Mycena chlorophos

Mycena chlorophos is one of the best known bioluminescent members of the very large genus Mycena (family Mycenaceae), a group of small, fragile, bell-capped fungi with white spore prints and slender, cartilaginous stems. It was first described from Pacific island material in the nineteenth century and is now recorded across subtropical and tropical Asia, Oceania, and parts of the Americas.

What sets the species apart is its light. Fresh fruit bodies glow a soft yellow-green, brightest along the gills and the cap margin, and the glow is strongest during the first night after the cap opens, fading as the tissues age. In daylight the mushroom is easy to overlook: a translucent grey button no wider than a fingernail on a hair-thin stem.

In Japan the species is celebrated locally as yakoutake, and tourism around its seasonal appearance on Ogasawara and the Ryukyu islands has made it one of the most photographed small fungi in the world.

Cap

The cap is 6-30 mm across, at first hemispherical or bell-shaped and later flattening. It is translucent pale grey to grey-brown, often almost watery-white, and is distinctly sticky or slimy in wet weather. Deep radial grooves run from the margin nearly to the centre, so the cap looks pleated and the gills can be traced through the thin flesh.

Gills

Gills are whitish to pale grey, broad, rather widely spaced, and narrowly attached to the stem, frequently with a small decurrent tooth. They are the main source of the greenish glow, which is visible only after several minutes of dark adaptation.

Stipe

The stem is 6-30 mm long and under 1 mm thick, hair-fine, translucent grey to brownish, minutely downy under a lens, and expands at the base into a small flattened disc that is glued to the wood or leaf substrate.

Flesh and spore print

The flesh is membranous and almost non-existent. The spore print is white; spores are smooth, ellipsoid and amyloid, a standard combination for the genus.

Mycena chlorophos fruits on dead plant material in warm, very humid forest: fallen bamboo culms, decaying palm petioles and leaf sheaths, twigs, bark, and accumulated hardwood litter. It appears in flushes after heavy rain when night temperatures stay warm, mainly from early summer into autumn in the subtropics and through the wet season in the tropics.

The recorded range includes Japan (notably the Bonin and Ryukyu islands), Taiwan, southern China, Southeast Asia, India, Sri Lanka, Micronesia and Polynesia, Australia, and scattered locations in tropical South America.

The species is a saprotroph, digesting the cellulose and lignin of dead bamboo, palm and hardwood debris and helping recycle woody litter on the forest floor. Colonies often fruit in dozens of tiny caps from a single decaying culm, indicating an extensive mycelium inside the substrate.

Its luminescence is produced by a luciferin-luciferase reaction tied to the fungus's own metabolism and continues in the mycelium as well as the fruit body. A widely discussed hypothesis is that the glow attracts night-flying insects that then carry spores away, though the light may also simply be a by-product of oxidative metabolism.

Other glowing Mycena species overlap in range. Mycena lux-coeli and Mycena luxaeterna have similarly small grooved caps but differ in cap colour and in the gelatinous coating of the stem, and both tend to grow directly on decaying hardwood rather than bamboo. Mycena illuminans is noticeably larger, with a more yellow-brown cap and a longer stem.

Panellus stipticus, another luminescent fungus, is unmistakable by its shape: a tough, kidney- or fan-shaped bracket attached sideways to wood with a stub of a stem or none at all, rather than a central-stemmed parasol. Non-luminescent grey bonnets in the same habitat generally have drier, less deeply pleated caps and thicker, opaque stems.