Lepra corallina is a species of saxicolous (rock-dwelling) crustose lichen in the family Pertusariaceae. First described by Carl Linnaeus in 1767 as Lichen corallinus, it was long known as Pertusaria corallina until molecular studies showed that Pertusaria in its traditional sense was not a natural group; the species was transferred to Lepra in 2016. The lichen forms a thick whitish to grey crust on acidic, lime-free rocks and is characterized by abundant coral-like isidia that often cover much of the surface. Fertile structures are rare. When present, they appear as small wart-like swellings containing a few apothecia with very large, two-spored asci. The lichen's chemistry is dominated by thamnolic acid, a secondary metabolite that contributes to its characteristic reactions to chemical tests. Lepra corallina is widely distributed across temperate and boreal regions of the Northern Hemisphere, with records from western Europe, Scandinavia, Russia, Greenland, and eastern Asia. It grows mainly on steep or vertical faces of hard siliceous rock, from coastal outcrops to alpine ridges. The lichen actively weathers its rock substrate by excreting oxalic acid, which dissolves minerals and produces calcium oxalate crystals within the thallus. On manganese-rich ores it forms manganese oxalate, the first documented natural occurrence of that mineral. Field studies in Norway and Wales have shown it to be a habitat generalist and a relatively strong competitor, capable of overgrowing neighbouring crustose lichens, though its dominance can be transient. Established thalli also serve as hosts for several lichenicolous fungi.
Taxonomy The species was scientifically described by Carl Linnaeus in 1767 as Lichen corallinus. In the original description, Linnaeus characterized it as a white, densely tufted leprose lichen from rocks in Sweden and associated it with material received from Johan Zoëga. The original species epithet was corallinus; the now-familiar form corallina arose later when the species was transferred to genera treated as feminine. Because no original Linnaean specimens of this saxicolous crust survive in the Linnaean Herbarium, Jørgensen, James, and Jarvis designated the Ehrhart-annotated specimen LINN 1273.17 as neotype in 1994, thereby fixing the application of the name to the species long known as Pertusaria corallina. During the nineteenth century the species passed through several genera, including Isidium and Variolaria, before Ferdinand Arnold published the combination Pertusaria corallina in 1861. That name remained the standard usage for more than a century. Its present placement reflects the break-up of the traditional broad concept of Pertusaria. Molecular studies showed that Pertusaria in the broad sense was polyphyletic, and that the disc-bearing "Variolaria group", which includes this species, was more closely related to Ochrolechia and its allies than to Pertusaria in the strict sense. Beyond the molecular evidence, workers treating the "Variolaria group" as Lepra also pointed to a distinctive combination of characters. These characters include unusual disc-like ascomata that may become heavily pruinose, one- or two-spored asci, very large thin-walled ascospores, multilocular pycnidia with short rod-like conidia, and a chemistry lacking chlorinated xanthones and orcinol depsides. In 2015 the species was briefly recombined as Marfloraea corallina, but this treatment was not retained. Josef Hafellner transferred it to Lepra in 2016, using the older available generic name for the group. A subsequent proposal to conserve the familiar name Variolaria over Lepra was rejected by the General Committee in 2024, leaving Lepra in place. Higher-level classifications of the genus have not been entirely consistent: some modern database-driven sources continue to place Lepra in Pertusariaceae, whereas specialist lichen treatments have placed it in Ochrolechiaceae, and Wei and coauthors left the genus as incertae sedis within Pertusariales pending further resolution.
Description
Lepra corallina forms a thick, crustose thallus that is pure white to pale or dark grey. The thallus is usually continuous but often cracked into small, irregular plates (rimose-areolate), and is bordered by a conspicuous white prothallus. It is characteristically covered by abundant isidia, 0.1–0.3 mm in diameter, which begin as simple cylindrical outgrowths and often become more branched and coral-like. These isidia are mostly erect or more or less lying flat on the surface, and may form a dense layer that obscures much of the underlying thallus. Fresh thalli of Lepra corallina are greyish white, but older parts often become more or less reddish with age, apparently reflecting variation in the concentration of thamnolic acid. Fertile structures are rare. When present, they form wart-like swellings 1.5–2 mm in diameter that are strongly constricted at the base and often have an uneven or warted upper surface. Each of these warts contains a small group of apothecia, typically 2–4 but occasionally up to 11. The apothecial discs are relatively small (0.3–0.5 mm across), flat to slightly convex, pale to dark brown, and often faintly dusted with a white pruina. The asci in these apothecia usually contain two large ascospores, measuring 80–140 μm (occasionally up to 230 μm) × 40–80 μm, with a wall 2–3 μm thick that lacks any obvious zoning or striation. In standard chemical spot tests (used to identify lichen substances) the thallus is C–, reacts K+ (yellow to yellow-orange), KC+ (yellow), and Pd+ (yellow-red), and is UV–. The main lichen substance detected is thamnolic acid.
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