Arthonia (aka “Comma Lichens”)
Description: Crustose lichens, with thalli usually very thin or actually growing beneath the upper layers of the substrate. Fruiting bodies (ascomata) irregular to elongate & branching, or round & disk-like; asci broudly club-shaped or balloon-shaped with a thickened summit, K/I-, lying in a more or less uniform layer of branched & tangled hyphae making up the hymenium, which is usually K/I+ blue (hemiamyloid); without an exciple.
Spores: Colourless, ellipsoid to fusiform, usually with one end broader than the other, 2-8 cylindrical, thin-walled cells, often with 1 or 2 of the cells conspicuously larger than the others (a distinguishing characteristic).
Photobiont: green (Trentepohilia or unicellular species similar to Desmococcus).
Chemistry: Most species have no lichen substances & react PD-, K-, KC-, C-, but some have orange, K+ red-purple, anthraquinone pigments on the fruiting bodies.
Substrate: On bark, wood, or– less frequently– rocks. Some species are entirely parasitic on other lichens.
Range: Most species are from tropical or temperate latitudes.
Lookalikes: This very large genus is greatly in need of study. The genus Arthothelium is very similar & closely related, but it generally has larger fruiting bodies & larger, muriform spores. The script lichens (Graphis & Phaeographis) have lirellae with a well-developed exciple, 7 the locules of the spores are lens-shaped. Most Arthonia species grow on tree bark, but some are restricted to rock. Arthonia phaeobaea, for example, forms large colonies on maritime rocks along both coasts. It has a thin, orangish to brown thallus with round, black, convex ascomata resembling apothecia, 0.15 – 0.3 mm in diameter, & 3-6 celled, somewhat constricted, tapering spores.
Bibliography: Lichens of North America, by Brodo, Sharnoff, & Sharnoff
Database Entry: Distance Everheart 12-26-13