Gregarious amongst Sphagnum and general vegetation in the mire, on twigs and woody debris of Pinus and Quercus. In Spain and Portugal on twigs and fallen leaves of Cistus salviifolius and Eucalyptus globulus, but also on leaves of Quercus orocantabrica with presence of Pinus pinaster and Sphagnum sp. So far only recorded in Uk, Spain and Portugal. Autumn.
Pileus 2-10 mm across, hemispherical, broadly campanulate to plano-convex, initially barely mamelonate and usually depressed in the centre, sulcate, translucent-striate, initially pruinose then glabrous, pale beige at centre with pinkish tinge, then gradually dull white towards margin, with the centre and striation dark pinkish brown. Lamellae 10-18 reaching the stipe, adnate to broadly adnate, white, edge concolorous. Stipe (8-)20-80 x 1-2 mm, fragile, cylindrical, pruinose at the apex, more glabrous below, white to beige at the apex then gradually darker to grey-brown towards the base (purplish brown below according to the type protologue), attached to the substrate with a dense patch of radiating, long, coarse whitish fibrils. Smell and taste indistinct.
Basidia 14-18 x 6-7.5 µm, narrowly clavate, 4-spored with sterigmata 3-4 µm long. Spores (6.3-)8-9(-9.8) x 3-4.6 µm, Q (1.7-)1.9-2.3(-2.6) according to Villarreal et al. 2024) Q av 2.3-2.7 (Cullington 2023), pip-shaped, amyloid. Cheilocystidia(23-)50-60(-70) x (9-)10(-11) µm, forming a sterile band, smooth, markedly lageniform, the wider lower section (often with pedical below it) narrowing abruptly to an elongated to lanceolate extension, occasionally forking, 2-3 µm wide at the apex, more rarely pyriform to spherical cystidia, 17.5 x 12.5 µm, smooth or covered with some short excrescences in the upper part 1.5-3 x 0.5-1 µm. Pleurocystidia not seen. Lamellar trama dextrinoid. Hyphae of the pileipellis 2.5-5 µm wide, densely covered with simple or branched excrescences 1.5-7 x 0.5-2 µm, with numerous erect lageniform to cylindrical, occasionally septate dermatocystidia up to 95 µm long, emerging from the subpellis. Hyphae of the stipitipellis (1.8-)3-5 µm wide, covered with simple to branched excrescences (1.3-)3.5-13 µm long. Caulocystidia 40-75 x 3.3-8 µm, mostly present near the stipe apex, similar to cheilocystidia. Clamps present in all tissues.
The description is based on Maas Geesteranus & Læssøe 1992, Cullington 2023 and Villarreal 2024. I have not seen this species myself.
When Maas Geesteranus & Læssøe (1992) described the new species from Surrey in England, it was based on very scanty material. Two of the specimens were said to have been infected by a parasitic fungus, whose hyphal ends protruded everywhere, simulating cheilocystidia and caulocystidia. Today we know that this was a misinterpretation. They described the cheilocystidia as short, clavate to obpyriform, covered with excrescences, typical for most species in Sect. Polyadelphia. Almost 30 years later, Penny Cullington rediscovered the species at Burnham Beeches in Buckinghamshire (Cullington 2023), and based on several collections she was able to provide a more detailed description in addition to three sequences. One important observation was that the "parasitic fungus cells" in fact are genuine cheilocystidia, and that the short "hedgehog" cystidia that were described by Maas Geesteranus & Læssøe (1992) more rarely are present too.
Villarreal et al. (2024) reported on several collections from Spain and Portugal and provided more information about the variation of this rare species. They also made a phylogenetic analysis, indicating that M. dasypus should be placed in sect. Fragilipedes and that it is not a member of sect. Polyadelphia, which was claimed by Maas Geesteranus & Læssøe (1992). Sect. Fragilipedes is the largest section in the genus Mycena and probably a polyphyletic section. Mycena dasypus seems to occupy a unique place within the section because of the peculiar cystidia. The combination of smooth lageniform-lanceolate cheilocystidia and short, pyriform to globose cystidia with short excrescences is not found in any other species.
Due to the sparse description of the holotype and uncertainty about the identity of the coarse "cystidia" that had been interpreted as result of an infection, Aronsen and Læssøe (2016) only mentioned M. dasypus in the section for "Taxa not treated in the main text". They mentioned three more British collectins and recommended that these be revised and sequenced. This has now been carried out.
I am very grateful to Penny Cullington for loan of photos and her kind help and cooperation.
Additional photos:
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