Lithocalla

Lithocalla species form crustose (crust-like) lichens with a leprose texture, meaning they have a powdery or granular appearance without a clearly defined structure. The thallus, which is the main body of the lichen, develops from small, separate granules that eventually merge to form a continuous crust. In mature specimens, this crust can reach up to 2 mm in thickness and may develop cracks. The granules making up the thallus are very fine, measuring 60–100 μm in diameter. They lack a cortex (a protective outer layer) and do not have projecting hyphae (fungal filaments). The lower parts of the thallus consist of decolorised granules, while a medulla (inner layer) and hypothallus (underlying structure) are absent.^[3]

In some shaded environments, a prothallus (a thin, spreading structure that precedes the development of the main thallus) may be visible, though it is usually sparse or absent. The thallus colour is typically pale yellow-green, although it can appear blue-grey in deeply shaded areas. This colouration is due to the presence of usnic acid, a characteristic lichen substance found in Lithocalla species.^[3]

Lithocalla lichens do not produce visible reproductive structures. Both ascomata (fungal fruiting bodies) and conidiomata (structures producing asexual spores) are unknown in this genus, indicating that these lichens likely reproduce primarily through vegetative means, such as thallus fragmentation. The photobiont, or algal partner in the lichen symbiosis, is a green alga, though the specific genus has not been identified.^[3]

Lithocalla comprises two known species, each with distinct geographical distributions and slightly different habitat preferences. Lithocalla ecorticata is predominantly found in Great Britain, particularly in the north and west. It has also been confirmed in southwestern Norway. This species favours siliceous (silicon-rich) rocks, typically inhabiting rain-sheltered areas. It is particularly adept at colonising deeply sheltered cavities, often forming pure stands in these locations. L. ecorticata can be found from sea level up to at least 480 m (1,570 ft) altitude in North Wales, with confirmed Norwegian specimens occurring as high as 640 m (2,100 ft). It is frequently observed on drystone walls in woodland settings, where it characteristically highlights recesses with its pale yellow coloration.^[3]

Lithocalla malouina, in contrast, is endemic to the Falkland Islands. It occupies a similar ecological niche to its British counterpart, growing on shaded and rain-sheltered siliceous rocks and stones. However, L. malouina shows a preference for south-facing overhangs, where it can often be found on rocks and stones beneath these protective structures.^[3]

While reports of Lithocalla (particularly under its former classifications) have emerged from various parts of the world, including other European countries, Asia, North America, and Antarctica, these records require confirmation. The true global distribution of the genus remains uncertain, and further research is needed to verify its presence in these regions.^[3]