Typically growing on bark of Alnus, but has also been reported growing on a fallen branch of Corylus avellana, moss-covered bark of Salix, Quercus and Castanea and under Larix and Tilia (Ronikier et al. 2006). Recorded from Denmark growing on fallen, decaying leaves of Salix (Læssøe 2011), but not yet found in Norway. Summer to autumn. Recorded from Germany, Italy, Slovakia, Switzerland, the Netherlands and Denmark (Ronikier et al. 2006, Læssøe 2011, Favre 1957).
Pileus 0.5-4 mm across, covered with a separable gelatinous pellicle which is covered with white to watery greyish white, acute hairs or spinules visible under a lens, hemispherical, conical to convex, translucent-striate, fairly dark grey when very young, then pale grey, watery greyish white to watery white, slightly darker at the center. Flesh very thin, watery white.Lamellae 6-13, ascending, narrowly adnate to forming a pseudocollarium, pale greyish to white, the edge concolorous. Stipe 10-30 x 0.3-1 mm, filiform, cylindrical, equal or widening below, curved, puberulous, glabrescent but retaining the puberulous surface in lower parts, pale greyish to watery white, springing from a swollen, pubescent, white basal disc 0.3-1 mm broad. Odour indistinctive or very faintly nitrous. Taste not recorded.
Basidia 17-27 x 7-8 µm, narrowly clavate, 4-spored. Spores 8-10 x 4-6 µm, Q = 1.6-2.2, Qav = 2.0, pip-shaped, smooth, weakly amyloid. Cheilocystidia 10-23 x 6-15 µm, forming a sterile band, broadly clavate to ovoid, covered with comparatively few, narrow, cylindrical excrescences 2-8 x 0.5-1 µm. Pleurocystidia absent. Lamellar trama dextrinoid, violaceous brown in Melzer's reagent. Hyphae of the pileipellis 2-7 µm wide, densely covered with warts and unbranched, cylindrical excrescences 2-7 x 0.5-1 µm, overlying a 80-120 µm thick gelatinous layer with irregularly interwoven, more or less smooth hyphae. Pileal spinules composed of erect, conical fascicles of a few agglutinated hyphae similar to those of the pileipellis, gradually tapered to an acute apex, densely to sparsely covered with 2-5 µm long excrescences. Hyphae of the cortical layer of the stipe 3-5 µm wide, smooth; caulocystidia present in the lower part of the stipe, 40-250(-900) x 8-9 µm, cylindrical, narrowing towards the apex, smooth, rarely forked. Clamp connections present in all tissues.
Although it has been recorded from several countries, M. tenuispinosa is not very well known, and the infraspecific variation is probably not completely understood. It belongs to sect. Basipedes together with M. mucor, M. rhenana and M. stylobates. The former grows on fallen leaves of Quercus and differs in many microscopic features, as the cheilocystidia and the hyphae of the pileipellis. M. rhenana lacks cheilocystidia, and both hyphae of the pileipellis and caulocystidia are different. M. stylobates too differs in completely different cheilocystidia and hyphae of the pileipellis.
According to Maas Geesteranus (1983a: 414) the type was devoid of clamps. In a later paper (Maas Geesteranus 1991: 562), however, he referred to new information that clamps are present after all. The few other reports on this species, all claim that clamps are present. In the Danish collection, kindly put to my disposal by Mr. Th. Læssøe, clamps are present; they occur scattered but are not easily seen.