrotus francisanus and Strongylocentrotus purpuratus) cohabit the west coast of North America and exhibit behavioral switching between sheltering, when food is abundant, and emergence and overgrazing, when food is scarce. To better understand individual urchin foraging behavior, we conducted a series of time-lapse behavioral studies within and at the edge of a resilient sea urchin barren. Photographs were taken at 15-min intervals for weeklong periods to observe behavior (1) in different microtopographic settings, (2) in response to food additions, and (3) along a spatial gradient from the leading edge of a sea urchin grazing front to ∼100 m behind it. Movement was limited for both species when crowded or in complex microtopography. Consistent differences in sheltering behaviors and diel movement patterns were observed between species in the presence and absence of food indicating behavioral niche differentiation. Red sea urchins responded to food falls at distances of at least 3m and exhibit an ability to return to shelters at similar distances. Both species exhibit (1) local movement for up to weeklong periods indicating constraints on bulk movement and grazing front formation, (2) decreased movement rates owing to crowding and microtopography analogous to traffic jams, and (3) consistent instraspecific differences in individual movement behaviors (i.e., personality). We propose how small-scale behavioral modes may scale to larger-scale local population movements and affect the dynamics of sea urchin overgrazing.
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Vol. 36 • No. 2