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Marasmius rotula is a species of fungus in the Marasmiaceae family of mushrooms. It is commonly known variously as the pinwheel mushroom, the pinwheel Marasmius, the little wheel, the collared parachute, or the horse hair fungus. It is a widespread and common fungus, and is the type species of the genus Marasmius. The fruit body is characterized by its whitish, thin, membranous cap, its long and slender but tough black stem, and widely-spaced white gills that are attached to a collar encircling but not touching the stem. The fungus grows on decaying wood and leaves. Unlike other mushrooms known to release spores in response to an internal timer, or circadian rhythm, spore release in M. rotula is dependent on rain.
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The species was first named by Italian mycologist Giovanni Antonio Scopoli as Agaricus rotula in 1772. In 1821 Elias Magnus Fries redescribed the mushroom,[1] and later sanctioned the name as Marasmius rotula in 1838.[2] In 1946 Smith and Singer proposed to conserve the genus name Marasmius over Micromphale, which had nomenclatorial priority, with M. rotula as the lectotype.[3] The genus Marasmius, with M. rotula as the lectotype species, was later conserved by the 1954 Paris Congress on Botanical Nomenclature.[4][5]
It is commonly known as the "pinwheel mushroom", the "pinwheel Marasmius",[6] the "collared parachute",[7] or the "horse hair fungus".[8] The name "little wheel fungus" is suggestive of the collar to which the gills are attached like the spokes of a wheel.[9] The specific epithet is derived from the Latin word rota meaning "wheel".[6]
The cap of the fruit body is thin and membranous, measuring 3 to 20 mm (0.1 to 0.8 in) in diameter.[10] It has a convex shape, and is slightly depressed in the center. The cap color is whitish to light pinkish-white, slightly darker at the center, and smooth. The cap margin has scalloped edges and furrows that outline the shape of the gills on the underside; the radial furrows extend inward to one-third to three-quarters of the cap radius from the edge.[11] The flesh is thin and white.
The gills are attached to a collar, and are free from direct attachment to the stem, although some specimens may have the collar pressed close to the stem and this characteristic will be less obvious.[6] The gills are whitish-pallid, widely-spaced; there are typically between 16 and 22 of them.[12] The stem is 2 to 5 cm (0.8 to 2.0 in) long and up to 0.1 cm (0.04 in) thick, with a smooth, sometimes shiny surface. It is tough, hollow, slightly blackish-brown below, with a lighter apex that is almost translucent.[13] The base of the stem may be connected to black root-like rhizomorphs. There is no partial veil on the stem.[8]
Viewed in deposit, such as with a spore print, the spores of Marasmius rotula appear white or pale yellow.[11] Viewed with a light microscope, the spores are hyaline (translucent), teardrop or pip-shaped, and have dimensions of 7–10 by 3–5 µm.[8] The basidia (the spore-producing cells) are four-spored. Along the edge of the gill, interspersed among the basidia, are sterile (non-reproductive) cells called cystidia; they are club-shaped and have rough wart-like protuberances on the surface.[12]
Marasmius rotula is considered inedible,[6] but not poisonous. The mushroom has no distinguishable odor, and the taste is mild or bitter.[8] The fruit bodies have the ability to bioaccumulate the heavy metal cadmium; a study of the metal concentration of 15 wild Indian mushroom species showed that M. rotula had accumulated the highest concentration of cadmium.[14]
In 1975 American mycologist Martina S. Gilliam investigated the periodicity of spore release in M. rotula and concluded that spore discharge did not follow the regular circadian rhythm like other typical agaric and bolete mushrooms,[15] but rather, was dependent on rain.[16] Like many other species of Marasmius, the fruit bodies of M. rotula can dry out in dry periods, and revive when sufficient moisture is available in the form of rain or high humidity. In her study, fruit bodies were capable of discharging spores over a period of at least three weeks.[16]
Marasmius capillaris has a roughly similar appearance, but with a pale tan cap with a white center, and it grows on oak leaves, and is never clustered.[10] Further, its cap is evenly rounded, unlike M. rotula, which is pleated and furrowed.[17] Marasmius nigripes has white caps that are 5 to 10 mm (0.2 to 0.4 in), widely-spaced gills, and a dark stem covered with what appears to be a white powder.[18] Mycena corticola is smaller than Marasmius rotula, has a pale pink-brown cap, and can is usually found growing singly or in groups on bark near the base of living trees.[6] M. rotula is distinguished from M. bulliardi by its larger size, and greater number of gills.[12]
Marasmius rotula is a saprobic species,[17] and obtains nutrients by decomposing dead organic matter. It grows on rotten leaves, dead wood (especially beech), or other woody debris in deciduous forests. The fruit bodies are easily overlooked because of their diminutive size.[9]
The fungus is widespread and common in the British Isles, and has been collected in Shetland (Scotland),[19] Nigeria,[20] India,[14] China,[13] and Poland.[21] In North America it is more commonly found in the east.[8]
