One hundred years later, resurrection of <i>Tydemania gardineri</i> A. Gepp &amp; E. Gepp (Udoteaceae, Chlorophyta) based on molecular and morphological data

Weber-van Bosse is a genus belonging to the family Udoteaceae (Bryopsidales, Chlorophyta) and currently thought to be monospecific throughout its distribution range in the Indo-Pacific. We tested the assumption that <i>Tydemania</i> is a single species using species delimitation methods, morphological observations and phylogenetic reconstructions of large datasets. Our molecular and morphological data recovered two distinct groups, which we argue are the type species <i>T. expeditionis</i> Weber-van Bosse and <i>T. gardineri</i> A. Gepp &amp; E. Gepp. The latter is currently considered a synonym of the former and we resurrect this name from synonymy. <i>T. gardineri</i> is distinguished morphologically from <i>T. expeditionis</i> by the complete absence of glomeruli, whereas <i>T. expeditionis</i> can have glomeruli and flabella, glomeruli alone, or flabella alone. The two species can also be distinguished by the shape and length of the stalks at the base of the flabellum, the diameter of the main axis and of the flabella siphons at the apices. In addition, they have different geographic distributions, with <i>T. gardineri</i> restricted to the Western Indian Ocean, and <i>T. expeditionis</i> extending from the Red Sea, throughout the Indian Ocean and into the West Pacific, including the Coral Triangle, the Philippines and Japan. From our dataset and literature search, we hypothesize that its south-western distribution limit is in northern Madagascar. Finally, we confirm the synonymization of <i>T. mabahithae</i> with <i>T. expeditionis</i> based on specimens from the type locality of the former and the lack of flabella in several other specimens of <i>T. expeditionis.</i>