Caught in the act: calcareous algae creating undescribed morphologies of mesophotic algal reef

The exploration of the Linosa Island shelf (Sicily Channel, Mediterranean Sea) by remotely operated vehicle surveys revealed a previously undescribed morphotype of coralligenous algal reef. These biogenic solid substrates are characterized by planar to conical shapes, ranging from single to multilayered structures, with an elevation of 20–30 cm and a concave to convex arrangement. Such coralligenous assemblages primarily cover the seafloor at depths between 80 and 100 m, developing on a sedimentary substrate rich in biogenic components, particularly abundant rhodoliths. We document the role of crustose coralline algae as autogenic engineers, in transforming mobile biogenic sediments into stable substrates by algal reef formation, as theorized by benthic bionomics for the coralligène de plateau.