GRAY, A.C.*; KLINGER, T.; Georgia Southern University, Statesboro; University of Washington, Seattle: Does limpet grazing restrict the abundance of Fucus on oyster shells?

The oyster Crassostrea gigas is commonly found in the rocky intertidal zone of the San Juan Islands, where it is an invasive species. Various species of native limpets are often found on the surface C. gigas, while the seaweed Fucus is very rarely seen growing on the oyster valve. To determine whether macroalgal distributions on oysters are influenced by grazers, we quantified the number of limpets and littorine snails on oyster valves and the surrounding rocky substrate. In addition to the field study, a laboratory experiment was performed to determine whether limpets exhibit substrates preferences. Culture experiments were performed to determine whether Fucus germlings grown in the presence of aqueous extracts of C. gigas shells, barnacle tests, or enriched seawater (control) exhibited differential growth rates. We found that limpets but not littorine snails were more dense on oyster valves than on surrounding hard substrates. Laboratory results indicated that limpets but not littorines prefered the valve of C. gigas over Mytilus trossulus, Semibalanus cariosus, and rock substrates. Limpet grazing therefore may be limiting Fucus distribution on oyster valves. Culture experiments demonstrated that germination and growth of Fucus in the oyster extract was equal to or better than growth in the enriched seawater. This implies that there is no negative effect of aqueous-phase oyster shell chemicals on Fucus germination and growth, and suggests that grazers rather than shell chemistry may be limiting the distribution of Fucus on oyster shells.