Feeding preference of Littorina snails (Gastropoda) for bleached and photosynthetic tissues of the seaweed Mazzaella parksii (Rhodophyta)

Marine invertebrate herbivores occasionally prefer particular tissues of a given seaweed depending on tissue age. For the intertidal alga Mazzaella parksii (=M. cornucopiae), however, important tissue differences result from abiotic stress: distal tissues of many fronds become bleached under strong irradiance and desiccation during spring and summer low tides. Snails of the genus Littorina (periwinkles) are significant grazers of M. parksii. Through a multiple-choice feeding-preference experiment, we found that periwinkles actively consume bleached tissues, but almost no photosynthetic tissues. This suggests that bleached tissues of M. parksii may support Littorina populations to a good extent during spring and summer. In addition, since photosynthetic tissues are basal, the impact of periwinkles on frond mortality might be lower than if they preferred photosynthetic tissues. Photosynthetic tissues are actively consumed by periwinkles when they are the only food choice, as shown by other researchers. Therefore, retaining bleached tissues for some time might have evolved in M. parksii to divert grazing pressure away from vital tissues.