Calcareous tubes of the micro-serpulid Hyalopomatus madreporae n. sp. (Polychaeta, Serpulidae) are described from the central Mediterranean. Like congeners, the new Hyalopomatus species has a deep-water distribution (497 to 1146 m). Only empty tubes have been collected, from Madrepora oculata (Linnaeus, 1758) mounds and from muddy sediments associated to the coral framework. The species has been described considering characters of the tube which is small-sized, circular in cross-section and free on its distal end, with a smooth shiny surface. Tube is rather flexible owing to some disruptions and its parts are kept joined by the inner chitinous layer. Way of life of H. madreporae n. sp. is put in relation to such tube feature. Micromorphology of the outer surface and ultrastructure of the tube wall are also described. Morphological characters observed on more than 150 tubes of H. madreporae n. sp., permitted to keep H. madreporae n. sp. apart from the other known Hyalopomatus species. Tubes of H. claparedii Marenzeller, 1878 and H. marenzelleri Langerhans, 1884 from Icelandic waters have been compared with those of H. madreporae n. sp. X-ray microanalyses showed that the organic content in H. madreporae n. sp. is more abundant than in H. claparedii tubes. The discussion covers aspects of tube features used for taxonomy within the genus.
Annelida, Polychaeta, Serpulidae, Hyalopomatus madreporae n. sp., bathyal corals, Madrepora oculata, tube morphology, micromorphology, ultrastructure, microanalysis, new species.