Fruit bodies 2-10 cm broad, consisting of clustered, convoluted folds with blunt margins; surface yellow, to yellowish-orange, shiny when wet, otherwise dull; context gelatinous, drying to a stiff, hard crust, reviving after periods of moisture; odor and taste not distinctive.
Spores 6.0-9.5 x 6.0-7.5 µm, subglobose to ovoid, smooth, thin-walled with a conspicuous hilar appendage; basidia longitudinally septate, typically stalked, 10-14 µm broad.
Gregarious on downed hardwoods in coastal forests as well as low elevations of the Sierra Nevada; parasitizing Stereum hirsutum; fruiting throughout the mushroom season; common.
Edible, but without flavor.
Tremella aurantia resembles and is often confused with Tremella mesenterica. Both species have convoluted yellowish-orange fruiting bodies, are similar in size, occur on hardwoods, and like all Tremellas, are parasitic on other fungi. As is often the case with closely related taxa, a combination of characters is needed for identification. Here we've relied on the work of British mycologist Peter Roberts to separate the species. Tremella aurantia is most easily distinguished by its host preference, attacking the common wood rotter, Stereum hirsutum, while Tremella mesenterica parasitizes species of Peniophora, a resupinate fungus. Tremella aurantia is also somewhat larger and has a duller surface when dry. Microscopically, it has smaller spores, and stalked, not sessile basidia. The ubiquitious nature of Stereum hirsutum, helps explains its abundance. The frequency of Tremella mesenterica, on the other hand, is unclear and remains to be determined.
Besides the above species, a third "Witch's Butter," Dacrymyces palmatus, is also regularly encountered. It typically is more orange, and unlike the Tremella species, is saprobic on conifer wood. A helpful fieldmark is a whitish attachment point to the substrate. Microscopically, tuning-fork type basidia differentiates it from the longitudinally, septate, cruciate type basidia of Tremellas.