Morphometric Taxonomy of Siphonous Green Algae: a Methodological Study within the Genus Halimeda (bryopsidales)

Species-level taxonomy of Bryopsidalean genera is often based on quantifiable morphological characters. Yet there are relatively few examples of statistically founded morphometric studies within this group of siphonous algae and macroalgae in general. Molecular phylogenetic studies have revealed cases of cryptic diversity in several Bryopsidalean genera and call for new approaches toward taxonomy.We present a combinedmolecular andmorphometric approach toward Halimeda taxonomy using a selection of specimens representing the five natural lineages within the genus. A phylogeny was inferred from partial nuclear rDNA sequences (30 end of small subunit, internal transcribed spacer region 1, 5.8S, internal transcribed spacer region 2, and 50 end of large subunit) from our and previously studied specimens. Segment size and shape descriptors were acquired using different techniques, including landmark analysis and elliptic Fourier analysis. A broad range of anatomical structures was measured. Taxonomic utility of the different methods and characters was assessed using predictive discriminant analysis. Molecular data were used to delimit species groups. Segment morphological characters proved fairly good predictors for species membership, but anatomical variables yielded the best results. The good performance of morphometric taxon predictors offers perspectives, not only for future taxonomic case studies within problematic species complexes, but also for thorough examinations of the rich fossil record of Halimeda. Statistically founded morphometric studies can probably help elucidate taxonomic issues within other Bryopsidalean genera as well. Key index words: anatomy; Bryopsidales;Halimeda; morphology; morphometrics; phylogeny; siphonous algae; taxonomy