Tremella encephala

Tremella encephala is a mycoparasitic jelly fungus of conifer wood, forming rounded, convoluted cushions that look like small brains — hence both the specific epithet and its usual English name. Colour runs from translucent whitish through flesh-pink to a warm pinkish brown, darkening as the fruit body ages.

What sets this species apart from every other Tremella is its internal structure. Slice a fruit body across and it is not gelatinous all the way through: a dense, firm, chalky-white sphere sits at the centre, surrounded by a jelly rind. That core is not the parasite at all — it is the compressed tissue of its host, walled off and enclosed as the Tremella grows around it.

The fungus belongs to the Tremellomycetes and shares the group's characteristic four-celled, longitudinally septate basidia. It is widespread across the conifer forests of Europe, northern Asia and North America, fruiting mainly in the colder half of the year when its wobbly cushions are among the few fungi active on fallen spruce and pine.

Fruit body

Cushions are 1–3 cm across, occasionally coalescing into larger masses, and are rounded to pulvinate with a deeply folded, convoluted, brain-like surface. They sit tight against the wood with a broad attachment and have no stalk. The surface is smooth, waxy and glistening when moist.

Colour

Young fruit bodies are translucent whitish to pale flesh-pink; with age they become pinkish buff, dull ochre-pink, or brownish, and finally shrink to hard dark crusts. The gelatinous rind is semi-transparent, so the pale core can sometimes be glimpsed as a ghostly centre before the specimen is cut.

Section, flesh and spore print

A vertical cut is the decisive test. The outer 2–5 mm is rubbery jelly; inside sits a firm, opaque, white to cream sphere of host tissue with a distinct boundary. No liquid is released and no colour change occurs on cutting. The spore deposit is white, and the basidia are longitudinally divided into four cells.

The fungus is confined to dead conifer wood — spruce, pine, fir and larch — including fallen trunks, stumps, and lopped branches, usually where the bark has been lost. It appears specifically on wood already carrying the thin greyish sheets of its host crust fungus, so scanning likely logs for that crust is the most efficient way to find it.

Fruiting runs from autumn through winter into early spring, peaking in cool, wet weather; the species tolerates frost and often looks freshest after a thaw. It occurs throughout the boreal and montane conifer zones of Europe, Asia and North America.

Tremella encephala is a mycoparasite of Stereum sanguinolentum, the Bleeding Conifer Crust. It invades the host's mycelium within the wood and then fruits directly on or beside the host's fertile sheets, encasing a knot of host tissue that becomes the familiar firm white core.

Because the jelly cannot decay wood itself, its life cycle is entirely dependent on the prior colonisation of a log by Stereum. The pairing effectively makes the presence of the jelly an indicator of that host, and the two are so consistently found together that experienced observers look for one to locate the other. The gelatinous rind dries to a hard nodule during cold dry spells and rehydrates to resume spore release when moisture returns.

Tremella mesenterica is superficially similar in shape but bright golden-yellow to orange, grows on broadleaf wood in association with Peniophora crusts, and is gelatinous throughout with no firm core. Phaeotremella foliacea and its relatives on conifer wood are brown to reddish-brown and form loose frondose lobes rather than compact brains, again without a core.

Exidia nucleata is closest in colour, being translucent whitish and blob-like, but its internal white bodies are numerous small hard granules scattered through the jelly, not a single large central sphere, and it grows on broadleaf wood. Exidia thuretiana is chalky white, flatter and more spreading, with a uniformly gelatinous interior. In all cases a single clean cut through the fruit body resolves the question.