Caulerpa taxifolia var. distichophylla (Sonder) Verlaque, Huisman & Procaccini, a new invasive species on the French Mediterranean coast

In June 2025, we discovered the invasive and non-indigenous alga Caulerpa taxifolia var. distichophylla (Sonder) Verlaque, Huisman & Procaccini in the Bay of Villefranche (Alpes-Maritimes, France). This alga is native to Western Australia where it was originally described under the name of Caulerpa distichophylla Sonder. In the Mediterranean Sea, the alga was first recorded from Syria in 2003, and since spread to southern Turkey, Greece (Rhodes), Cyprus, Malta, Italy (Sicily, Calabria, and southern Sardinia), Libya and Tunisia and most recently to southern Spain (Andalusia). The previous closest record is located over 500 km from Villefranche-sur-Mer Bay, where we found over one hundred small colonies, each covering less than 0.1 m², at depths between 1.5 and 5 m, less than 60 m from the shoreline and spanning approximately 600 m of coastline. Caulerpa taxifolia var. distichophylla began to colonise the bay in which two other invasive species of Caulerpa J.V.Lamouroux had previously become established. We have been following their progress since they arrived in this bay as well as that of a native species of Caulerpa. Their spread has been characterised by phases of rapid expansion followed by abrupt and still-unexplained declines. This context enables us to formulate hypotheses regarding the potential for colonisation and invasive behaviour of C. taxifolia var. distichophylla.