First record of the colonial ascidian Didemnum vexillum Kott, 2002 in the Mediterranean: Lagoon of Venice (Italy)

Numerous colonies of the invasive colonial ascidian Didemnum vexillum Kott, 2002 have been found in the Lagoon of Venice (Italy) in 2012, overgrowing fouling organisms on maritime structures such as docks, pilings, and pontoons. This is the first record for the Mediterranean Sea. A survey conducted in July 2012 revealed that D. vexillum is present in the euhaline and tidally well flushed zones of the lagoon, whereas it was absent at the examined estuarine tracts and at the zones surrounding the saltmarshes. Suitable climatic, physiographic and saline features together with a high volume of international maritime traffic make the Lagoon of Venice a perfect hub for the successful introduction of temperate non-native species.