Arthonia ilicinella
Arthonia ilicinella is a species of crustose lichen belonging to the family Arthoniaceae. First described in 1867, it is characterised by its immersed thallus that appears white-grey to cream-white, and its small black reproductive structures. The species has a widespread global distribution, occurring across Europe, South America, and parts of Asia. In the British Isles, it serves as an ecological indicator species used in environmental assessments, and has been noted in Scotland as an old-growth forest indicator that has significantly declined over recent decades.
Taxonomy
Arthonia ilicinella was described as a new species in 1867 by the Finnish lichenologist William Nylander. In his original description, Nylander noted its similarity to A. ilicina, but distinguished it as typically smaller with smaller spores. He also observed that when treated with iodine, the hymenial gelatin turned wine-red without any visible blue colouration. Nylander documented its habitat on holly bark in Killarney, Ireland, where it was collected by Isaac Carroll. He further characterised the species as having spores with 3–6 septa, usually well-developed with the larger ones being 5-septate. He compared it to A. cinnabarina, noting differences, and mentioned that in A. ilicina, the hymenial gelatin turns persistently blue with iodine. Another distinguishing feature he recorded was that the epithecium turns a dirty blue colour when treated with potassium hydroxide solution.Description
Arthonia ilicinella is a crustose lichen with a thallus is immersed within its substrate rather than sitting prominently on the surface. The thallus appears either as a diffuse growth or with defined boundaries marked by a brown line, and is coloured white-grey to cream-white. The reproductive structures measure between 0.1–0.6 mm in diameter, though some may be elongated, reaching about 0.8 mm long by 0.15–0.3 mm wide. These structures are irregularly rounded, relatively flat, and black in colour without any powdery coating. The apothecia are 80–95 micrometres tall.Internally, the lichen has a layered structure: the is red-brown and turns pale green when exposed to potassium hydroxide solution ; the hymenium measures 40–60 μm in height, is pale red-brown, and also turns pale green with K; the hypothecium is 10–25 μm tall and matches the colour of the hymenium. The are about 1 μm in diameter, with brown-walled and slightly thicker structures in the, occasionally showing caps at their tips.
The typically measure 18–23 by 7–9 μm and have a cylindrical to egg-shaped form with 3 or 4 cross-walls or septa, with enlarged cells at the tips. Older spores turn brown and have a surface that is smooth or very slightly textured when viewed at high magnification.
Small asexual reproductive structures produced measure 7–9 by roughly 1 μm. Chemical analysis did not detect any secondary metabolites in this species.