Form Meets Function: Role of Macroalgal Morphology in Structuring Mobile Epifaunal Assemblages in Selected Algae from the Central Mediterranean

Quantitative measurement of macroalgal thallus morphology and its link with epifaunal communities remains largely unexplored in the central Mediterranean. This paper explores the link between thallus morphology and the associated mobile epifaunal assemblages of three common macroalgae native to the Maltese Islands (central Mediterranean Sea): Ellisolandia elongata, Jania rubens, and Dictyota spp. (D. dichotoma, D. mediterranea, and D. fasciola in mixed assemblages). We hypothesised that macroalgal species exhibiting distinct morphological characteristics support mobile epifaunal assemblages that differ significantly in structure and composition. Using a full factorial framework, macroalgal beds dominated by each target species were sampled from shallow water in three localities on the northeastern coast of Malta, using SCUBA diving. Dry biomass and fractal dimensions were used to characterise the distinct morphological traits of the three macroalgal species. A total of 85 epifaunal taxa were associated with Dictyota spp., 98 taxa with E. elongata, and 112 taxa with J. rubens. Excluding the disproportionately abundant invasive foraminiferan Amphistegina lobifera, 911 individuals were recorded on E. elongata, 1224 on Dictyota spp., and 2233 on J. rubens. Of the 152 taxa identified across all macroalgae studied, Arthropoda (44 taxa) and Mollusca (39 taxa) dominated, with individuals occupying the small inter-frond spaces as shelters that provide refuge from predators and hydrodynamic stress. Multivariate analysis revealed that the structure and composition of mobile epifaunal assemblages were associated with the interaction between macroalgal morphology and site-specific environmental conditions. The more structurally complex, articulated, calcified, and densely branched forms of E. elongata and J. rubens supported more varied epifaunal communities that differed from those associated with the compressed and leafier thalli of Dictyota spp., which were instead dominated by surface-associated taxa. Notably higher richness and diversity values were recorded in sheltered sites, especially in the case of J. rubens, showing that the effect of habitat architecture may be modulated by wave exposure.