Effects of structural complexity on epifaunal assemblages associated with two intertidal Mediterranean seaweeds

Brown foundation seaweeds are key elements increasing substrate heterogeneity and shaping the biodiversity in rocky coastal ecosystems. They are, however, vulnerable species that are declining due to multiple anthropogenic and climate change stressors, leading to a shift to less structural complex habitats. We investigate the role of structural attributes of two intertidal macroalgae, Ericaria amentacea and Laurencia obtusa, in shaping the abundance and diversity of their associated epifaunal assemblages. For this aim, we measured seaweeds' biomass, thallus volume and length (used here as proxy of substrate complexity), and explored which seaweeds' substrate attribute explained better variation of epifaunal assemblages. Results showed that E. amentacea was more complex than L. obtusa and hosted a higher number of epifaunal individuals. However, unlike that expected, the epifaunal assemblage of L. obtusa was more structured with higher Shannon-Wiener diversity and Pielous' evenness. Our findings indicate that, besides seaweed's substrate attributes, other mechanisms such as wave action and chemical defense might play a role in structuring epifaunal assemblages. We suggest that a shift from E. amentacea to L. obtusa population could have effect on structure and abundance of associated epifaunal assemblages. Certainly, further investigations are needed to clear up the consequences of these changes.