Disentangling the mesophotic zone: algal and invertebrate bioconstructions host distinct benthic assemblages in the Mediterranean Sea

A comprehensive overview of the benthic assemblages associated with bioconstructions in the mesophotic zone of the southern Adriatic Sea is provided through a comparison of algal- and invertebrate-based bioconstructions. To characterize these bioconstructions, sampling was conducted at six sites along the Apulian coast (Italy). Algal-based bioconstructions were found in shallower areas and corresponded to coralligenous sensu stricto, while two distinct invertebrate-based bioconstructions, built by scleractinians and bivalves, were observed at greater depths. A multi-taxa approach, which recorded 511 benthic taxa, revealed significant differences in the taxonomic composition of their associated benthic assemblages, although similar species richness values were observed across all types of bioconstructions. Given the marked difference in terms of primary constructor species between the coralligenous sensu stricto and invertebrate bioconstructions, we propose referring to the latter as Mesophotic Coral Bioconstructions (MCB) and Mesophotic Oyster Bioconstructions (MOB). beta-diversity analysis identified a turnover in benthic fauna along the North-South gradient, with higher beta-diversity values between distant sites and lower values between nearby sites, likely driven by North-South circulation dynamics in the region. Additionally, the distinctions observed along the depth gradient are consistent with the decrease in irradiance, which causes a shift from photoautotrophic to heterotrophic builder species. These findings underscore the role of mesophotic bioconstructions along the Apulian coast as biodiversity hotspots and confirm their importance in understanding mesophotic ecosystem dynamics in the Mediterranean Sea.