According to the recent lichen checklist published by Hur et al. [¹], nine fruticose lichen genera have been reported from South Korea, Bryoria Brodo & D. Hawksw., Cladina (Nyl.) Nyl., Cladonia P. Browne, Oropogon Th. Fr., Pilophorus Th. Fr., Ramalina Ach., Stereocaulon Hoffm., Sulcaria Bystrek and Usnea Dill. ex Adans. The current study revealed a new, rare fruticose cyanobacterial lichen genus Polychidium (Ach.) Gray for the South Korean lichen flora.

The genus Polychidium includes 15 species worldwide [²]. However, the number of species recorded has varied among studies [^3-5], and no recent revisionary work of the genus has been conducted. Recently, four species belonging to the genus Polychidium [⁶, ⁷] together with Leptochidium M. Choisy and Massalongia Körb. were transferred to a new family, Massalongiaceae Wedin, P. M. Jørgensen & E. Wiklund based on the parsimony and Bayesian analysis of the mitochondrial small subunit (mtSSU) and nuclear large ribosomal subunit (nuLSU) rDNA sequence [⁸].

The genus Polychidium includes the following characteristic features. Thallus very small and non-stratified, dichotomously, evenly or isotomically branched, fruticose, shrub like, attached to substrate by a basal holdfast. Upper surface or thallus is greyish to brownish or brownish black in color and round in cross-section, very slender, shiny, gelatinous, brittle and solid, corticate. Cortex well developed, cellular, one to several layers thick, surrounding a medullary hyphae. Within the cortex, cyanobacteria (Nostoc or Scytonema) form the core of the branch. Isidia, soredia and pseudocyphellae absent. Ascomata apothecia, biatorine, mostly lateral, disc pale to dark brown and sunken, very common on some species and rare on others. Thalline exciple absent, true exciple present, paraphyses unbranched, septate. Spores 1 or 2 celled, colorless, ellipsoid to spindle shaped, fusiform, thin or thick walled, eight per ascus. Conidiomata pycnidia, bacilliform conidia. No lichen substances on thin layer chromatography (TLC). Cosmopolitan genus, but generally at higher elevations in tropical regions [^9-12].

In the East Asian region, Polychidium species have been recorded from four countries, China (P. dendriscum) [⁷], Japan (P. muscicola) [¹³], Taiwan (P. stipitatum) [¹⁴], and Papua New Guinea (P. dendriscum, P. muscicola and P. stipitatum) [¹⁵]. According to Jørgensen [¹⁶], P. muscicola is considered as a red listed species since it to be endangered in particular habitat. This is the first record of the genus Polychidium from South Korea. This genus representing species P. muscicola was found on a moss together with a thallus of Myelochroa irrugans. Here, the thallus is described in full and figures and taxonomic details pertaining to P. muscicola are presented.

This study was based on the voucher specimen deposited in the herbarium of the Lichen and Allied Bioresource Center at the Korean Lichen Research Institute (KoLRI), Sunchon National University, South Korea. The external morphology of the thallus was observed under a dissecting microscope (Nikon SMZ645; Nikon, Tokyo, Japan) and photographs were taken using a Hi-SCOPE HC-100A camera. A compound microscope (Zeiss Scope. A1; Carl Zeiss, Oberkochen, Deutshland, Germany) was used to study the anatomy of the thalli and photographs were taken using an AxioCam ERc 5s camera (Carl Zeiss) fitted to the light microscope. All measurements were made from materials mounted in water and stained with lactophenol cotton blue. Spot test reactions were carried out on the thallus and TLC was performed in solvent system C (toluene : acetic acid = 85 : 15) as described by Orange et al. [¹⁷].