A temperate rainforest species of smooth bark in lichen rich old woods, which is widespread in core area of temperate rainforest in the west Highlands and inner Hebrides. Occasionally found further east in the Highlands in humid old growth woods in the transition zone to sub-oceanic woodland. Resembles Thelotrema lepadinum, but with a grey-green thallus and with apothecia with a narrower opening. Microscopic examination is necessary for certain identification, however, the large, dark ascospores can be visible with a ×20 lens and are often scattered over the surface of the thallus and particularly around the ostioles looking like tiny mouse droppings.
Thallus pale grey-green, often somewhat white-marbled when fresh, becoming ochraceous when dry, forming small to large superficial patches which are rather thick, (to 130 µm in thickness); medulla with coarse calcium oxalate crystals. Apothecia (0.7–) 0.8–1.2 (–1.5) mm diam., immersed in hemispherical warts, somewhat flattened at their summits, urceolate, the ostiolar opening usually narrow, 0.2–0.4 (–0.7) mm wide; true exciple colourless, inconspicuous when seen through the ostiole; thalline margin entire; hymenium (100–) 140–200 µm tall. Asci 130–170 × 40–55 µm, 1- (to 2-) spored. Ascospores (70–) 90–120 (–160) × (15–) 25–40 (–50) µm, broadly fusiform with rounded ends, opaque grey-brown when mature, with 16–22 (–24) transverse and numerous longitudinal septa, I–, with a gelatinous coat. Thallus and medulla C–, K–, KC–, Pd–, UV– (lichen products not detected by TLC).
Resembles Thelotrema lepadinum, but with a grey-green thallus and with apothecia with a narrower opening; microscopic examination is necessary for certain identification. However, the large, dark ascospores can be visible with a ×20 lens and are often scattered over the surface of the thallus and particularly around the ostioles looking like tiny mouse droppings. Previously referred to in Europe as T. monosporum but that is a species of tropical S.E. Asia.
A hyperoceanic species found on smooth bark, especially twigs of Hazel and Rowan, in sheltered boggy undisturbed ancient woodlands in the temperate rainforest zone, a rare relict species in Ireland.
Very local, W. Scotland, rare in W. Ireland.
Widespread in high quality woods within the core area of temperate rainforest in the west Highlands and inner Hebrides. Occasionally found further east in the Highlands in humid old growth woods in the transition zone to sub-oceanic woodland. It appears to be a rare relict species in Ireland.
Britain: International Responsibility species
Scotland: Priority Taxon for Biodiversity in Scotland
Aptroot, A., Weerakoon, G., Cannon, P., Coppins, B., Sanderson, N. & Simkin, J. (2023). Ostropales: Graphidaceae, including the genera Allographa, Clandestinotrema, Crutarndina, Diploschistes, Fissurina, Graphis, Leucodecton, Phaeographis, Schizotrema, Thelotrema and Topeliopsis. Revisions of British and Irish Lichens 36: 1-23.
Text by Neil A. Sanderson based on Aptroot et al (2019)