Which structural traits in Sargassum species really matter for caprellid assemblages?

Algal complexity is an important driver of the benthos in coastal marine ecosystems. Distinct algal architectures host communities with different structures. To assess how amphipod assemblages relate to algal complexity, we studied caprellids in four Sargassum species (S. cymosum, S. filipendula, S. furcatum, S. stenophyllum). Ten fronds from each algae species were collected from rocky shores in the coast of São Paulo (southeastern Brazil) in December 2009 and March 2010. In each frond, caprellids were identified and counted, and algal traits (length, number of branches, and epiphytic algae and hydroid cover) were measured. Sargassum architecture clearly differed among species. Caprellid density and richness significantly varied among Sargassum species and sampling dates, and amphipods showed a clear preference for specific algal traits. Preference for specific structures within the same habitat generates higher caprellid diversity, since it decreases competition for space in different microhabitats created by the structural complexity of algae. Presence of hydroids is the main driver of caprellid assemblage structure, with the former functioning as biogenic sources and influencing qualitative aspects of the habitat.