Roundspored Oysterling

The Roundspored Oysterling (Crepidotus cesatii) is one of a large group of very small, sideways-growing gilled fungi placed in the genus Crepidotus (family Crepidotaceae, sometimes treated within the Inocybaceae). Members of the genus are known collectively as oysterlings because their fan-, shell- or kidney-shaped fruit bodies attach laterally to wood without a central stalk, superficially recalling a miniature oyster mushroom.

What sets Crepidotus apart from true oyster mushrooms is the spore colour: oysterlings drop a brownish rather than white or lilac print, and their mature gills flush a dull ochre or cinnamon tone. C. cesatii is among the smallest and whitest of the group, rarely exceeding the width of a fingernail, and is easily overlooked unless the underside of a fallen branch is deliberately turned over.

The species name refers to the round spores that give the fungus its English name. Under a microscope the spores are globose to subglobose and finely spiny, whereas most of its closest relatives have distinctly elongated, ellipsoid spores. In the field the fungus is recognised as a Crepidotus by shape and substrate; separating it from its nearest relatives is largely a microscopic exercise.

Cap

The fruit body is 0.5–2 cm across, kidney-shaped, semicircular or shell-like, attached by a narrow point at the rear or upper edge. The surface is white to cream, sometimes ageing pale buff, and is finely downy or minutely hairy, especially near the point of attachment. The margin is thin, often slightly inrolled when young and becoming wavy or lobed with age. In damp weather the flesh becomes soft and translucent.

Gills

The gills radiate outward from the attachment point like the ribs of a fan. They begin white or cream and gradually take on a pale ochre, tan or dull cinnamon tint as the spores ripen. They are moderately crowded, fairly narrow, and the edges may appear finely fringed under a lens.

Stipe

A stipe is effectively absent. At most there is a tiny lateral nub or a whitish felty pad where the fungus is fixed to the wood. Because the fruit body grows sideways from its substrate, specimens on the underside of a branch hang with the gills facing downward, while those on a vertical surface may curve to reorient themselves.

Flesh & spore print

The flesh is thin, soft, white and watery, without colour change when broken. There is no marked bruising reaction. The spore print is pale ochre-brown to clay-brown. Microscopically the spores are globose to nearly globose and minutely echinulate (spiny), a feature that is diagnostic within the genus and gives the fungus its common name.

Crepidotus cesatii is a wood-inhabiting fungus of damp, shaded woodland and hedgerows. It fruits on small-diameter dead material — fallen twigs, thin branches, bramble and rose stems, old herbaceous stalks and occasionally fern rachides — most often on broadleaved hosts such as ash, hazel, willow, elder and oak. It has a strong preference for material lying on the ground where humidity remains high, and often appears in troops of many fruit bodies scattered along a single fallen stick.

Fruiting runs from summer through to late autumn, peaking after prolonged wet spells. The species is widespread across Europe and is also reported from North America and other temperate regions, though because it is small and requires microscopy to confirm, it is almost certainly under-recorded across much of its range.

The fungus is a saprotroph, obtaining its nutrition by decomposing dead plant tissue rather than by forming any partnership with a living plant. It specialises in the fine woody debris that accumulates on the woodland floor — the twig layer that larger bracket and polypore fungi generally ignore — and contributes to the early and middle stages of breakdown in that material, softening bark and outer wood and returning nutrients to the soil.

Mycelium spreads unseen through the thin twig until conditions are damp enough to trigger fruiting, at which point a flush of tiny brackets can emerge along the whole length of the stick within a few days. The fruit bodies are short-lived, collapsing quickly once they dry, and rely on the sheltered underside position to keep spore release going through humid spells.

Crepidotus variabilis is the closest match and is effectively identical to the naked eye — a small white lateral fan on twigs with pale brown gills. It is separated only by its spores, which are elongated and ellipsoid rather than round, so field determination between the two is unreliable. Crepidotus mollis is much larger, up to 6 cm across, with a brownish, distinctly gelatinous cap cuticle that can be stretched elastically before it tears, and it grows on substantial logs and stumps rather than twigs.

Crepidotus applanatus is also larger, whiter and has a translucent, strongly striate cap margin, typically clustering in overlapping tiers on rotting stumps. True oyster mushrooms (Pleurotus species) are far bigger, fleshier, and drop a white to pale lilac spore print rather than a brown one, and often have a short lateral stem. Small white Resupinatus and Hohenbuehelia species can occupy the same twigs but give white spore prints and, in Resupinatus, a more cup-like or gelatinous fruit body.