Macroalgae morphological complexity affects the functional diversity of epifaunal annelid assemblages

Host structural complexity influences the diversity of associated epifaunal species, but its role in shaping functional trait diversity remains underexplored. We developed a trait-based framework to assess whether macroalgal structural complexity significantly influences the functional assembly of marine annelid epifauna in a sandstone reef system at Enseada dos Corais Beach (NE Brazil). Sampling was conducted in December 2018, February 2019, April 2019, and June 2019. Ten fronds from each of four macroalgal species—Gelidiella acerosa and Palisada perforata (corticated), Padina gymnospora and Ulva lactuca (foliose)—were collected to describe the associated annelid fauna. Structural complexity was quantified using the interstitial space index (ISI), height, and the fractal dimensions of frond area (Da) and perimeter (Dp). Based on body length, feeding strategy, and larval development, the functional trait diversity of annelid assemblages was analyzed using Rao’s Quadratic Entropy (Rao’s Q) and RLQ analysis. Corticated algae species hosted more functionally dissimilar annelid assemblages than foliose ones. Moreover, morphological traits of macroalgae influenced epifaunal functional trait composition, particularly during the rainy season, when hydrodynamics are more intense. Our findings thus supported the hypothesis that increased habitat complexity positively influences functional trait diversity in marine macroalgal phytal communities.