# Aspilidea

Aspilidea is a fungal genus of uncertain familial placement in the subclass Ostropomycetidae. It contains the single species Aspilidea myrinii, a saxicolous (rock-dwelling) crustose lichen with a circumpolar distribution.

## Taxonomy

The species Aspilidea myrinii was first formally described by Elias Magnus Fries, who classified it as member of the genus Parmelia. Josef Hafellner transferred it to the newly circumscribed genus Aspilidea in 2001. The genus name is a combination of Aspicilia and Lecidea, reflecting the species' mix of characteristics that are reminiscent of both genera.
Aspilidea myrinii can be distinguished from species in the genus Aspicilia by several characteristics. When tested with iodine, the inner layer (medulla) of A. myrinii's thallus shows a pale violet colour, which is not seen in Aspicilia. The reproductive layer (hymenium) of A. myrinii turns blue when treated with iodine, while in Aspicilia, this reaction is less intense. There are also subtle differences in the structure of the spore-producing sacs (asci). A. myrinii has distinctive structures for asexual reproduction (pycnidia) with specific types of spore-producing cells (conidiophores). Additionally, A. myrinii hosts a different set of fungi that grow on lichens (lichenicolous fungi) compared to those found on Aspicilia species.

## Description

Aspilidea myrinii has a crustose thallus, which forms a crust-like layer that adheres closely to the surface it grows on (episubstratic). The thallus is composed of small, irregularly shaped sections (areolate) and is bordered by a dark prothallus, which is a preliminary growth that outlines the main body of the lichen. The thallus has a cortex, or outer layer, made up of elongated fungal cells arranged in a tightly packed manner (paraplectenchymatous). Sometimes, this layer is covered by a thin epinecral layer, which consists of dead or dying cells. Beneath the cortex is the medulla, a white layer that turns pale blue when exposed to iodine.
The apothecia (fruiting bodies) are cryptolecanorine-aspicilioid in form, meaning they have a black disc at the centre with a poorly developed thalline rim—essentially a faint, indistinct border derived from the thallus. The proper exciple, which is the outer layer of the apothecium, is also poorly developed. It appears brownish-black on the outside but is colourless on the inside. The uppermost layer of the apothecium (epithecium) ranges in colour from olive-green to green-black.
The hymenium, the spore-producing layer within the apothecium, is colourless and reacts to iodine by turning blue (euamyloid, I+ blue). The hymenium contains paraphyses, which are filament-like structures that can be either simple or branched and tend to intertwine in the upper part. The tips of these paraphyses are either not swollen or only slightly enlarged. Beneath the hymenium lies the hypothecium, which can be colourless or pale yellow.
The asci, which are the cells that produce spores, typically contain eight spores and are club-shaped (clavate). They are thickened at the apex and show a faint blue reaction in the lower outer part when treated with iodine. The ascospores themselves are one-celled, colourless (hyaline), broadly ellipsoid in shape, and do not react with iodine (I-).
Aspilidea also has pycnidia, which are tiny, flask-shaped structures involved in asexual reproduction. These are embedded in the thallus, with the upper part of the pycnidium wall being green-black and the rest colourless. The conidia, or asexual spores, are short and rod-shaped (bacilliform).
The lichen's photobiont, or photosynthetic partner, is a chlorococcoid green alga . Chemically, Aspilidea produces norstictic and connorstictic acids, compounds commonly found in some lichen species.

## Habitat and distribution

Aspilidea myrinii is a lichen species that primarily inhabits cold, mountainous regions around the world. It is commonly found in arctic and alpine areas, particularly in locations close to or above the tree line. This species prefers to grow on specific types of rocks, favouring crystalline schists and acidic, silica-rich stone surfaces.