Arthonia circinata Th. Fr.

Bot. Notiser: 100, 1865.
Description: Thallus inapparent, not lichenized, lichenicolous on the thalli of Umbilicaria-species, causing orbicular infection spots with a concentric zonation (a brownish black, sometimes bleaching outer zone, a reddish brown median zone with ascomata arranged in a ring, a pale brown to bleaching and necrotic median zone, often with reddish grains). Apothecia arthonioid, black, round but often confluent, 0.1-0.25 mm across, sessile, with a rough surface, 90-110 μm tall, usually arranged in rings of 10-30. Proper exciple poorly developed, resembling the epithecium; epithecium brown, 8-30 μm high; hymenium colourless to brownish with age, 20-30 μm high, I+ reddish, K/I+ deep blue, sometimes with orange-red crystals reacting K+ violet-red; paraphysoids c. 2 μm thick, branched and anastomosing, the apical cells 4-6 μm wide, with a dark cap; hypothecium colourless, turning pale brown with age, 30-40 μm high. Asci 8-spored, clavate, semi-fissitunicate, with a large apical dome and a distinct ocular chamber, lacking I+ or K/I+ structures in tholus. Ascospores 1-septate with slightly unequal cells, hyaline, ellipsoid to oblong ovoid, 11-14 x 4-6 μm. Photobiont absent. Spot tests: orange crystals K+ violet-red. Chemistry: unknown.
Note: a characteristic lichenicolous fungus growing on the thalli of Umbilicaria-species. Known from Spain and Austria, to be looked for in the Italian Alps.
Growth form: Lichenicolous fungus
Substrata: rocks
Reproductive strategy: mainly sexual
paras Umbilicaria-species

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Predictive model

NTNU-Vitenskapsmuseet, 7012 Trondheim, Norway CC BY-SA NC