Thallus effuse, usually forming small patches (<5 mm diam.) among other crusts, dull olive-grey, olive- or grey-brown, thin, minutely scurfy-granular; granules when discrete c. 20-50 μm diam. Photobiont chlorococcoid, cells 6-14(-17) μm diam.
Anamorph: conidiomata pycnidia, occasional, ca 40 μm diam., the wall pale brown, K+ olive. Conidia 3-4 × ca 0.5 μm, bacilliform.
Teleomorph: ascomata apothecia, 50-250 (-350) µm diam. and 60-100 µm tall, irregularly convex, glossy black, not pruinose. Epithecium 5-7 μm thick, pale green or red-brown, K+ pale green, composed of 2-3 rows of periclinally arranged dark-walled hyphae 1.5-2 μm diam. Hymenium 25-30 μm thick, hyaline or pale green. Hypothecium 30-70 μm thick, reddish brown, K+ dull olive-brown. Interascal tissue of very scanty pseudoparaphyses 0.5-1 μm diam. Asci 22-35 x 10-14 µm, clavate, short-stalked, the apex broad and rounded, very thick-walled, 8-spored. Ascospores 8-12 (-14) × 2.5-4 μm, 1-septate, ovoid, the upper cell broader and shorter than the lower, often with a slightly pointed apex, hyaline, fairly thick-walled, smooth, without a gelatinous sheath or appendages.
Chemistry: lichen products not detected by TLC.
Assessed by Woods & Coppins (2012) as of Least Concern, but listed as Nationally Scarce. The species is very inconspicuous and is probably under-reported.
The rare species Arthonia apatetica is externally similar but has slightly larger ascospores (12-15 × 4.5-5 µm fide Coppins & Aptroot (2009). It is known only from aspen twigs in central Scotland. Morphs of A. muscigena occurring on rock are easily confused with A. lapidicola; that also has larger ascospores (11-15(-18) × 4-7 μm). Both of these species are congeneric with Bryostigma muscigenum, but the necessary combinations have not yet been made.
Scattered throughout GB and Ireland from Cornwall and the Scilly Isles in the south west as far north as Sutherland. BLS map here.
On twigs, especially of Sambucus, or tree trunks (e.g. Acer, Ulmus), sometimes over bryophytes, less often on Abies needles, fence posts and shaded, ± siliceous rocks. Considered to be tolerant of suburban conditions.