Typically on decaying knopper galls on Quercus robur acorn cups or on fallen catkins of Alnus glutinosa, but also found on fallen debris from Castanea sativa and Corylus avellana (hazel nut). Late summer to autumn. Known from Belgium, France, Germany, the Netherlands, Spain, and UK. Not recorded in Norway.

Pileus 4-10 mm across, at first subglobose, ellipsoid or ovoid, then expanding to campanulate or convex, finally flattened, mostly conspicuously depressed at the centre, white-furfuraceous to hairy-floccose (coprinoid), with a separable, gelatinous pellicle, often with deflexed, dentate-fimbriate  margin, white to greyish white with a yellowish to  grey-brown centre. Lamellae 14-30 reaching the stipe, ascending, narrowly adnate, sometimes forming a pseudocollarium, ventricose, narrow, white, becoming cream with age, margin concolorous.  Flesh very thin, odour nitrous but sometimes very weak or apparently absent. Stipe 10-27 x 0.3-0.75 mm, fragile, terete, straight to somewhat curved, equal with somewhat bulbous base, entirely pruinose to puberulous, more hairy at the base, generally rising from a very small white-pubescent basal disc.

Basidia  12-20 x 5-9 µm, clavate, 4-spored (a few 2-spored seen). Spores 6-8.5 x (3-) 3.5-4.5 (-5) µm, Q 1.5 – 2.3, Qav ≈ 1.8, ellipsoid, smooth, hyaline, amyloid. Cheilo- and pleurocystidia absent. Lamellar trama dextrinoid. Hyphae of the pileipellis 2-6 µm wide, smooth to sparsely diverticulate, embedded in a gelatinous matter, cylindrical to slightly inflated, excrescences 2-13 x 1 µm, cylindrical, straight to curved; with acanthocyst terminal cells 20-63 x 8-28 µm, clavate, cylindrical, subglobose or fusiform, densely covered with warts up to 2.5 x 1 µm; the fusiform type mostly found at the margin of the pileus, and mostly with a smooth rostrum, sometimes even with a long, single or forked, needle-like extension up to 80 x 1.5 µm. Stipitipellis smooth, caulocystidia 17-95 x 10-22 µm, thin-walled, conical to broadly conical, with acute, sometimes rostrate apex, sometimes with a needle-like extension 30-100 x 1-1.5 µm, less frequently broadly clavate, ellipsoid or ovoid. Clamp connections present.

Microphotos of the hyphae of the pileipellis 1

Microphotos of the hyphae of the pileipellis 2

Microphoto of the caulocystidia 1

Microphoto of a caulocystidium 2

Mycena rhenana was described as a new species by Maas Geesteranus & Winterhoff (1985). The essential characters of the new species were the tiny size; presence of a small basal disc; nitrous smell; conspicuous acanthocyst terminal cells of the pileipellis; large, conical, smooth caulocystidia; and - most of all - absence of cheilocystidia. Moreover it was said to grow exclusively on fallen debris of Alnus glutinosa.

A few years later van den Berg et al. (2000) described another new species, M. cecidiophila, growing on knopper galls on the cups of Quercus robur. This species had many features in common with M. rhenana, for example absence of cheilocystidia and presence of acanthocyst terminal cells and large conical caulocystidia, but the authors never considered the two taxa to be conspecific. The reason was probably that they did not observe the gelatinous pellicle of the pileus, and the lamellar trama was said to be non-dextroid. They did not observe a basal disc of the stipe, and besides, the habitat was different.

Noten & Vannieuwerburgh (2009), however, reported collections of both 'M. rhenana' and 'M. cecidiophila' and showed that the two species were identical. They observed both a basal disc and a nitrous smell also in the taxon growing on knopper galls. The microscopic details were the same in both taxa. The hyphae of the pileipellis are embedded in a gelatinous matter also in the taxon growing on knopper galls, and the trama is in fact dextrinoid. Moreover, they reported the same species from fallen debris of other trees: Castanea sativa and Corylus avellana.

I have examined 10 collections from different habitats, kindly put to my disposal by L. Vannieuwerburgh, and can not find any differences between them. M. cecidiophila should be regarded as a later synonym of M. rhenana.

Maas Geesteranus & Winterhoff (1985: 296) stated that "In allen Merkmalen ist M. rhenana eindeutig eine Art der Sektion Basipedes, ist jedoch insofern abweichend, indem ihr ein wichtiges Merkmal fehlt. Trotz gezielter Suche, (....) konnten keine Cheilozystiden gefunden werden". van den Berg et al. (2000: 484) proposed the new section Cecidiophilae to accommodate their new species. This section should be rejected.

M. rhenana has features in common with sect. Sacchariferae Kühner ex Singer too, such as the pileipellis hyphae that are terminated by acanthocysts. Abberant features are the gelatinous pellicle of the pileus and the absence of cheilocystidia. Desjardin et al. (2007a), however, suggested that it is better accommodated in sect. Exornatae Maas Geest.