Identity and origin of a slender Caulerpa taxifolia strain introduced into the Mediterranean Sea

On the basis of morphological and molecular studies, we identified the Australian endemic green alga known as Caulerpa distichophylla along the coasts of Sicily (Italy, Mediterranean Sea). The slender Caulerpa previously reported as C. taxifolia from southeastern Turkey (Gulf of Iskenderun) also belongs to C. distichophylla. Morphologically, C. distichophylla clearly differs from C. taxifolia in its slender thallus and the lack of large rhizoidal pillars. However, genetic data do not provide undisputed evidence that the species are distinct. Sequences of the tufA cpDNA gene and of the cp 16S rDNA intron-2 sequences separated the two taxa by only one single nucleotide mutation, whereas ITS rDNA sequences did not clearly distinguish them. The new combination Caulerpa taxifolia var. distichophylla is therefore proposed. Western and eastern Mediterranean populations of C. taxifolia var. distichophylla are probably the result of introduction events from southwestern Australia. Although the vector of primary introductions remains unknown (aquarium trade or shipping), maritime traffic appears to be the most likely vector of secondary dispersal. C. taxifolia var. distichophylla is closely related to C. taxifolia, hence interbreeding with the other C. taxifolia strains in the Mediterranean Sea might be expected to occur