Biodiversity of Mediterranean mesophotic rhodolith beds: Macrofaunal community composition and structure (SW Italy)

Rhodolith beds are structurally complex biogenic habitats that support high biodiversity of associated benthic organisms. In the Mediterranean Sea, they are recognized and protected under European Community legislation, yet their structural complexity and associated biodiversity remain poorly documented, particularly in deeper environments. This study investigates rhodolith beds and their associated macrofaunal assemblages at four mesophotic sublittoral sites (50–71 m) along the Tyrrhenian coast of the Campania Region (Capri Island, Secchitiello Shoal, Ischia Island, and Cilento Coast). Significant differences among sites were observed in terms of rhodolith cover and morphotype diversity, quantified through a 3D Complexity Index as a proxy of habitat structural complexity, but no significant differences in terms of live/dead rhodolith ratio were found. Overall, the rhodolith-associated community was dominated by Polychaeta, Mollusca, Crustacea, and Echinodermata. Polychaetes accounted for the highest total abundance (1299 individuals across 65 species), while molluscs were the most species-rich group (109 species from 518 individuals). Crustaceans and echinoderms contributed less to overall diversity but were consistently present across sites. The Cilento site exhibited the highest rhodolith cover and complexity (3D Complexity Index = 1.83 ± 0.62) and correspondingly supported the most abundant and species-richest benthic assemblage (131.50 ± 41.83 individuals; 47 ± 4.15 species). A general positive correlation was found between rhodolith cover and macrofaunal abundance and richness, highlighting the importance of rhodoliths in enhancing benthic biodiversity. In terms of functional trophic structure, carnivores and deposit feeders emerged as the dominant feeding guilds (DI = 38% - DQ = 37%; DI = 33% - DQ = 23%, respectively), reflecting a complex food web supported by both prey availability and the capacity of the habitat to retain detrital material. Beyond their physical role as habitat-forming, rhodoliths also contribute functionally by facilitating the accumulation of particulate organic matter and supporting a detritus-based trophic web. In fact, trapped detritus and surface biofilms fuel their basal trophic levels, where deposit feeders act as key energy processors driving also a bottom-up control of the benthic food web. These results confirm that rhodolith beds act as ecosystem engineers and biodiversity enhancers in Mediterranean coastal ecosystems. Given their ecological significance and vulnerability, especially in underexplored mesophotic environments, targeted conservation efforts are urgently needed to protect these benthic biodiversity hotspots.