Underrated deep-water scleractinian species: an ecological study along the Italian coasts (Mediterranean Sea)

Abstract Scleractinians are key components of Mediterranean mesophotic and bathyal benthic ecosystems, comprising 30 species known to occur below 40 m. A large ROV dataset, including 625 dives, each corresponding to a distinct geographical site, conducted between 40 and 1825 m across the Italian seas, was used to assess the diversity of this taxon, focusing on the distribution, habitat preferences, and population density of four Data Deficient, yet well-represented species: solitary Caryophyllia cyathus , Paracyathus pulchellus , and Javania cailleti , and colonial Thalamophyllia gasti . The study recorded 18 scleractinian species (representing 60% of the known deep ones) and analysed almost 2600 records of the four target species. Caryophyllia cyathus , Paracyathus pulchellus, and Thalamophyllia gasti were the most common, occurring between 80 and 150 m on inclined outcropping rocks encrusted with coralline algae along the continental platform, with major hotspots in the Tyrrhenian Sea and average densities ranging between 2.4 and 36 individuals or colonies m −2 . Thalamophyllia gasti was observed for the first time in a cave habitat in Southwest Sardinia, forming large colonies. Javania cailleti was recorded at only 4% of the investigated sites, mainly along the Northeast Sardinian canyons, Corse Channel, Dohrn Canyon, and on seamounts between 400 and 550 m. This species, thriving on Fe–Mn-rich rocks, thanatocoenoses, and coral frameworks, tolerated high levels of silting and occasionally formed dense populations, averaging 12 individuals m −2 . This study expands biogeographic and ecological knowledge of these four species, highlighting their environmental preferences and distribution hotspots and confirming their significant yet underappreciated role in benthic communities in the Mediterranean mesophotic and bathyal zones.