Anisomeridium robustum

The thallus of A. robustum is thin, pale grey to whitish, and often cracked, containing the orange alga Trentepohlia as its photosynthetic partner (photobiont). Sexual reproductive structures (ascomata) have not been seen. Instead, the species is recognised by its conspicuous asexual fruiting bodies (pycnidia). These appear as black, spherical (globose) to conical structures, 0.2–0.6 mm in diameter and up to 0.5 mm high, usually with a narrow neck that may extend to 1.5 mm. Conidia (asexual spores) are colourless, simple, pear-shaped to ellipsoid with a truncate base, measuring 4.5–6 × 2.5–3 micrometres (μm). They are sometimes extruded in masses, forming visible whitish strings (cirri) up to 20 μm wide. No microconidia have been detected. Standard chemical tests have shown no secondary metabolites (lichen products).^[2]

Anisomeridium robustum grows on the bark of mature broad-leaved trees, particularly sycamore (Acer pseudoplatanus), ash (Fraxinus), and oak (Quercus), but it can also occur on hazel (Corylus), willow (Salix), or over mosses on bark and rocks. It favours old woodland and parkland habitats and sometimes establishes on bryophyte mats covering tree trunks or boulders. Its European distribution is concentrated in western Britain, from Cornwall northwards to Wester Ross, with additional records from Ireland.^[2] In 2018, the known range of the lichen was extended considerably when it was recorded from Jeju Island in South Korea.^[3] A. robustum has also been found on Santa Maria Island in the Azores archipelago, growing on Metrosideros.^[4]