Cyphellostereum ushimanum is a species of lichen-forming fungus in the family Hygrophoraceae. This basidiolichen was discovered and described in 2022 from a single location on Amami à Âshima island in Japan, where it grows on the bark of Japanese cedar trees. The species has microscopic feeding structures (haustoria) that penetrate its cyanobacterial partner, a feature that required an update to its genus, Cyphellostereum. The lichen produces small, white, paper-thin reproductive structures adjacent to its bluish-green thallus.
The species was formally introduced and described as Cyphellostereum ushima by Hiroshiâ¯Masumoto and Yousukeâ¯Degawa, who based the epithet on an indigenous pronunciation of the island name "à Âshima". Their morphological study, combined with internal transcribed spacer rDNA data, placed the fungus in a strongly supported clade within the genus Cyphellostereum, sister to C. unoquinoum and C. phyllogenum. Together these taxa share the absence of clamp connections and an incomplete hyphal sheath around the cyanobacterial trichomes, yet C. ushima alone have intracellular tubular haustoria.
The discovery required a slight revision of the generic concept. Classical Cyphellostereum species were thought to lack haustoria and to form stipitate, cupâÂÂlike fruiting bodies, whereas the new species demonstrates that resupinate basidiomata and haustorial penetration of the both fall within the phylogenetic limits of the genus. As a consequence, the authors emended the of Cyphellostereum to encompass a broader range of thallus architectures and hymenophore morphologies.
Macroscopically, the lichenised thallus forms a fragile, green to bluishâÂÂgreen mat of loosely interwoven fungalâÂÂcyanobacterial fibrils on the bark of Cryptomeria japonica. In microscopic section the thallus is up to 80â¯micrometres (üm) thick, with occasional tufts of compact fibrils projecting to roughlyâ¯300 üm. Each Rhizonema filament (aboutâ¯2.7âÂÂ3.5 üm diam.) is only partly ensheathed by hyphae, leaving visible interspaces. Under the light microscope and in TEM micrographs, tubular haustoria (aboutâ¯2.5âÂÂ3.4 üm diam.) are seen entering the trichome and running through consecutive cells, a feature otherwise associated with the related genus Dictyonema.
The basidiomata develop on the wood surface adjacent to the thallus, especially along the margins or on the underside of overhanging bark. They are paperâÂÂthin, membranous and white, rarely exceeding a few millimetres across. The hymenium is â¤â¯40 üm thick and composed of hyaline, thinâÂÂwalled hyphae 3âÂÂ4 üm wide; clamp connections and cystidia are absent. Basidia are shortâÂÂclavate, 8.6âÂÂ13.0â¯ÃÂâ¯5.4âÂÂ6.5 üm, bearing four sterigmata 2.2âÂÂ3.6 üm long. Basidiospores are ellipsoid to slightly elongate, 5.3âÂÂ6.3â¯ÃÂâ¯3.4âÂÂ4.0 üm, smooth, thinâÂÂwalled and nonâÂÂamyloid. When grown axenically on potato dextrose agar at 20 ðC the forms slowâÂÂgrowing, pale pink to pale brown colonies reaching 4.0âÂÂ4.5 mm diameter after two months.
As of its original publication, Cyphellostereum ushima was known to occur only in the type locality, a lowâÂÂelevation (15 m) coastal forest in Setouchi town, Amami à Âshima, Kagoshima Prefecture. The climate there is warm and humid yearâÂÂround, supporting subtropical evergreen broadâÂÂleaved vegetation in which Cryptomeria japonica plantations are common. The lichen grows epiphytically on the shaded, occasionally rainâÂÂwashed trunks and branches of this conifer, where mesic conditions favour cyanobacterial partners. Field observations indicate that the species is locally rare but may be overlooked because its inconspicuous thallus blends into algal films and bryophyte patches, while its basidiomata are minute and transient.