Immersaria fuliginosa

The lichen body (thallus) forms a crust of scattered to locally merging patches (areoles, typically 0.1–0.4 mm across) that are pale to red-brown and flat to shallowly concave. Each patch is edged by a dark zone derived from the black border layer (prothallus), which breaks apart into tiny dispersal granules (thalloconidia). Towards the thallus margin, the areoles may become larger and merge into a more continuous crust. In cross-section, the outer skin (cortex) is weakly developed (about 10–20 µm thick) beneath a conspicuous clear dead layer (epinecral layer, typically about 70–120 (less often up to 150) µm).^[2]

The algal partner (photobiont) is a green, rounded (chlorococcoid) alga, arranged as a distinct horizontal layer near the base of the areoles; the algal cells are thick-walled and about 7–12 µm in diameter. The inner tissue (medulla) stains violet with iodine (I+ violet).^[2]

The fruiting bodies (apothecia) are infrequent and of the lecideine type (with a fungal rim rather than a thallus-derived rim), remaining sunken in the thallus. The disc is dark brown to black and concave, and may appear browner when wet. The asci are of the Porpidia-type, and the ascospores are simple (undivided), colourless (hyaline), thinly surrounded by a clear halo (halonate), and measure about 16–18 × 6–8 µm. Secondary metabolites reported from the thallus include 2-O-methylperlatolic acid and confluentic acid (detected by thin-layer chromatography).^[2]

Immersaria fuliginosa is known only from two collections made in mountainous parts of West Falkland, where it grows on exposed rock outcrops at roughly 610–700 m elevation. Both gatherings came from open, high-ground habitats (including feldmark at the second locality), demonstrating a preference for cool, windswept sites above the coastal lowlands.^[2]

At the type locality it was recorded with several other saxicolous lichens, including Rhizocarpon geographicum agg., a species of Lecidea, Pertusaria spegazzinii, and Poeltidea perusta. Fryday remarked that it can resemble P. perusta in overall appearance and may occur alongside it, but differs in the thalloconidial prothallus and in having smaller, hyaline ascospores.^[2]