Herpothallon

The genus was circumscribed in 1930 by German lichenologist Friedrich Tobler, with Herpothallon sanguineum assigned as the type species. Tobler erroneously believed that the fungus was a member of the Basidiomycota.^[3][4] After it was recognized as an ascolichen, it was referred to either Chiodecton (family Roccellaceae) or Cryptothecia (family Arthoniaceae).^[5]

In 2009, Herpothallon was resurrected following a publication by André Aptroot, Göran Thor, Robert Lücking, and John Elix, in which they recognized 29 species worldwide.^[6] The type species is now known as Herpothallon rubrocinctum,^[7] or in the vernacular as the "Christmas lichen".^[8]

Herpothallon is characterized by the byssoid (a wispy or cottony texture) prothallus (i.e., the first purely fungal layer upon which an algae-containing thallus develops) and hypothallus (i.e., a growth of undifferentiated purely fungal mycelium present as a distinct layer on the underside of the thallus). The texture of the thallus is somewhat felt-like, and its form is heteromerous, meaning that more or less distinct tissues are present, in particular, the mycobiont and photobiont components occur in well-marked layers, with the photobiont in a more or less distinct zone between the upper cortex and the medulla. Another thallus feature common to all species is the felty pseudoisidia; these are isidia-like outgrowths that, unlike true isidia, lack internal differentiation and have no distinct cortex. The thallus also has pustules (blister-like elevations), and granules ranging in form and size from soredia-like to minute. The photobiont partner of the Herpothallon lichen is from Trentepohlia, a genus of green algae.^[9] In Herpothallon, the asci do not develop in true ascomata. Only two members of the genus, H. fertile and H. inopinatum, are known to be fertile.^[10]

As of July 2024^[update], Species Fungorum accepts 49 species of Herpothallon.^[11]

• Herpothallon adnatum G.Thor (2009)^[12]
• Herpothallon alae Sipman (2018)^[13] – Vanuatu
• Herpothallon aurantiacoflavum (B.de Lesd.) Aptroot, Lücking & G.Thor (2009)^[14]
• Herpothallon australasicum (Elix) Elix & G.Thor (2009)^[15]
• Herpothallon biacidum Frisch, Elix & G.Thor (2010)^[16] – Australia
• Herpothallon brialmonticum Aptroot & Elix (2009)^[17]
• Herpothallon capilliferum P.F.Chen & L.L.Zhang (2022)^[18] – China
• Herpothallon cinereum G.Thor (2009)^[17]
• Herpothallon confluenticum Aptroot & Lücking (2009)^[19]
• Herpothallon confusum G.Thor (2009)^[20]
• Herpothallon corallinum G.Thor (2009)^[21]
• Herpothallon coralloides Jagad.Ram (2014)^[22] – Andaman Islands
• Herpothallon echinatum Aptroot, Lücking & Will-Wolf (2009)^[23]
• Herpothallon elegans G.Thor (2009)^[24]
• Herpothallon fertile Aptroot & Lücking (2009)^[25]
• Herpothallon flavominutum Jagad.Ram, G.P.Sinha & Elix (2009)^[9] – India
• Herpothallon furfuraceum G.Thor (2009)^[26]
• Herpothallon glaucescens L.L.Liu & Lu L.Zhang (2023)^[27] – China
• Herpothallon globosum G.Thor (2009)^[28]
• Herpothallon globuliferum Jagad.Ram (2014)^[22] – Andaman Islands
• Herpothallon granulare (Sipman) Aptroot & Lücking (2009)^[29]
• Herpothallon granulosum Jagad.Ram & G.P.Sinha (2009)^[5] – India
• Herpothallon himalayanum Jagad.Ram & G.P.Sinha (2009)^[9] – India
• Herpothallon hypoprotocetraricum G.Thor (2009)^[29]
• Herpothallon hyposticticum Bungartz & Elix (2013)^[30] – Galápagos Islands
• Herpothallon inopinatum Frisch & G.Thor (2014)^[10] – Mexico
• Herpothallon isidiatum Jagad.Ram & G.P.Sinha (2009)^[5] – India
• Herpothallon japonicum (Zahlbr.) G.Thor (2009)^[31]
• Herpothallon kigeziense Frisch & G.Thor (2013)^[32] – Uganda
• Herpothallon lilacinum L.L.Liu & Lu L.Zhang (2023)^[27] – China
• Herpothallon lutescens Jagad.Ram (2014)^[22] – Andaman Islands
• Herpothallon minimum Aptroot & Lücking (2009)^[33]
• Herpothallon minutum Jagad.Ram (2014)^[22] – Andaman Islands
• Herpothallon nigroisidiatum G.Thor (2009)^[34]
• Herpothallon philippinum (Vain.) Aptroot & Lücking (2009)^[35]
• Herpothallon polyisidiatum P.F.Chen & L.L.Zhang (2022)^[18] – China
• Herpothallon psorpseudisidiatum Aptroot, L.A.Santos & M.Cáceres (2024)^[36] – Brazil
• Herpothallon purpureum Aptroot & M.F.Souza (2021)^[37] – Brazil
• Herpothallon pustulatum G.Thor (2009)^[38]
• Herpothallon queenslandicum (Elix) Elix (2009)^[39]
• Herpothallon roseocinctum (Fr.) Aptroot, Lücking & G.Thor (2009)^[40]
• Herpothallon rubrocinctoides (G.Thor) Aptroot, Lücking & G.Thor (2009)^[41]
• Herpothallon rubrocinctum (Ehrenb.) Aptroot, Lücking & G.Thor (2009)^[42]
• Herpothallon rubroechinatum Frisch & G.Thor (2010)^[43]
• Herpothallon rubromaculatum G.Thor (2009)^[44]
• Herpothallon saxorum Bungartz & Elix (2013)^[30] – Galápagos Islands
• Herpothallon subglobosum P.F.Chen & L.L.Zhang (2022)^[18] – China
• Herpothallon tomentosum L.L.Liu & Lu L.Zhang (2023)^[27] – China
• Herpothallon tricolor Aptroot & M.Cáceres (2017)
• Herpothallon viridi-isidiatum P.F.Chen & L.L.Zhang (2022)^[18] – China
• Herpothallon weii Y.L.Cheng & H.Y.Wang (2012)^[45] – China