Agonimia octospora

Agonimia octospora is a minute lichen species that forms small, scattered squamules (scale-like structures) on the bark of aged broadleaf trees, particularly oak trees. This species has several distinctive characteristics. The squamules are typically 0.05–0.1 mm thick and 0.05–0.25 mm wide, appearing pale grey-green when dry and brightening to a glaucous (bluish-green) colour when wet. These squamules are usually closely attached to the substrate, sometimes simple in form but occasionally irregularly divided or finger-like, with rounded or elongated shapes. Their margins may be entire or notched, and their upper surfaces are convex with a matte or slightly shiny appearance.^[2]

The reproductive structures (perithecia) are relatively rare, scattered among the squamules, and measure 0.5–0.8 mm in diameter. They are partially embedded in the substrate, appearing spherical or somewhat pear-shaped. These structures are black with a matt or slightly shiny surface that may be smooth or slightly roughened. The opening (ostiole) appears pink-grey in immature specimens but becomes black as the perithecia mature, and is often positioned off-centre.^[2]

Under microscopic examination, the squamules are composed mostly of pseudoparenchymatous tissue (cells arranged like a simple tissue) with only a few fungal filaments penetrating the substrate. The upper surface has a single cell-thick cortical layer with distinctive conical, peg-like projections measuring about 1–2 by 1–1.5 μm.^[2]

The wall of the perithecium comprise three distinct layers: an outer pigmented zone, an inner non-pigmented layer, and an innermost layer of inwardly directed, unbranched fungal strands. Each ascus (spore-producing structure) contains 8 colourless, ellipsoid spores that are densely muriform (divided by both transverse and longitudinal walls), typically measuring 60–75 by 20–26 μm.^[2]

The lichen is found in Europe and South America.^[3] In central Spain, A. octospora consistently appeared in minimally managed areas with dense shrub coverage and steeper terrain.^[4]