Solitary or in small groups on decaying vegetable matter as small twigs, conifer needles, rotten logs, fallen leaves, grass stems and fallen coniferous cones. Summer to autumn. Widespread but rare or absent in the northern lowlands (see Læssøe 2013 a). Not uncommon under Salix in alpine sites.

Pileus 3-8 mm across, hemispherical to broadly conical, sometimes becoming plano-convex, occasionally somewhat centrally depressed, translucent-striate, sulcate, pubescent to conspicuously setose, particularly at the centre, grey to grey-brown. Lamellae 12-15 reaching the stipe, ascending, narrowly adnate, free or forming a pseudocollarium, white to pale grey. Stipe 10-50 x 1-2(-3) mm, hollow, straight, terete, equal, entirely puberulous, grey; springing from a ca. 1 mm wide grey, grey-brown to whitish grey, pubescent basal disc. Odour none.

Basidia 15-22 x 6.5-8 μm, clavate, 4-spored. Spores 7.5-10.5 x 3.8-4.7 μm, Q 1.7-2.3, qav ~ 2.0, pip-shaped, smooth, non-amyloid. Cheilocystidia 11.5-30 x 5.5-13.5 μm, clavate or fusiform, apically broadly rounded or gradually drawn into one or two or three slender necks, or with one single to three finger-like, usually unbranched but occasionally branched, straight, curved or flexuous excrescences 4.5-33 x 1 μm. Pleurocystidia absent. Lamellar trama dextrinoid. Hyphae of the pileipellis 2-11 μm wide, embedded in gelatinous matter, covered with straight to curved, branched to unbranched, cylindrical excrescences 2-12 x 1 μm, with long, very thick-walled, one-celled hairs (pileo-setae) up to 200 μm long arising from the pilei trama. Hyphae of the cortical layer of the stipe 2-3.5 μm wide, smooth. Caulocystidia similar to the pileo-setae, up to 250 μm long, thick-walled to solid, with bulbous base. Clamp connections generally absent, but sometimes present at the base of basidia and cheilocystidia.

Mycena aciculata represents the common, widely distributed, temperate Northern Hemisphere species misidentified in the literature as M. longiseta. M. longiseta is a species restricted to southeast Asia. A commentary on the confusion around the species concept of M. longiseta was published by Desjardin & Horak (2002). M. aciculata differs from M. longiseta primarily in having inamyloid spores, distinctive cheilocystidia, more strongly gelatinized pileipellis tissue, shorter pileosetae, thick-walled caulocystidia and basal disc cystidia, and in lacking pileus marginal cystidia (Desjardin et al. 2002).

Mycena aciculata is readily identified on account of the setae at the pileus and the stipe. The small basal disc is not always easy to see. In the field it could possibly be confused with M. tenuispinosa that looks somewhat similar, including a small basal disc and acute squamules on the pileus. This species is easily distinguished, however, on account on amyloid spores, presence of clamps and complete lack of setae.

According to Maas Geesteranus (1992: 25-26) Mycena aciculata is devoid of clamps (with a question mark for basidia). That is my experience as well, but collections A 4/92 and A 28/93 showed scattered clamps at the septa of the basidia and the cheilocystidia.

Robich (2003c) described a new species from Italy, Mycena setulosipes, that was separated from M. aciculata on account of a more brownish pileus, a larger number of lamellae reaching the stipe, absence of basal disc, larger and almost subglobose spores, absence of pileosetae, and presence of clamps.

Microphotos of hyphae of the pileipellis

Microphotos of cheilocystidia

Microphoto of pileo-seta