Arrhenia acerosa
Arrhenia acerosa, commonly known as the moss oysterling, is a small agaric fungus in the family Hygrophoraceae. It produces fan‑shaped to kidney‑shaped fruit bodies directly on woody debris or bare soil along forest paths. Originally described in 1821, it occurs in Northern Europe and has been recorded from Sweden, Estonia and Norway.
Taxonomy
The species was first named Agaricus acerosus by Elias Magnus Fries in 1821 and has since been shuffled through several genera—including Pleurotus, Dendrosarcus, Pleurotellus, Omphalina and Panellus—before Robert Kühner placed it in Arrhenia in 1980. Because Fries did not designate a holotype, Petersen and Knudsen selected a neotype from a 1943 collection made near Bokhultet in Småland, Sweden, which matches Fries's original description in both habitat and morphology. Genetic data from the internal transcribed spacer region group this Swedish neotype with specimens from Estonia and Norway, supporting a Northern European distribution and confirming its identity across collections.Description
Fruit bodies of Arrhenia acerosa are small and sessile and up to 15 mm across. The thin‑fleshed cap is smooth and medium grey‑brown, with the margin incurved when young then straightening and becoming lobed; a white, fuzzy tomentum of hyphae marks the point of attachment to the substrate. The gills develop from the cap margin, are of normal spacing and shape, and are slightly darker than the cap surface.Under the microscope, spores measure on average 7.4 by 3.8 μm 6.8–8.7 by 3.4–3.9, are narrowly elliptical to tear‑shaped, smooth, transparent and bear a small apiculus. The spore‑bearing cells are club‑shaped, four‑spored and measure 21–25 by 6–7 μm. No cystidia occur. The cap cuticle is a cutis of radially orientated, cylindrical hyphae 3–10 μm wide, thin‑walled and yellow‑brown due to sparse incrustations of pigment. Clamp connections are present at the base of basidia and throughout all tissues, and pigment occurs both within cell membranes and as external incrustations.