Cryptothecia darwiniana


Cryptothecia darwiniana is a species of crustose lichen in the family Arthoniaceae. Found on the Galápagos Islands, it was described as a new species in 2013.

Taxonomy

Cryptothecia darwiniana belongs to the genus Cryptothecia, which is characterized by species that typically form fertile crusts with asci loosely dispersed or closely aggregated within areas rather than in distinct ascomata. The species is closely related to Cryptothecia assimilis and C. lichexanthonica, sharing similar morphological traits and containing the chemical compound lichexanthone, but is distinguished by having confluentic acid in addition to lichexanthone and larger ascospores.
The species was previously misidentified in historical collections. Weber reported it as Chiodecton effusum for the Galápagos, a species later cited as Syncesia effusa. Through detailed examination of specimens, researchers determined it was a previously undescribed species.

Description

Cryptothecia darwiniana forms a crust-like thallus that grows on bark and dead wood. The thallus is clearly defined by a distinct fuzzy border of white to brownish, radiating hyphae. The surface is smooth, shiny, and yellowish white, often becoming more intensely lemon yellow with age. It occasionally has sparse granular soredia but generally lacks the commonly found in the related genus Herpothallon. Its medulla is white and densely filled with minute colourless crystals that dissolve in potassium hydroxide.
A distinctive feature of this species is its reproductive structures. The areas develop within roundish, broad, lemon yellow, pruinose, irregular with a roughened surface. These pustules break open at maturity. The spore-producing asci are thick-walled with a thickened upper portion containing a small.
The asci occur isolated within thalline pustules and are entangled by strongly branched and interconnected, KI+ violet-blue . As the asci age, they become encapsulated by a brownish pigmentation, eventually becoming and forming . These locules are irregularly arranged or develop into striae, and when eroded at their surface, they may resemble ramified, carbonized .
The are colourless, turning pale olive-green in potassium hydroxide, ovoid, and with curved septa. They measure 60–85 by 32–40 μm. Each ascus typically contains eight spores, but frequently 1–2 spores are aborted, resulting in 6–7-spored asci.
In chemical spot tests, the thallus is P−, K−, C−, and KC−. It fluoresces bright orange under ultraviolet light, and its medulla reacts deep blue with Lugol's iodine solution. Chemical analysis reveals the presence of confluentic acid and lichexanthone.

Habitat and distribution

Cryptothecia darwiniana is known only from the Galápagos Islands and may be endemic to the archipelago. It is common throughout the dry zone and lower transition zone of the islands. The species has been documented on several islands within the Galápagos, including Isabela, Santiago, Santa Cruz, Pinta, Española, Floreana, and San Cristóbal.
It grows exclusively on the bark of native and endemic tree species, most commonly Bursera graveolens and Erythrina velutina, in sunny, wind- and rain-exposed habitats. Unlike many members of the genus Cryptothecia that prefer humid, sheltered environments, C. darwiniana is adapted to the relatively dry conditions of the Galapagos lowlands. Its dense thallus and high concentrations of xanthones may represent adaptations to high solar radiation in these exposed habitats.