The investigation was led by Alessandra Cosma of the University of Palermo and the University of Genoa, together with colleagues from institutions across Italy, including the Institute for Environmental Protection and Research, the Zoological Station Anton Dohrn, and the University of Cagliari. Rather than mounting new expeditions, the team mined an extraordinary archive of remotely operated vehicle, or ROV, surveys conducted between 2006 and 2022 in Italian waters. The dataset encompasses 625 dives, each corresponding to a distinct geographic site, spanning five macro-areas: the Ligurian Sea, the North-central and South Tyrrhenian Sea, the Sicily Channel, and a collection of offshore seamounts. In total, the researchers reviewed 640 hours of video covering roughly 682,000 square metres of seafloor, at depths ranging from 40 metres down to 1,825 metres.

The Mediterranean Sea hosts 36 species of scleractinian corals, the group of hard, calcium-carbonate-secreting animals that build many of the world’s tropical reefs. About 2 percent of global scleractinian diversity is packed into this semi-enclosed basin, and 30 of those species live below 40 metres, in the mesophotic zone between 40 and roughly 150 to 200 metres, or in the bathyal zone that extends from about 200 metres down to 3,000. Most scientific attention has focused on the famous cold-water coral reefs built by white corals such as Madrepora oculata and Desmophyllum pertusum, which are regarded as the climax communities of the bathyal zone. The smaller, non-structuring corals that live alongside them have been largely ignored, partly because their small size, cryptic habits, and inaccessible depths make them easy to miss.

The new study set out to change that by focusing on four target species: three solitary corals, Caryophyllia cyathus, Paracyathus pulchellus, and Javania cailleti, and one colonial species, Thalamophyllia gasti. All four are azooxanthellate, meaning they lack the symbiotic algae that power tropical reef corals and instead feed by capturing particles and plankton from the water with their tentacles. All four are also listed as Data Deficient on the IUCN Red List for Mediterranean Anthozoa, a designation reflecting how little is known about their distributions and population trends. Their bathymetric ranges, tiny dimensions, and tendency to hide in crevices have historically excluded them from exploratory surveys and conservation planning.

Identifying such small animals from video required considerable taxonomic care. The team relied on distinctive morphological traits visible in the footage, including corallite shape, septal structure, size, and colouration, cross-checking their identifications against specimens held in the ROV collection of the University of Genoa. Caryophyllia cyathus, the largest of the four at an average corallite diameter of 22 millimetres, was recognised by its oval corallite, numerous uniform septa, and pinkish-orange polyps with a reddish region around the mouth. Paracyathus pulchellus, at just 9 millimetres across, displayed a characteristic banded colour pattern with an opaque outer band over short peripheral septa and a translucent band over the central ones. Thalamophyllia gasti formed small white colonies of four to five slender corallites, while Javania cailleti was unmistakable thanks to its large, trumpet-shaped, flared corallite and vivid red polyps contrasting with a bright white skeleton. Each identification was assigned a confidence level, and records with uncertain visual identification were excluded from the analysis.

The results were striking. The surveys recorded 18 scleractinian species in total, representing 60 percent of all deep-water scleractinians known from the Mediterranean, and extended the known bathymetric range of nine of them. Scleractinians appeared at 69 percent of the surveyed sites. Caryophyllia cyathus was the most frequently observed species overall, present at 37 percent of sites, followed closely by Caryophyllia smithii, Paracyathus pulchellus, Dendrophyllia cornigera, and Thalamophyllia gasti. Javania cailleti proved the rarest of the target species, recorded at only 4 percent of sites, almost exclusively in canyon systems and on seamounts between roughly 400 and 550 metres, including the Northeast Sardinian canyons, the Corse Channel, and the Dohrn Canyon in the Gulf of Naples.

The three mesophotic species shared remarkably similar ecological preferences. Caryophyllia cyathus, Paracyathus pulchellus, and Thalamophyllia gasti were concentrated between 80 and 150 metres on inclined, outcropping rocks encrusted with crustose coralline algae along the continental shelf break, with major hotspots in the Tyrrhenian Sea, particularly the Tuscan Archipelago, the southern coasts of Sardinia, the Pontine Islands, the Gulf of Naples, and the coasts of Sicily. Average densities ranged from 2.4 individuals per square metre for Caryophyllia cyathus to a remarkable 36.3 colonies per square metre for Thalamophyllia gasti, whose densest patches reached more than 400 colonies per square metre at Carloforte Shoal. Linear mixed-effects models revealed that depth significantly reduced the densities of both Caryophyllia cyathus and Paracyathus pulchellus, while Thalamophyllia gasti responded positively to steeper slopes and negatively to silting, explaining its strong preference for vertical walls and overhangs where sediment cannot accumulate.

Perhaps the most spectacular discovery concerned Thalamophyllia gasti. In Southwest Sardinia, the team observed this species for the first time in a cave habitat, where two enormous colonies, each bearing hundreds of corallites, grew upside down from an overhang associated with a red coral, Corallium rubrum, facies. The larger colony measured 13.1 centimetres in height and 27.8 centimetres in width, dimensions never before documented for this species. The researchers suggest these colonies may constitute a biogenic framework, meaning the corals themselves create a three-dimensional structure that provides shelter for other organisms. This finding adds a new entry to the rich catalogue of structured biogenic habitats known from the Mediterranean and hints that, under favourable conditions, even small colonial corals can become ecosystem engineers.

Javania cailleti told a completely different story. Unlike its three counterparts, which were virtually absent below 250 metres, this strictly bathyal species peaked between 250 and 500 metres and tolerated conditions the others could not. It settled on a diverse array of substrates, including outcropping rocks, white coral frameworks, and thanatocoenoses, the accumulated skeletal remains of dead organisms, and showed the highest tolerance for silting of any species examined, with nearly 20 percent of records occurring under high sedimentation. Intriguingly, dense populations averaging about 12 individuals per square metre thrived on rocks encrusted with iron-manganese mineral crusts, particularly on the Cialdi Seamount and within the Capo Coda Cavallo Canyon, suggesting these mineral-rich hardgrounds may promote settlement. The species was also a frequent companion of the cold-water corals Madrepora oculata and Desmophyllum dianthus and a dominant component of upper bathyal black coral forests dominated by Parantipathes.

The study’s authors emphasise that these four species, while not primary habitat-formers, are significant contributors to the complex suspension-feeding communities that characterise Mediterranean mesophotic and bathyal ecosystems, appearing across 21 recognised habitat facies and alongside 54 recurrent associated invertebrate species. Deep coral populations benefit from more stable trophic conditions than shallow-water assemblages, since persistent currents at depth deliver a steadier flux of organic matter than the seasonally stratified surface waters. All four species receive indirect protection under the EU Habitats Directive through the rocky and biogenic reef habitat type and are listed on CITES Appendix II, yet critical knowledge gaps remain regarding their reproductive biology, genetic connectivity, growth rates, and resilience to impacts. By mapping distribution hotspots and defining environmental optima across a 600-nautical-mile latitudinal gradient, this research provides the baseline that future monitoring and conservation planning in the deep Mediterranean has been missing, and a reminder that some of the sea’s most important architects are also its most overlooked.

Subject of Research: Ecology and distribution of deep-water scleractinian corals along the Italian Mediterranean coasts

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

Article References: Cosma, A., Toma, M., Costa, A., Bavestrello, G., Angiolillo, M., Cau, A., Canese, S., & Bo, M. (2026). Underrated deep-water scleractinian species: an ecological study along the Italian coasts (Mediterranean Sea). Coral Reefs. https://doi.org/10.1007/s00338-026-02967-z

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02967-z

Keywords: scleractinia, deep sea, Mediterranean Sea, mesophotic, bathyal, ROV, coral reefs, Caryophyllia cyathus, Javania cailleti, Thalamophyllia gasti, benthic ecology, conservation