Harbouring the enemy: kelp holdfasts protect juvenile sea urchins from predatory crabs

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Predation is an important agent of post-settlement mortality of sea urchins that is mediated by the availability and suitability of spatial refuges, particularly during the vulnerable juvenile stage. In laboratory and field caging experiments, we show that holdfasts of a dominant kelp, Saccharina latissima, provide a spatial refuge for juvenile sea urchins Strongylocentrotus droebachiensis (< 20 mm, test diameter) from crabs Cancer borealis and C. irroratus, considered to be the dominant predators of sea urchins in kelp-bed ecosystems in the northwestern Atlantic. In treatments with individual crabs of either species, the presence of holdfasts reduced predation on juvenile sea urchins by ~20 to 30% compared to treatments with no refuge. Crabs consumed juve- niles (from 5 to 19 mm) in each of three 5 mm size classes in proportion to their abundance, regard- less of treatment. In kelp beds in St. Margarets Bay, Nova Scotia, Canada, the number of juvenile sea urchins per holdfast ranged from 0.3 to 0.9, with juveniles in holdfasts accounting for two- thirds of the total urchin population density at one site. Up to 4 juveniles occurred within a single holdfast, and there was a significant positive relationship between juvenile size (but not number) and holdfast volume. Small adult sea urchins were not found within holdfasts in kelp beds and rarely occupied holdfasts presented to them in laboratory cages. Our findings indicate an ontoge- netic shift in sea urchin-kelp interactions, whereby kelp facilitates recruitment of its major grazer.