Multiclavula petricola

Fruit bodies of M. petricola are scattered over the rock surface, simple and unbranched (rarely forked), clavarioid to fusiform, and 2.9–6.1 mm tall by 0.3–0.6 mm thick. They are pure white, fleshy when fresh, and become slightly translucent towards the base; the apex tapers to a fine awl-shape. The stipe, up to 2.7 mm long, is composed of parallel, heavily agglutinated, thin-walled hyphae (1.5–3.5 μm wide) with frequent clamp connections. A hymenium 40–60 μm thick lines the outer surface of the fertile portion. Basidia are 26–49 × 5–8 μm, clavate with a tapering base, and bear (2)4–6 sterigmata that are 2.9–5.3 μm long. Basidiospores are hyaline, smooth, thin-walled, elongate to slightly cylindrical, 6.1–11.3 × 3.1–4.6 μm. The lichenised thallus forms minute, gelatinous globules (12–39 μm diameter) that surround the bases of the fruit bodies. Each globule comprises a handful of more or less spherical to broadly ellipsoid green-algal cells (6–10 μm diam.) containing a conspicuous pyrenoid; these are enveloped by a lace-like sheath of fungal hyphae 1.2–4.6 μm thick. In culture, basidiospores germinate readily on water agar, and multispore isolates grow slowly on potato dextrose agar, forming whitish, felted colonies 5–7 mm across after two months at 15 °C.

The species is known only from its type locality, a shaded, north-facing talus slope on the flank of Mount Akagi, near Katashina village. The site lies within a cool, moist coniferous forest dominated by Abies species, where constant seepage keeps the volcanic andesite perpetually damp. Fruit bodies appear in early autumn, erupting directly from moss-free rock surfaces, while no thalli have been observed on adjacent soil or vegetation. Ecologically, M. petricola occupies a niche quite unlike that of most other Multiclavula species, which are typically associated with soil, decaying wood, or bryophytes. The reliance on permanently wet rock implies sensitivity to desiccation and suggests the lichen could be vulnerable to microhabitat disturbance or shifts in moisture regime. Future surveys in comparable subalpine habitats of central Honshū may clarify whether the distribution is genuinely narrow or merely under-recorded.

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