Herbivory and community organization on a subtidal cobble bed

We examined the role of molluscan mesograzers (periwinkles Littorina littorea, limpets Testudinalia testudinalis and chitons Ischnochiton ruber) in mediating macroalgal succession after a mass mortality of sea urchins Strongylocentrotus droebachiensis in a subtidal cobble bed in Nova Scotia, Canada. Experimental exclusions of mesograzers from cobbles encrusted with smooth (Phymatolithon) or rugose (Lithothamnion) coralline algae resulted in the establishment of a variety of ephemeral filamentous algae, suggesting that these small (2 to 5 mm) but abundant herbivores are the dominant agents controlling early succession and community organization in this habitat. In removing inhibitive effects of early successional species, molluscan grazing facilitated the establishment of fleshy perennial species Fucus evanescens and Chondrus crispus, which occasionally escaped grazing at small size to attain a growth refuge on the cobbles. Algal biomass and species richness were greater on Lithothamnion than Phymatolithon, suggesting that rugose crusts provide more favourable microhabitats for recruitment or survival of various macroalgae. The distribution of mesograzers differed between coralline types, likely influencing the rate and intensity of grazing: periwinkles foraged on both types of crust, whereas limpets occurred primarily on Phymatolithon and chitons on Lithothamnion. All non-coralline algae were consumed when urchins were experimentally reintroduced to the cobble bed, returning the assemblage to the urchin barrens state.