Invasion alert: rapid range expansion of Caulerpa taxifolia var. distichophylla in Maltese waters (central Mediterranean).
The alien alga Caulerpa taxifolia var. distichophylla (Sonder) Verlaque, Huisman and Procaccini was first discovered in Malta in June 2013 and had spread to three sites spanning some 8 km by 2014. A monitoring programme in combination with a citizen science reporting initiative revealed a dramatic expansion in the distribution of this species, which was found at 23 sites over a 45 km stretch of coastline in 2016 and at four additional sites in 2017 (including three sites where it was previously absent in 2016). The alga is now distributed along the entire northeastern coasts of the Maltese Islands, as well as in northwestern and southeastern Malta, with mean frond densities of 91-972 fronds/m2 and mean total frond lengths (including the rachis) of 12.3-41.8 mm. It was found at depths down to 40 m, predominantly on rocky substrata intermixed with other macroalgae (especially when the rocky substratum had a thick layer of sediment covering it) or on heterogeneous bottoms, including at the borders of Posidonia oceanica patches, but not within dense meadows of this seagrass. The remarkably rapid spread of C. taxifolia var. distichophylla and its ability to colonise several different habitats indicate that this alga should be regarded as invasive in Maltese waters.