Caulerpa taxifolia in the northwest Mediterranean: Introduced species or migrant from the Red Sea?

Specimens of the green alga Caulerpa mexicana Sender ex Kutzing were collected at Messina (Sicily) and transported to Monaco in February 1995. Five days after transplantation into aguaria, the samples began metamorphosing into typical northwest Mediterranean C. taxifolia (Vahl) C. Agardh. Accompanying metamorphosis was novel production of the sesquiterpene secondary metabolite, caulerpenyne, which was absent in pre-metamorphosed samples. These results support the contention that C. mexicana and C. taxifolia are ecads of a single species; the former of which has existed in the eastern Mediterranean since 1939. Biogeographic dispersion of the mexicana ecad, combined with the morphogenetic transformation reported here, may explain the occurrence of C. taxifolia in the northwest Mediterranean. Northwesterly migration of this species and others previously localized in the Levant Sea, such as Caulerpa racemosa (Forsk.) J. Agardh and the seagrass Halophila stipulacea (Forsk.) Ascherson, may have been assisted by recent warming of the western Mediterranean basin.