Growing singly or in groups on fallen Corylus fruits or rarely on other substrates as twigs and fallen leaves. Very rare. I have earlier considered it a synonym of M. tenerrima (but see below).

Pileus 1.5 - 3 mm across, at first conical, becoming hemispherical-convex or parabolical, translucent-striate when old, shallowly sulcate, furfuraceous or floccose, becoming puberulous to glabrous, white. Lamellae 9-13, (13-20 according to the type description), ascending, the edge convex, narrowly adnate, not always reaching the stipe, white. Stipe 8-20 mm long, filiform, straight to flexuous, equal, the base somewhat bulbous, entirely pubescent or hairy, glabrescent, watery greyish, with age more whitish and shining, with a very small basal disc, which is almost disappearing with age.

Basidia 12-24(-32) x 8-10 µm, clavate, 4-spored, with sterigmata 3-5 µm long. Spores 8-10.5(-11) x (3.5-)4-5.3 µm, Q 1.9-2.5, Qav 2.1-2.3, pip-shaped to almost cylindrical, amyloid, smooth. Cheilocystidia 12-26 x 5.5-15 µm, numerous but not forming an entirely sterile band (?), clavate, densely covered with evenly spaced, short excrescences in the apical part, with or without a shorter or longer, conical to bulbous, smooth rostrum up to 25 µm long. Pleurocystidia absent. Hyphae of the pileipellis 3.5-15 µm wide, densely covered with narrow, short excrescences, not gelatinous; terminal cells acanthocysts, broadly clavate to globose, up to 30 µm wide, densely covered with short excrescences 0.5-1.5 x 0.5 µm. Hyphae of the cortical layer of the stipe smooth, caulocystidia 23-100(-200) µm long, smooth, thin-walled, lanceolate, acute, with a broadened base up to 14 µm wide. (In one collection a few caulocystidia were apically forked or branched with excrescences up to 30 mm  long.) Hyphae of the basal disc terminated by densely spinulose cells 20-49 x 6-15 µm, clavate, fusiform to subcylindrical, excrescences 0.5-6 x 0.5-1 µm, or mixed with smooth cells with rather coarse excrescences apically, excrescences 8-9 x 2-4 mm, some cells resembling the caulocystidia but shorter. Clamps apparently absent or very rare (a few seen at the base of the cheilocystidia).

Aronsen 99/23 with microphotos of cheilocystidia, basidium, pileipellis, and basal disc hyphae

Aronsen 13/12 with microphotos of basal disc hyphae

Aronsen 105/23 with microphotos of basal disc hyphae

Aronsen 107/23 with drawings of cheilocystidia and basal disc hyphae

Aronsen 112/23 with microphotos of spores, cheilocystidia and pileipellis

Image 1	Image 2	Image 3
Image 4		Image 5

Maas Geesteranus (1991b) and Desjardin (1995) discussed the differences between M. nucicola and M. tenerrima. The differences seemed to be marginal and were even less trustworthy when Maas Geesteranus claimed that the generally 2-spored M. tenerrima also could be 4-spored (as M. nucicola) and occasionally occur on hazel nuts too. Referring to Aronsen & Larsson (215), who had sequenced specimens identified as M. nucicola and M. tenerrima and found them identical, Aronsen & Læssøe (2016) reduced M. nucicola to a synonym of the latter.

While working with the new species Mycena amoena, Jagers et al. (2023) discovered two different phylogenetic species of what they identified as M. tenerrima and called them Mycena tenerrima I and Mycena tenerrima II, both occurring on hazel nuts. That raised the question if M. nucicola could exist as a species of its own after all.

Deceuninck (2023) had already investigated this question (pers. comm.) and published a preliminary study, where she listed the differences, more or less in accordance with Desjardin (1995). She inspired me to investigate the question myself, and I gathered a number of collections on hazelnuts in addition to collections from twigs, branches and bark of Salix sp. Also, I grew some hazelnuts in moist chambers, which also produced fruiting bodies for further investigation. Finally, I examined collections of M. tenerrima from the fungarium in Oslo.

There are, indeed, two species involved. All collections that I found on hazel nuts were microscopically different from M. tenerrima in several aspects. It's likely that this other species is identical with Hujisman's Mycena nucicola, but the protologue is somewhat confusing and the holotype is said to be in very poor condition. It was studied by both MaasGeesteranus and Desjardin, but their observations were somewhat divergent. Tentatively, I hereby use the name Mycena nucicola for the species that I have found on hazel nuts. It appears that it not always grows on hazel nuts as I have collected it on twigs and fallen leaves of Salix, and Mycena tenerrima apparently very rarely can occur on hazel nuts.

Huijsman (1958) separated M. nucicola from M. adscendens (= M. tenerrima) based on the cheilocystidia of M. nucicola lacking an apical rostrum and the caulocystidia being subglobose and densely spinulose. Desjardin (1995), who re-examined the holotype, showed that both observations were erroneous. My experience is that many cheilocystidia are clavate, obtuse and without a rostrum, but many cheilocystidia also have a shorter or longer, single rostrum. The caulocystidia are lanceolate and smooth. My observations don't quite fit with Desjardin's either. He claimed that the terminal cells of the basal disc hyphae were of two types: 1) acanthocysts similar to those on the pileipellis, and 2) elements similar to the caulocystidia. I've seen a few scattered acanthocysts on the basal disc, which I believe are leftovers from the pileipellis in the primordia. Admiddetly, the terminal cells are very variable in shape, and of more than two kinds, but mainly they are densely spinulose. Nor were the observations of Maas Geesteranus (1991b) on the holotype entirely in accord with mine from Norwegian collections. Maas geesteranus also claimed that M. tenerrima can be 4-spored, but I have not been able to confirm this.

Tentatively Mycena nucicola and M. tenerrima can be separated as follows:

The terminal cells of the basal disc hyphae seem to be the most reliable character to separate the two species.

A nitrous smell was noted on several Danish collections on hazel nuts that otherwise matched 4-spored M. tenerrima. Another 4-spored Danish collection on hazel nuts had ochraceous granules on the cap (Aronsen & Læssøe 2016).

NORWAY, FÆRDER, TJØME, Moutmarka 26. Aug. 2012..
Two specimens on fallen fruit of Corylus avellana, leg. A. Aronsen.