Thallus thin, finely rimose-cracked or smooth, sometimes totally immersed, white, grey, pale green-pink or rusty ochraceous, often with abundant lipid-rich cells which stain in Sudan Black, 5-10 µm diam. Prothallus inconspicuous, soredia and isidia absent. Photobiont Trentepohlia.
Anamorph: conidiomata pycnidia, to ca 300 µm diam., frequently absent. Conidia 4.5-7 × 0.5-1.5 µm, bacilliform, or rarely ellipsoid, straight, hyaline, aseptate, thin- and smooth-walled, without an epispore, gelatinous sheath or appendages.
Teleomorph: ascomata apothecia, lirellate, very variable in form, 600-1200 (-2000) × (100-) 150-300 µm, 40-120 µm tall, sessile or rarely semi-immersed, black, not pruinose, scattered or contiguous, simple, sometimes branched or stellate or in complex clusters. Disc slit-like, sometimes partially exposed with age. Exciple K+ olive-green; epithecium brown, K+ greenish; hymenium 80-100 µm tall. Interascal tissue of pseudoparaphyses, septate, branched, often richly anastomosed, the apices not or rarely slightly swollen. Asci clavate-cylindrical, thick-walled and fissitunicate with a well-developed apical cap and ocular chamber, the lower part of the cap staining weakly blue with iodine, 8-spored. Ascospores 16-20 (-24) × 4-5 (-6.5) µm, clavate, 3-septate, hyaline, dextrinoid when young, fairly thick-walled, smooth, without an epispore, gelatinous sheath or appendages.
Chemistry: thallus C–, K–, KC–, Pd–, UV– (no lichen products detected by TLC).
Assessed by Woods & Coppins (2012) as of Least Concern. The species is common and widespread.
Similar to Opegrapha dolomitica, but with smaller ascospores and a greenish exciple, epithecium and hypothecium in K. The lichenicolous O. rupestris, with rounded apothecia, also occurs on thalli of Verrucariaceae on limestone.
The wide circumscription of Arthonia calcarea adopted here encompasses several morphotypes. The most distinctive of these have been named as: Opegrapha conferta auct., the commonest, has a thin, inapparent to evanescent, pale grey thallus, often confined to minute fissures in neutral to siliceous rocks, especially near the coast; the lirellae are variable in form, sometimes scattered, irregularly nodular or deformed or often forming knot-like heaps to 7 mm across. O. chevallieri resembles O. conferta auct. but has a conspicuous, superficial, pure white, chalk-like thallus with conspicuous, loose to compact heaps of lirellae, sometimes to 5 mm diam.; it is widespread on cement, mortar and plaster walls, especially in churchyards and on derelict buildings. These morphotypes appear to intergrade with the typical state of the species and confusing intermediates occur; a detailed study of this complex is required.
Molecular data strongly suggest affinities with Arthonia (in fact the species appears to be close to the type of that genus), but morphologically Arthonia calcarea matches species of Opegrapha where it is traditionally classified. Further research is needed.
Throughout the British Isles, especially in lowland areas with suitable habitat and near the coast. BLS map here.
On damp, shaded limestone, mortar, bone or base-enriched rocks.