Pyrenocarpon

Pyrenocarpon thelostomum forms a very thin, crust-like thallus that lies flush against the rock, rarely exceeding 0.2 mm in thickness. Seen through a hand lens the surface looks minutely cracked into tiny, angular patches (an areolate texture) and ranges in colour from chestnut to reddish-brown. The lichen lacks an outer cortex, so the photobiont—minute, spherical green algae 5–7 micrometres (μm) across—is visible in section. Each algal cell sits within a faint, pale-brown, jelly-like sheath that helps retain moisture, giving the thallus a slightly gelatinous feel when damp.^[4]

Reproduction occurs in plentiful, distinctive fruit bodies that at first resemble submerged beads. These apothecia are perithecioid: initially hemispherical and completely covered by thallus tissue, they eventually open to reveal a small, paler, reddish-brown disc only about 0.3 mm wide. A conspicuously thick, pale rim of fungal tissue (the true exciple) encircles each disc, creating the fish-eye appearance by which the species is most readily recognised. Inside the cavity the spore layer does not react with iodine, and it is threaded by branched, net-like paraphyses whose tips remain narrow rather than swollen. The spore sacs (asci) are slender clubs with thin walls and lack the amyloid caps common in many lichens; each contains eight colourless, single-celled ascospores measuring roughly 17–20 × 9–12 μm. No asexual propagules or secondary lichen products have been detected.^[4]

Ecologically the species is restricted to hard, often calcareous rocks that are kept constantly wet by flowing water. In Britain it has been recorded only from shaded stream beds in Exmoor, the Pennines, north-west England, and parts of the Scottish Highlands, where its distinctive apothecia stand out against the dark, crustose thallus.^[4]