Pyrenula occidentalis


Pyrenula occidentalis

(R.C. Harris) R.C. Harris (1987)
Pyrenula laevigata auct. p.p.
Pyrenula neglecta auct.
Conservation Status
LC Sc IR (Key)
BLS Number
Taxon Photo
General Description

A characteristic Pyrenula of smooth bark communities in temperate rainforest habitats. Distinguished by the yellow-brown to orange-brown thallus, often with small K+ purple rusty patches,  without pseudocyphellae, with neat round perithecia with central ostioles. Locally abundant in western Scotland and in parts of western Ireland, but rare in the west beyond and local in western Ireland.


Thallus immersed, yellow- to orange-brown; pseudocyphellae absent; surface K+ purple-red in parts in section. Perithecia 0.4–0.8 mm diam., forming projections in the thallus; exciple usually with K+ purple-red pigment associated with the upper outside wall; hymenium densely inspersed with minute oil droplets 0.5–2.5 μm diam., thus appearing cloudy; anthraquinones absent (K–). Ascospores (1-) 3-septate, (17–) 18– 22 × (7–) 8.5–10.5 (–11) μm. Conidia ca 20 μm long, curved. Thallus C–, K+ yellowish to + purplish in parts, KC–, Pd+ pale orange, UV– (unidentified anthraquinones).

Anthraquinones are responsible for the K+ purple-red reaction in the thallus and on the outer surface of the exciple (but not the inner surface); small rusty patches of anthraquinone can sometimes be seen on the thallus surface under a dissecting microscope. This species has been much confused with P. laevigata, which differs in the paler thallus, absence of anthraquinones, and the hymenium which is not inspersed with droplets. P. acutispora has ostioles that are mainly lateral and the hymenium is not inspersed.


On smooth bark, particularly of Hazel, Holly and Rowan, in more or less shaded, constantly humid woodland and sheltered stream valleys valleys and characteristic of temperate rainforests.

Distribution Map
Key to map date classes

Local, rare in S.W. England, Mid & N. Wales and Cumbria, abundant W. Scotland and local W. Ireland.

Threats & Status

A characteristic and constant species of undescribed hyperoceanic Graphidion communities confined to temperate rainforest habitats. The lichen has very strong populations in the western Highlands and isles, but rare and threatened beyond in Britain. 

Britain: Notable, International Responsibility species

Scotland: Priority Taxon for Biodiversity in Scotland 

Wales: Vulnerable


Cannon, P., Coppins, B., Aptroot, A., Sanderson, A. & Simkin, J. (2023). Perithecial genera I, including Acrocordia, Alloarthopyrenia, Anisomeridium, Antennulariella, Arthopyrenia, Celothelium, Cyrtidula, Dichoporis, Eopyrenula, Julella, Leptorhaphis, Leptosillia, Lithothelium, Mycomicrothelia, Mycoporum, Naetrocymbe, Pyrenula, Rhaphidicyrtis, Sarcopyrenia, Swinscowia and Tomasellia. Revisions of British and Irish Lichens 37: 1-59.

Text by Neil A Sanderson

Lichenicolous Fungi
Skyttea pyrenulae Diederich, Etayo & Coppins
Stigmidium cf. pyrenulae Van den Boom