Littorinids control high intertidal biofilm abundance on tropical, Hong Kong rocky shores

Abstract

Biotic interactions in the high shore are assumed to be of little importance as compared to the role of adverse physical factors, despite the fact that these shore levels support dense numbers of grazing littorinids worldwide. In Hong Kong, three species, Nodilittorina trochoides, N. radiata and N. vidua, are abundant in the high shore and graze the epilithic biofilm, which is primarily composed of cyanobacteria (mostly Gloeocapsa and Dermocarpa species). When littorinids were excluded from the high shore (2.00-2.25 m above C.D.) using gum barriers, growth of the biofilm as measured by chlorophyll a levels was enhanced after 42 days at three different sites. Whilst the overall pattern of increasing chlorophyll a levels in exclusion areas was the same for all three sites, there was between-site variation possibly due to different littorinid densities and/or rock types. Exclusion of grazers, however, revealed that even at high shore levels on tropical shores biotic factors can control biofilm development. It is suggested that this influence acts in synergy with physical factors such as tidal inundation and frequency of wave splash and storms which can directly control littorinid grazing activity and presumably biofilm growth.