Rhodolith formation in the deep water off Marettimo, Egadi Islands, Sicily

During the research for identification and monitoring of priority marine benthic habitats, an extraordinary rhodolith bed has been discovered at the Egadi Islands, Sicily, Southern Italy. Its exploration has been conducted, onboard the R/V Minerva Uno in July 2016, using a Multibeam Echosounder coupling with observations by ROV dive (video recording of > 1 km of seafloor) and direct sampling by Van Veen grab (70 l, 3 samples). Remote data and ROV inspections have been processed in order to produce a precise cartography of the habitat extension. ROV video and grab samples have been used to describe i) main sedimentary and biological features of the bed, ii) rhodolith morphotypes, iii) dominant algal species and iv) associated biodiversity. The rhodolith bed extends up to 7 km2 southeast of Marettimo Island and extended toward Favignana Island, at depth ranging between 90 and 100 m. Living rhodoliths cover up to 95% of the explored area. Patches of mobile biogenic sediments, mainly sand and gravel, are scattered within the living bed. Sedimentary structures, like megaripples and bioturbations by Spatangus sp. and Cydaris sp. are commonly visible on ROV video frames. Pralines are the dominant rhodolith morphotype, with medium size of L axis of 1.1 cm (max 3.4 cm), and an ellipsoidal to discoidal shape. The nucleus is formed by terrigenous clasts, or rarely, by biogenic components. Living calcareous algae are Phymatolithon lenormandii (Areschoug) Adey 1966, Lithothamnion spp. and L. valens Foslie 1909. The algal association at the nucleus of rhodoliths is characterized by different species, namely Lithophylloidea and Titanoderma spp.. The latter are representative of a shallower setting, perhaps the one occurring when the bed started to develop. Radiocarbon dating of the nucleus are still in progress, aimed at verifying this hypothesis and place the process of rhodolith formation in the correct temporal frame.