Fauna associated with non-native Sargassum muticum (Fucales, Phaeophyceae) vary with thallus morphology and site type (sounds and bays)

The invasive brown alga Sargassum muticum was first recorded in Norway in 1988 and is presently common in the sublittoral fringe on the sheltered coast in South Norway. Here, results from a study on mesofauna associated with S. muticum in an archipelago on the southwest coast of Norway are presented, with two stations placed in sounds exposed to tidal currents and two in sheltered bays. Sargassum muticum occurred with a patchy distribution and an estimated density of less than 7 individuals m−2 at three of the stations (two in sheltered bays and one in a sound) and 42 individuals m−2 at the fourth station (placed in a sound). A total of 102 distinct faunal taxa were identified, with an average of 31 taxa per sample. Crustacea and Gastropoda constituted around half of the identified taxa. Fauna richness showed no relation to site type but had a curvilinear correlation with habitat size (dry weight of thallus). Total and sessile fauna richness showed a significant negative correlation with branching density. Vagile faunal density showed a curvilinear correlation with weight of epiphytes and was significantly related to site type with higher densities in the bays. The community composition of vagile fauna reflected this and was significantly related to site type, but not to epiphyte weight or branching density. When including sessile taxa in a community analysis the stations became less separated, and the community was significantly influenced by branch density. Thus, both site characteristics and morphology of S. muticum influenced the associated fauna community