Unveiling a novel species, <i>Pachymeniopsis shinchonai</i> sp. nov., from Korea, with reference to phylogenetic relationships within the Grateloupiaceae (Halymeniales, Rhodophyta)

Abstract Recent comprehensive phylogenetic and morphological analyses have led to the recognition of the Grateloupiaceae as a distinct family within the Halymeniales. Despite strong support for its monophyly, the genera within the Grateloupiaceae display diverse morphological traits and share external features that prevent accurate generic identification without additional molecular analyses. This study conducted detailed morphological observations and assessed the phylogenetic relationships of a Grateloupia -like species found in Korea, which superficially resembles “G”. imbricata . We describe Pachymeniopsis shinchonai sp. nov., characterized by erect, compressed axes branching dichotomously to subdichotomously, maturing into a dome-shape thallus. This species is distinguished from related species by its densely arranged thallus, which forms a hemispherical shape, its abundant secondary attachments adhering to adjacent branches and the substratum, and its phylogenetic position in the rbc L tree. This species contributes to the biodiversity of the intertidal zone on Jeju Island, Korea. Our phylogenetic analysis confirms the monophyly of nine clades within the Grateloupiaceae and underscores the need for further studies to resolve the generic status of Pachymeniopsis, Prionitis, and Phyllymenia , which cannot be readily distinguished by morphological criteria alone. Due to the lack of support for several genera, comprehensive collections and multi-gene analyses are essential to accurately delineate the internal generic relationships within the family.