Cyathodium tetrasporum (Cyathodiaceae, Marchantiopsida), a new species of liverworts with spores permanently united in tetrads

The genus Cyathodium (Marchantiophyta: Cyathodiaceae), a phylogenetically distinct lineage of cave-adapted liverworts, is characterized by a female involucre on the ventral surface of thalli and a distinctive green luminescence of thalli. Globally, 12 species of this genus are currently accepted, with a species diversity center in the Himalayas. During our taxonomic survey of the Chinese bryoflora, we found a remarkable undescribed species that exhibits spores permanently united in tetrads. Molecular evidence of chloroplast markers (psbA-trnH, rbcL and rps4) resolved it as sister to Cyathodium tuberosum Kashyap. Here, this plant is described as Cyathodium tetrasporum sp. nov. Cyathodium becomes the third genus of the class Marchantiopsida with species characterized by spores permanently united in tetrads, after Riccia and Sphaerocarpos. Detailed morphological descriptions, comparative analyses with congeners, SEM images of spores, and a provisional IUCN conservation assessment are provided.