Bitter Oyster Identification Guide

The first impression

A fallen oak trunk in a damp wood, and along one flank of it, running for a metre or more, a dense pavement of small tan brackets stacked in overlapping tiers. Each one is a centimetre or two across, kidney-shaped or shell-shaped, attached to the bark by one edge, and the whole colony looks tiled, like scales on a fish.

Pick one off. It is not soft. The flesh is tough, pliant and leathery, and it tears rather than snapping. It also revives: a colony that looked like dried brown flakes in a dry week will be plump and flexible again a day after rain, and the same fruitbodies may persist for many months through repeated wetting and drying.

Reading the cap

The cap is 1 to 3 cm across, occasionally slightly larger, and shaped like a kidney, fan or scallop shell because it is attached at one side rather than centrally. The margin is inrolled when young, and stays somewhat incurved, often becoming lobed, scalloped or shallowly notched with age.

Colour runs from buff and pale ochre to cinnamon, tan or clay, sometimes with a slightly pinkish or greyish tone in shaded material and paler at the margin. The surface is dry and, crucially, finely scurfy, mealy or granular, particularly towards the point of attachment, where it may crack into small scaly patches. Run a thumb over it and it feels faintly gritty rather than smooth or silky.

The spore-bearing surface

The underside carries true blade gills, which is worth confirming before anything else. They radiate from the attachment point rather than from a central hub, and they are narrow, crowded and shallow. Their colour is a warmer orange-tan, cinnamon or pale rust that contrasts noticeably with the paler cap, and that contrast is one of the quickest field cues.

Under a lens the gills frequently show fine cross-veins or interconnections near the base, and the edges are even. Because the fruitbody is tough, the gills stay intact through drying and rehydration rather than collapsing.

Stipe and attachment

There is a stipe, but it is short and easy to overlook: 0.5 to 1.5 cm long, stout, lateral or strongly eccentric, often flattened, and paler than the cap, whitish to buff with a fine downy or velvety covering. It flares outward into the cap so that the two are continuous.

On the vertical face of a log the stem is usually distinct. On the upper surface of a log the fruitbody may sit almost directly on the wood with the stem reduced to a broad attachment pad. Look at several fruitbodies in a colony before deciding whether a stipe is present.

Flesh and reactions

Slice a fruitbody in half through the attachment. The flesh is thin, whitish to pale buff, and tough and elastic like thin rubber or leather. It does not blue, redden or blacken when cut, and there is no latex, no gel layer and no watery exudate. Handling leaves no stain on the fingers.

That toughness is itself a taxonomic signal: a soft, watery, easily broken fan on wood is far more likely to belong to a different group.

Taking a spore print

A lateral fruitbody needs a slightly different arrangement. Cut a mature cap with a short piece of its stem, then prop it so the gills face straight down over the paper, either by resting the stem stub on a matchstick or by pinning the cap upright against a small support. Use dark card, since the deposit is pale, and cover the whole thing with a bowl.

Leave it overnight. The print is white to very pale cream. This single result eliminates a large slice of the possible confusions in one move, because most of the small brown fans on dead wood belong to genera with clay-brown or rust-brown prints. Under the microscope the spores are small, narrow and sausage-shaped, and the tissue contains distinctive gelatinous elements, but the macroscopic white print is usually decision enough.

Substrate, ecology and season

The Bitter Oyster is a white-rot saprotroph of dead hardwood, especially oak, but also beech, birch, hornbeam, maple and many others. It colonises fallen trunks, standing dead stems, stumps, cut ends of logs and large fallen branches, generally with the bark still attached, and it fruits in huge numbers on a single piece of wood.

It is one of the most persistent wood-decay agarics in temperate forest, fruiting from late summer through autumn and into winter, and old fruitbodies survive on the wood essentially year-round. Its ability to dry to a hard flake and reconstitute after rain is a genuine ecological adaptation to exposed bark surfaces that wet and dry repeatedly.

One more ecological point of interest: populations in eastern North America and parts of East Asia are strongly bioluminescent, their gills emitting a steady green glow visible to a dark-adapted eye at night, while European populations generally do not luminesce. It is one of the classic sources of what old woodsmen called foxfire.

Separating it from look-alikes

Crepidotus species (C. mollis, C. variabilis and others). The main confusion group. They are soft rather than tough, often flabby or with a gelatinous layer in the cap, usually paler and whitish, and their gills turn clay-brown as the spores ripen. Their spore print is brown, not white, and most lack any real stipe.

Schizophyllum commune. Whitish grey and densely hairy on the upper surface, and its underside is not made of true blades: each fold is split lengthwise along its edge and the two halves roll apart when dry. A hand lens on the gill edge settles it at once.

Plicaturopsis crispa. Common on birch. The cap is orange-brown with concentric zones and a crisped white margin, and the underside consists of blunt, forking, wrinkle-like folds rather than knife-edged gills.

Panellus serotinus (Sarcomyxa serotina). Far larger at 3 to 10 cm, with a viscid olive-green to yellow-green cap and a thick yellowish stipe, appearing late in the season.

Resupinatus applicatus. Tiny, dark grey to blackish, gelatinous, with sparse gills and no stipe, growing flat against the wood.

Young oyster mushrooms (Pleurotus). Much larger even when small, with soft thick white flesh, broad widely spaced decurrent white gills, and a smooth grey or brown cap with no scurf.

Lentinellus species. The gill edges are jagged and saw-toothed under a lens, and the flesh is tough but the fruitbodies are usually larger and paler.

Field checklist

Small tan kidney- or fan-shaped brackets in overlapping tiers on dead hardwood; dry scurfy cap surface; crowded narrow orange-tan gills contrasting with the paler cap; short lateral downy stipe; tough leathery flesh that revives after rain; white spore print.