Small macrobenthic invertebrates affect the mortality and growth of early post-settlement sea urchins and sea stars in subtidal cobble habitat

Early post-settlement events can have a large impact on the successful recruitment of benthic invertebrates. A field caging experiment was conducted in 2007 to examine whether predation by, and/or competition with, small macrofauna affects mortality or growth of recently settled sea urchins Strongylocentrotus droebachiensis or sea stars Asterias spp. Kelp was added to half of the sea urchin cages to test whether the addition of this food source (including its associated biofilms) altered growth or survival. Sea urchins survived 25% better in cages where the other organisms were removed, indicating that predation or bulldozing likely plays an important role. The growth of sea urchins was greatest in cages without other organisms and lowest in cages with other organisms and with food, possibly indicating competition that affects sea urchin behaviour. Sea stars showed the opposite trend in survival. A greater proportion of sea stars survived in cages where the other organisms were present (31.3 vs. 11.5%), presumably due to them being a food source for the sea stars, indicating that starvation, cannibalism and/or competition for food are likely important for recent settlers of this taxon. The declines in abundance in the cages were greater than those in the natural environment for sea urchins, but similar for sea stars, suggesting that caution is required when extrapolating experimental results to the field. These results indicate that multiple factors, which differ between these species, affect early post-settlement growth and mortality.