Type. ITALY. Val Cherpa, Bolca, ad Corylus, 1869, Massalongo, s.n. (VER, lectotype designated in Tehler 1990).
Description. Lichenized fungus. Thallus crustose, endophoedal, thin, continuous, whitish, vegetative diaspores absent. Thallus anatomy homoiomerous (not layered; photobiont throughout); photobiont Trentepohlia alga, cells coccoid. Ascomata irregular to polygonal, immersed, up to 2 mm diam., frequent, often separated from surrounding thallus by cracks. Disk black, epruinose; margin absent; epithecium reddish brown, up to 80 μm thick; hymenium brownish with pigments uneven, up to 150 μm high, hemiamyloid (I+ red, K/I+ blue); hypothecium brown, integrated with bark fragments; paraphysoids branched, anastomosing. Asci scattered, subglobose, 55–75 x 40–50 μm, 8-spored. Ascospores colorless or brownish, ellipsoid, muriform, 25-37 x 12-15 μm. Pycnidia not seen.
Chemistry. Spot tests all negative; secondary metabolites not detected.
Substrate and Habitat. On bark of hardwood trunks, notably Carya species in mixed and hardwood forests.
Distribution. North America, Eurasia, South America (Brazil); in North Carolina throughout.
Notes.Arthothelium spectabile has been proposed for IUCN Fungal Red List inclusion in July 2020 due to a single occurrence in Brazil, marking it as data deficient for that country. Outside of Brazil, A. spectabile can be classified as least concern; however, it may be extinct in the British Isles (Coppins 2009).
Coppins, B.J. (2009) Arthothelium A. Massal. (1852). Pp. 158-161 in Smith, C.W., A. Aptroot, B.J. Coppins, A. Fletcher, O.L. Gilbert, P.W. James & P.A. Wolseley (eds.). The Lichens of Great Britain and Ireland. The British Lichen Society, London.
Grube, M. & M. Giralt. (1996) Studies on some species of Arthothelium occurring in the western Mediterranean. Lichenologist 28(1): 15-36.
Massalongo, A.B. (1852) Ricerche sull'autonomia dei licheni crostosi. Pp. 1-221 (original description).
Tehler, A. (1990) A new approach to the phylogeny of euascomycetes with a cladistic outline of Arthoniales focussing on Roccellaceae. Canadian Journal of Botany 68: 2458-2492.