Yellow Brain

Yellow Brain (Tremella mesenterica) is one of the most instantly recognisable of the jelly fungi, forming bright egg-yolk to golden-orange gelatinous masses on dead attached branches. The fruiting body has no cap, gills or true stem in the conventional sense; instead it is a convoluted mass of lobes and folds, often compared to a brain, a pile of scrambled egg, or a blob of melted butter caught on a twig.

The genus Tremella belongs to the Tremellomycetes, an evolutionary line quite separate from the ordinary gilled mushrooms. Its members are distinguished microscopically by basidia that are divided by longitudinal or oblique cross-walls into four cells, a feature shared by no other common group of larger fungi. Almost all Tremella species are mycoparasites, growing on the mycelium of another fungus inside the wood rather than digesting the wood themselves.

What makes Yellow Brain especially distinctive is its dramatic response to moisture. In dry conditions the fruiting body shrinks to a thin, dark, varnish-like scab that is easy to overlook; within hours of rain it rehydrates and swells back to full size and colour. This is why the species is recorded in almost every month of the year in mild oceanic climates.

Fruiting body

Irregular gelatinous masses 2–10 cm across and up to about 4 cm tall, built from crowded, sinuous, flattened lobes that fuse into a brain-like or seaweed-like clump. There is no distinction between an upper and lower surface: the whole outer face is fertile. The texture is soft, slippery and elastic, springing back when squeezed, and the tissue is somewhat translucent so that light passes through the thinner lobes.

Colour

Bright golden-yellow to deep orange-yellow when fresh and well hydrated, often paler and more watery-yellow in shade or after prolonged wet weather, and darkening to orange or almost brick-red as the tissue dries. Dried specimens collapse to a thin, hard, dark orange-brown film pressed against the bark. Colour changes come from drying rather than from bruising; the flesh does not stain a new colour when cut.

Attachment and flesh

The mass is broadly attached to the bark over a small area, sometimes emerging from a bark crack or from beneath a loose flake. There is no ring, no volva and no distinct stipe. The flesh is uniformly gelatinous throughout, without a fibrous core.

Spore print

White, and difficult to obtain because the fertile surface is diffuse; spores are best collected by resting a fresh lobe on dark glass in a humid container overnight.

Yellow Brain grows on dead but still attached branches and twigs of broadleaved trees and shrubs, with gorse, hazel, ash, beech and oak among the most frequent supports. Fallen brash and stacked branch wood also produce it. It rarely appears on conifer wood, which helps separate it from several superficially similar orange jellies.

It is a cool-season fungus, most abundant from autumn through to early spring and often conspicuous in mid-winter when little else is fruiting. Distribution is very wide across the temperate Northern Hemisphere, including Europe, North America and northern Asia, with records from parts of the Southern Hemisphere as well.

Despite appearing to grow on wood, Tremella mesenterica does not decay wood directly. It is a mycoparasite of crust fungi in the genus Peniophora, whose thin resupinate fruiting bodies are frequently visible as smooth pinkish, lilac or grey patches on the same branch. The Tremella mycelium invades the Peniophora hyphae by means of specialised cells and draws nutrients from them, so the jelly is effectively a fungus feeding on a fungus that is feeding on the wood.

Because of this relationship, the presence of Yellow Brain is a reliable clue that a Peniophora crust is colonising the branch, and looking a few centimetres along the same twig will often reveal the host. Spores are released from the exposed gelatinous surface and, in this genus, may bud into yeast-like cells that persist and disperse before eventually infecting a new host colony.

Tremella aurantia (Golden Ear) is a deeper orange, firmer and more distinctly ear- or petal-lobed fungus that parasitises Stereum hirsutum; look for the hairy, zoned brackets of that host on the same log. Dacrymyces chrysospermus (Orange Jelly) is brighter orange, typically grows on barkless conifer wood, and has a conspicuous whitish attachment point where it grips the wood, unlike the evenly coloured base of Yellow Brain.

Dacrymyces stillatus (Common Jellyspot) is far smaller, forming scattered blobs only a few millimetres across rather than large convoluted masses. Phaeotremella foliacea (Leafy Brain) shares the folded structure but is brown to reddish-brown rather than yellow, and its lobes are broader, thinner and more leaf-like.