Pyrenocarpon
Pyrenocarpon is a fungal genus in the family Lichinaceae. It is monospecific, containing the single species Pyrenocarpon thelostomum, a lichen. This rare lichen is found only in shaded stream beds in parts of Britain, including Exmoor, the Pennines, and the Scottish Highlands. It was first described as a distinct genus in 1855 by the Italian lichenologist Vittore Trevisan based on its unique waxy fruiting structures. The species grows as a thin, reddish-brown crust on hard rocks that are kept constantly wet by flowing water.
Taxonomy
The genus was circumscribed by the Italian lichenologist Vittore Benedetto Antonio Trevisan de Saint-Léon in 1855. Trevisan distinguished Pyrenocarpon from related genera such as Verrucaria, Biatora, and Lecidea based on the waxy-membranaceous and coloured nature of the perithecia, which possess simple ostioles, in contrast to the horny- and black perithecia found in Verrucaria. The genus is characterised by having eight-spored asci with club-shaped paraphyses, ovoid simple hyaline ascospores, and a uniform crustose thallus. Trevisan noted that the genus belongs to the Verrucariarum tribe and is related to the subgenus Sphaeromphalaearum that he had previously proposed.Description
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 and ranges in colour from chestnut to reddish-brown. The lichen lacks an outer cortex, so the —minute, spherical green algae 5–7 micrometres 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.Reproduction occurs in plentiful, distinctive fruit bodies that at first resemble submerged beads. These apothecia are : initially hemispherical and completely covered by thallus tissue, they eventually open to reveal a small, paler, reddish-brown only about 0.3 mm wide. A conspicuously thick, pale rim of fungal tissue 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 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.