Bitter Oyster

The Bitter Oyster (Panellus stipticus) is a tiny but extremely common shelf fungus that colonises dead hardwood in dense, overlapping ranks. Individual fruitbodies are rarely more than 3 cm across, but they crowd together in rows of dozens along the edge of a log, each a small tan kidney with a finely velvety upper surface and a short off-centre stalk.

It is the type species of Panellus, a genus of small, tough, pleurotoid agarics with amyloid spores, and it is one of the most-studied fungi in mycology because of its bioluminescence. North American populations produce a steady, faint green glow from the gills and mycelium, visible after several minutes of dark adaptation, while most European collections of the same species are non-luminescent — a striking geographic split in a single morphological species.

The flesh is tough and leathery rather than soft, and dried fruitbodies revive on wetting, which is why the species can be found in almost any month of the year. Its abundance, small size and distinctive suede texture make it a good introduction to the small pleurotoid fungi.

Cap

Caps are 1–3 cm across, kidney-shaped, fan-shaped or semicircular, convex and often shallowly furrowed at the margin. The surface is dry, matt and finely velvety to scurfy — a suede or kid-leather texture — and coloured buff, tan, cinnamon-buff or pale ochre, sometimes with a slightly paler margin. Old caps become paler and more brittle, and dried ones curl inward.

Gills

Gills are narrow, crowded and pale buff to cinnamon or orange-tan, radiating from the point of attachment and distinctly interconnected by low cross-veins near the base — the fine veining between gills is a helpful character with a hand lens. They attach to the stub of stalk and may run very shortly down it.

Stipe

The stipe is very short and lateral or strongly eccentric, typically 0.3–1 cm long, thick relative to its length, pale buff and finely downy or scurfy, flaring where it joins the cap. It is tough and does not snap cleanly.

Flesh & spore print

The flesh is thin, tough, pliable and leathery, whitish to buff, and unchanging when cut. Dried specimens rehydrate and resume normal form. The spore print is white; spores are small, smooth, cylindrical to sausage-shaped and amyloid, staining blue-grey in iodine reagents. In luminescent populations, freshly collected gills emit a continuous dim green light.

Panellus stipticus grows on dead hardwood of many kinds — oak, beech, birch, maple, hickory, alder and others — on fallen logs, stumps, cut ends, standing dead stems and thick branches, and it particularly favours the exposed cut faces of stumps and the barkless flanks of old logs. Conifer substrates are uncommon. Fruitbodies almost always appear in dense overlapping rows or shingled clusters rather than singly.

It fruits at any time of year given adequate moisture, with the heaviest flushes in late summer and autumn; because the tough fruitbodies persist and rehydrate, they can be found in winter and spring as well. The species is cosmopolitan in the north temperate zone and is recorded widely in Europe, Asia, North America and Australasia, being one of the commonest small bracket-forming agarics in many forests.

The fungus is a saprotroph causing a white rot of hardwood, breaking down lignin and cellulose in dead trunks, stumps and branches. It is an effective early to mid-stage coloniser of exposed dead wood, and its mycelium can occupy substantial volumes of a stump, fruiting from the same substrate across many seasons. Tough, weather-resistant fruitbodies allow it to release spores intermittently over a very long period rather than in a single brief flush.

Its bioluminescence arises from an enzymatic reaction in living mycelium and gill tissue that produces continuous green light around 520 nanometres. The adaptive function is debated, with the leading hypothesis being the attraction of night-flying insects that carry spores away from the log. The trait is present in North American populations and largely absent in European ones, making the species a model system for studying how a biochemical pathway can be retained or lost within a single species across continents.

Crepidotus species form similar small stalkless fans on wood but are usually whiter, softer and smooth rather than suede-textured, and above all they leave a brown spore print that dusts fruitbodies growing beneath them. Panellus violaceofulvus and other small Panellus species differ in colour, being violet-grey to dark brown, and in their more strongly reduced stalks.

Schizophyllum commune superficially resembles it in size and shelf habit but has a densely white-hairy cap and, crucially, gill-like folds that are split lengthwise along their edges. Plicaturopsis crispa has wrinkled, forking, vein-like folds instead of true gills and a crisped orange-brown cap. Young Pleurotus species are far larger and fleshier with broad white gills. The combination of small size, suede cap, cross-veined buff gills, tough revivable flesh and white amyloid spores identifies P. stipticus reliably.