Bactrospora arthonioides

The thallus is a thin, whitish crust largely sunk within the outer bark (endophloeodal), forming an effuse film without obvious cortex or distinct margins. No lichen substances were detected by thin-layer chromatography.

The fruiting bodies (apothecia) are black, scattered and round to slightly irregular, about 0.2–0.4 mm in diameter. They begin immersed but soon break through the surface and sit flat on the bark; they lack a frosty bloom (epruinose), have no conspicuous margin (immarginate), and the discs are flat to slightly convex. The excipulum is very thin, about 15–25 micrometres (μm) laterally and 5–12 μm at the base, and open beneath the hymenium. The hymenium is about 75–80(–100) μm tall, and the subhymenium is pale brown (70–100 μm). Paraphysoid tips branch and interconnect to form a dark, reticulate cap (pseudoepithecium). Asci are slender, measuring 65–80 × 11–14 μm. Ascospores are of the patellarioides-type: long, narrow, and multi-septate, here 40–60 × 3–4 μm with 10–15 septa, and characteristically tapered at one end; in practice, they look like very fine, many-celled threads that may split into segments. Pycnidia were not observed.^[2]

The species was originally only known from the type locality in Tasmania.^[2] While initially described from a single locality, the species is now known to be locally common throughout the extensive rainforest areas of the western and north-western regions. It typically inhabits the relatively dry, cracked bark of mature trees, with the peeling bark of aged Nothofagus cunninghamii trunks being a characteristic substrate. In these locations, B. arthonioides frequently occurs alongside Lecanactis abietina, L. mollis, and various Micarea species. The species can be distinguished from the superficially similar coastal woodland species B. paludicola by having protruding apothecia, a thinner proper exciple, somewhat shorter ascospores, and by lacking pycnidia.^[3][4] In 2017, the lichen was reported growing on a trunk of Phyllocladus trichomanoides on New Zealand's North Island.^[5] The lichen has also been documented as part of an extensive corticolous lichen community growing on Pōhutukawa (the endemic New Zealand Christmas tree, Metrosideros excelsa).^[6]