Morphological characterization of lineages within the calcified tropical seaweed genus Halimeda (Bryopsidales

Halimeda Lamouroux constitutes a genus of calcified and segmented green seaweeds within the Bryopsidales. Molecular phylogenetic assessments have uncovered five principal monophyletic lineages within the genus. In the present study we define these lineages morphologically. We gathered morphological data from specimens used in the molecular analyses as well as from collections having a similar morphology and originating from the same geographical region. Starting from the lineages and their morphological synapomorphies, we define and illustrate five natural sections within Halimeda. All or most medullary siphons traversing the nodes between segments fuse into a single unit in specimens of lineage 1 (section Rhipsalis), and segments at the thallus base fuse with one another. Medullary siphons of specimens in lineage 2 (section Micronesicae) traverse the node without fusing. Medullary siphons of specimens in lineage 3 (section Halimeda) divide frequently below the nodes and become entangled among one another. The segments of specimens in this lineage possess a continuous uncorticated band along the distal perimeter instead of three or more pits as encountered in segments of specimens in all other lineages. Members of lineage 4 (section Pseudo-opuntia) possess club-shaped subperipheral utricles in their cortical region. Medullary siphons of specimens in lineage 5 (section Opuntia) fuse over only a short distance at the nodes and retain their identity. Apart from these synapomorphies, the lineages can be delimited further by a characteristic combination of symplesiomorphies and homoplasies. In addition we examined the morphology of H. bikinensis Taylor, a species no