Sciaphilous coral reefs in the southern Adriatic Sea

This study aimed to improve knowledge of Mediterranean coral bioconstructions by investigating two mesophotic reefs located on opposite shores of the Adriatic Sea, in Italy and Montenegro. Both reefs exhibited high morphological complexity yet differed markedly in structure and environmental setting. The Italian reef developed as a continuous framework in open waters, whereas the Montenegrin reef consisted of articulated coral blocks growing within a deep inlet, close to submarine freshwater springs. The latter occurred at shallower depths, likely reflecting local hydrographic and geomorphological conditions. At both sites, the scleractinian Polycyathus muellerae acted as the primary reef builder. Phyllangia americana mouchezii played a significant role in the Italian reef but was only sporadically present in Montenegro. The associated benthic assemblages also differed between sites. In both reefs, sponges represented the most diverse and abundant group. Overall species richness was higher in Italy, where polychaetes, bryozoans and molluscs were common, whereas in Montenegro cnidarians were more abundant, including the emblematic Savalia savaglia. Given the challenges in coral identification, an integrated morphological and molecular approach was applied to characterize P. muellerae populations. Specimens from the two sites shared key diagnostic traits, with only minor morphological variation. Molecular analyses using three markers (mitochondrial COI, nuclear 28S rDNA, and ITS2) confirmed that both populations belong to a single evolutionary lineage despite geographic separation. Circulation patterns of the Adriatic currents are likely to facilitate gene flow between them, although further studies employing more variable molecular markers (e.g. microsatellites or SNPs) are recommended to assess fine-scale population structure.