The presence of Phoronis australis (Phoronida) in southern Italian waters

Since the beginning of the 1990s records of the Phoronida species in the Mediterranean were limited to Phoronis muelleri Selys-Longchamps, 1903, P. hippocrepiaWright, 1856 and P. psammophila Cori, 1889. Thereafter, P. australis Haswell, 1883, P. ovalis Wright, 1856, and three species belonging to the genus Phoronopsis (P. albomaculata Gilchrist, 1907, P. californica Hilton, 1930 and P. harmeri Pixell, 1912) were recorded. P. australis, distributed in the Indo-Pacific and in the south-eastern Atlantic Oceans, was first recorded in the Mediterranean area along the coasts of Spain and Greece. Data reported in this paper have been collected since 2003 and show the current distribution of P. australis in southern Italian waters. Video-photographic records and the examination of the anatomical and ecological features of fifty specimens obtained by means of diving surveys confirmed the presence of the species. Stable populations of P. australis were observed at Milazzo, on the isle of Salina (Sicily) and also on the Calabrian side of the Strait of Messina. The species, which prefers to inhabit the mucous tube of Cerianthus spp., was always found in association with C. membranaceus (Spallanzani, 1784). As it has happened with other warm-temperate species, P. australis could have widened its distribution due to the warming of Mediterranean coastal waters, either through the Suez Canal and the Strait of Gibraltar (double-source hypothesis) or only through the latter (single- source hypothesis). The latter is supported by the earlier occurrence of the population on the Spanish coasts and its relative abundance compared to the Levantine coasts. In both cases, the present record supports the role of the Strait of Messina as a “biogeographic bridgehead” on the scale of the Mediterranean basin.