Arthonia cohabitans
A rare lichenicolous fungus of a rare lichen, Arthothelium macounii, which is itself confined Hazel stands in temperate rainforests, and so far only known from two sites, one not recorded recently. Can be spotted as fleck-like, brown-black, not pruinose, rounded apothecia on the thallus of the host.
Thallus absent, lichenicolous on the thallus of Arthothelium macounii. Apothecia scattered to clustered, developing below the surface of the bark but soon erumpent, fleck-like, brown-black, not pruinose, rounded and 0.06–0.16 mm diam., or elongate and to 0.3 (–0.4) × 0.08–1 mm; in section (50–) 60–70 μm tall, K+ purple due to yellow-orange pigment; epithecium often discontinuous, of pale yellow-orange pigment mixed with the dark brown walls of the paraphyses; hymenium 35–38 μm tall, pale yellow-orange, I+ blue; hypothecium 16–23 μm tall, mottled dark brown with yellow-orange background; paraphysoids rather scanty, sparingly branched, 1–1.5 μm diam., the apices more richly branched, brown-walled, 1.5–2 μm diam., sometimes with dark brown caps to 3.5 μm diam. Ascospores 10.5–14 × 4–5 μm, 1-septate, the upper cell broader and longer than the lower, cylindric-obovoid to cylindric-ellipsoidal or clavate, at first colourless with a thin perispore, later becoming brown and warted. Lichen products not detected by TLC.
The non-lichenicolous A. didyma, which confusingly can at times overgrow other lichens, is distinguished by the larger ascospores.
On thallus of Arthothelium macounii, commensalistic or weakly parasitic, rarely forming necrotic patches. In Atlantic Hazel woodlands within temperate rainforests.
Very rare western Highlands (Argyll, W. Inverness). Endemic.
A rare lichenicolous fungus of a rare lichen which is itself confined Hazel stands in temperate rainforests, and so far only known from two sites, one not recorded recently.
Britain: Vulnerable, International Responsibility species
Scotland: Priority Taxon for Biodiversity in Scotland
Cannon, P., Ertz, D., Frisch, A., Aptroot, A., Chambers, S., Coppins, B. J., Sanderson, N. A., Simkin, J. & Wolseley, P. (2020) Revisions of British and Irish Lichens Volume 1 August 2020 Arthoniales: Arthoniaceae. BLS
Text by Neil A Sanderson, based on Cannon et al (2020)