The tropical-subtropical red seaweed Asparagopsis Montagne (Bonnemaisoniales) constitutes the haploid, gametophytic phase in a heteromorphic diplo-haplontic life cycle. The diploid tetrasporophyte is known as the 'Falkenbergia' stage. The genus contains two species, A. armata and A. taxiformis, both present in the Mediterranean Sea where they are regarded as introduced. A. armata is morphologically distinct from A. taxiformis in that it possesses long stolons bearing harpoon-like hooks. The seemingly morphologically identical 'Falkenbergia' stages of the two Asparagopsis species and phenotypic variation within these species have caused taxonomic confusion. We defined species boundaries in the Mediterranean Sea by inferring phylogenies from sequence data from a variable region in the nuclear LSU rDNA gene, the plastid RuBisCo spacer, and the mitochondrial cox2 - 3 spacer of specimens from the Mediterranean, western Europe and the Canary Islands. Results indicate that A. armata and its 'Falkenbergia' tetrasporophyte are genetically distinct from A. taxiformis and its 'Falkenbergia' phase. No phylogeographic structure was detected within A. armata, whereas A. taxiformis seems to consist of at least two genetically distinct but morphologically cryptic species, an Atlantic one (from the Canary Islands) and a Mediterranean one. Hypothetical distribution patterns of the two species as reconstructed from critical temperature limits to growth, survival and reproduction and from the summer and winter isotherms in the Mediterranean Sea agree with the actual Mediterranean distribution patterns as gleaned from our data.
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