Grazing on green algae by the periwinkleLittorina littorea in the Wadden Sea

On sedimentary tidal flats in the Wadden Sea near the Island of Sylt, the periwinkleLittorina littorea occurred preferentially on clusters and beds of mussels and on shell beds (100 to 350 m⁻²), achieved moderate densities on green algal patches or mats (20 to 50 m⁻²), and remained rare on bare sediments (<5 m⁻²). Green algae covering>10% of sediment surface appeared in summer on approximately one third of the tidal zone, mainly in the upper and sheltered parts and almost never on mussel and shell beds. In feeding experiments,L. littorea ingested more of the dominant alge,Enteromorpha, than ofUlva, irrespective of whether or not algae were fresh or decaying. The tough thalli ofChaetomorpha were hardly consumed. Snails feeding onEnteromorpha produced fecal pellets from which new growth ofEnteromorpha started. In the absence of periwinkles,Enteromorpha developed on mussels and the attached fucoids. Experimentally increased snail densities on sediments prevented green algal development, but the snails were unable to graze down established algal mats. It is concluded that natural densities ofL. littorea hardly affect the ephemeral mass development of green algae on sediments. However, where the snails occur at high densities, i.e. on mussel beds, green algal development may be prevented.