“Anthurid invasion” along the Mediterranean coasts: the spreading of Mesanthura cfr. romulea and Paranthura japonica

The Anthuroidea is a superfamily of elongated isopods belonging to the mesofauna, mainly distributed in the austral hemisphere and able to colonize a variety of benthic habitats. In recent years, two species from the families Anthuridae and Paranthuridae have been reported as non-indigenous in the Mediterranean Sea: one species of Mesanthura, most likely conspecific with Mesanthura romulea native to Australia, and Paranthura japonica, native to the Asian Western Pacific. Both these species were initially recorded along the Italian coasts, in the harbour of Salerno (2000) and in the Lagoon of Venice (2005), respectively. Since these recordings, their distribution has been considerably expanding. In this study, we present an update on their current known distribution in the Mediterranean Sea from literature records and unpublished data acquired from a range of surveys conducted by the authors across the Mediterranean Sea. Thus far, M. cf. romulea has been found in 13 Italian localities, and dubiously in Tunisia. M. cf. romulea was found in harbours and marinas, as well as in natural habitats (such as seagrass meadows and algal canopies), suggesting shipping as the most likely vector of introduction and highlighting its capability of further expansion both by natural spread or secondary translocation. Furthermore, P. japonica has been reported in 19 localities from four countries (Italy, France, Greece and Tunisia), in several important shellfish farming localities (Venice, Taranto, Olbia, La Spezia, near Thau Lagoon), suggesting aquaculture to be the main vector of introduction for this non-indigenous species. In addition, the subsequent findings for these two species in ports and on boat hulls highlights the role of vessel traffic in their further spread across the Mediterranean. Despite their different introduction histories, the simultaneous occurrences of these non-indigenous anthurids in numerous Mediterranean localities may stem either from their recent spread in the basin, or from an improvement in taxonomic expertise. Since anthurid isopods are from a taxonomically challenging taxon, these two non-indigenous species may have gone either unnoticed or misidentified for a long time.