Lithothelium fluorescens

The lichen forms an ochraceous (yellowish-brown) corticate thallus (the main lichen body). Its perithecia (flask-shaped fruiting bodies) are solitary and mostly embedded in the thallus, but become visible as small, hemispherical warts. Individual perithecia are roughly 0.3–0.8 mm across and 0.2–0.4 mm high, and each has a tiny black ostiole (opening) that may be edged by a small grey to brown ring that sometimes protrudes from the surface (an annulus).^[2]

Microscopically, the tissue between the asci (the hamathecium) is clear rather than oil-filled and does not react with iodine (IKI–). The asci have a rounded ocular chamber and measure about 90–100 × 10–12 μm. Each ascus contains eight red-brown ascospores with three internal septa that are distoseptate (the septa form separate internal chambers), measuring about 13–17 × 7–8.5 μm. The spore compartments (lumina) are rounded in mature spores but can be angular in immature spores. The end compartments are smaller than the central ones, and the inner spore wall (endospore) is about 1–3 μm thick. Conidiomata (asexual structures) have not been observed. In lichen spot tests, the thallus is UV+ (yellow) and does not react with potassium hydroxide (K–), consistent with the presence of lichexanthone.^[2]

In the type locality, the species was found on bark on the lower trunk in a montane rainforest zone at about 1,800 m (5,900 ft), in an abandoned pasture with remnants of forest. The original description treated it as the first Lithothelium known to contain lichexanthone, and placed it in that genus rather than Pyrenula because its ascospores are red-brown and the lumina become rounded at maturity. It can be confused with Pyrenula dermatodes, which differs mainly in having ascospore lumina that remain angular when mature. Additional material was reported from the same province at about 1,600 m (5,200 ft), where it was collected in disturbed primary forest, again on bark on the lower trunk.^[2]