Arthonia excipienda


Arthonia excipienda

(Nyl.) Leight. (1871)
Arthonia dispersa var. excipienda
Arthonia hibernica
Conservation Status
NT NR Sc (Key)
BLS Number
Taxon Photo
General Description

A tiny non-lichenised fungus, most resembling Naevia punctiformis, and found in similar pioneer habitats. The apothecia, however, are narrower and occasionally branched and the one septate spores are very different from those of Naevia punctiformis, which are 3-septate. The fungus also occupies a much more restricted range, being confined to smooth bark in western temperate rainforests in Scotland and Ireland, with a single site found in Wales in 2022. 


Thallus immersed, often scarcely visible, effuse or sometimes delimited from adjacent crusts by a brown line; photobiont absent, but a few adventitious protococcoid or trentepohlioid cells are sometimes present. Apothecia 0.3–1.5 × 0.1–0.14 mm, often curved or flexuose, occasionally a few branched; ‘margins’ slightly raised towards a slit-like disc that sometimes expands in old apothecia. In section 40–50 μm tall, with a well- developed lateral dark brown (K+ pale green) exciple-like zone 12–18 μm thick; epithecium pale brown or olive; hymenium 30–40 μm tall, colourless; hypothecium indistinct, rarely over 5 μm tall, colourless; paraphysoids 1–1.5 μm diam., numerous, only rarely thickened by pigment above, apical caps not seen. Ascospores 15–17 (–18) × 5–7 μm, 1-septate, obovoid, colourless. Pycnidia not found. Lichen products not known.

Distinguished from superficially similar Arthonia spp. by the well-developed exciple-like margin. It is easily overlooked for Naevia punctiformis, which has 3-septate ascospores, but appears not to belong in that genus. Melaspilea atroides has a true black exciple and smaller, dark brown ascospores.


On smooth bark (e.g. Corylus) or the smooth surface of fissured bark; rather rare. Confined to temperate rainforests. 

Distribution Map
Key to map date classes

Western Scotland and W. Ireland, but not recorded from the latter recently. In 2022 found on Rowan in a ravine in the Vale of Ffestiniog, another addition to the remarkable assemblage of smooth bark hyper-oceanic specialist lichens found in the rainforests of this valley which are confined to this area in Wales.  

Threats & Status

Very localised and confined to temperate rainforest habitats on the west coasts.

Britain: Near Threatened

Scotland: Priority Taxon for Biodiversity in Scotland

Wales: not assessed but in 2022 known only from a single site


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)