Density, Distribution, and Feeding of the Marine Snail Norrisia norrisi (Mollusca: Gastropoda) on the Kelp Macrocystis pyrifera (Phaeophyta: Laminariales)

Grazing by the snail Norrisia norrisi on Macrocystis pyrifera was studied off Santa Catalina Island, California. Snails occurred on all parts of M. pyrifera plants and averaged 8.7 individuals/plant. Laboratory feeding experiments indicated that N. norrisi consumed significantly more sporophyll tissue (53%) than either mature (23%) or apical (24%) laminae. Snails consumed approximately 1-3% of the estimated daily M. pyrifera production. Consequently, grazing by these snails may not be significant under normal conditions. However, the strong preference for sporophyll tissue suggests that N. norrisi could have significant effects on the reproductive capacity of M. pyrifera, especially in kelp beds damaged by storms or other disturbances