Mediterranean Caulerpa taxifolia and C-mexicana (Chlorophyta) are not conspecific

In 1984, Caulerpa taxifolia (Vahl) C. Agardh was reported along the coast of Monaco, Over the past decade it has spread along 60 km of the Mediterranean coastline and presently represents a potential risk. to biodiversity, Several explanations have been advanced regarding the presence of C. taxifolia in the Mediterranean. One hypothesis maintains that the alga was introduced accidentally into the sea at Monaco, where it has been used as a decorative alga in aquaria. Caulerpa taxifolia has not been reported in earlier marine floras of the Mediterranean and ifs sudden appearance has suggested that it may be a recent introduction. Another hypothesis proposes that C. taxifolia and Caulerpa mexicana Sonder ex Kutzing are morphological variants of one another and hence conspecific taxa. Caulerpa mexicana has been found in the eastern. Mediterranean since at least 1941. In order to establish the taxonomic identities of these taxa, individuals from five populations of C. taxifolia and four populations of C. mexicana were collected from within and outside of the Mediterranean. Comparative DNA sequence analysis of the nuclear ribosomal cistron, including the 3'-end of the 18S, ITS1, 5.8S, and ITS2 regions, show clear phylogenetic separation of the two taxa using parsimony and maximum likelihood analyses. Separation is maintained whether the analyses are based on just the more conserved 18S data or just the fast-evolving spacers. The two species are thus not conspecific. For specimens of uncertain identity (i.e. taxifolia-mexicana intermediates), a PCR diagnostic amplification can easily be performed because the ITS1 in C. taxifolia is 36 nucleotides shorter than the ITS1 in C. mexicana. Whether or not C. taxifolia has been present for a longer period of time in the marine flora, either as a cryptic endemic species or as the result of one or more introductions, represents an additional hypothesis that will require identification of biogeographic populations from throughout the world, as well as a population-level study of the Mediterranean region.