P3.36 Friday, Jan. 6 Adventures in juvenile sea urchin ecology SALYERS, JM*; ALLEN, JD; College of William and Mary email@example.com
During the 1990’s, the population of the green sea urchin, Strongylocentrotus droebachiensis, decreased dramatically in the Gulf of Maine due to overfishing and has been slow to recover. Predation on juvenile urchins may be one cause of poor reestablishment, resulting in lasting ecological changes to community structure in the Northwestern Atlantic. To determine the effects of predation on juvenile urchin survivorship, we examined (1) the effect of juvenile urchin size on predation by American lobsters (Homarus americanus) and hermit crabs (Pagurus longicarpus) and (2) the effect of predator density on urchin survival in mesocosm pilot studies. In the first experiment, 10 juveniles of different sizes (0.5 -3.0 mm diameter) were simultaneously presented to three hermit crabs (16-36 mm shell length) or one juvenile lobster (16-32 mm carapace length). Hermit crabs showed size selective predation, feeding preferentially on smaller juveniles. Lobsters showed no size preference for recently settled urchins, frequently consuming all urchins within three hours. However, lobsters showed a strong preference across a wider size range (0.5 - 50 mm diameter) and did not eat urchins larger than 10 mm diameter. In the second experiment, we released 200 juvenile urchins (< 3 mm diameter) in each of two mesocosms, one with background predator densities and one with elevated predator densities. After 15 days, 36% of urchins exposed to background levels of predators were recovered but only 17.5% of urchins exposed to elevated predator levels were recovered. Increased predator density appears to reduce juvenile survivorship, even in the semi-natural environment of mesocosms. Both experiments indicate that predation on juvenile green urchins contributes to poor urchin population reestablishment in the Gulf of Maine.