Tremella mesenterica

Common Name: Yellow Brain (and lookalike Tremella aurantia)

(Old scientific names include: Tremella lutescens; Hormomyces aurantiaca the latter referring to asexual fruit bodies producing conidiophores)

This is a beautiful, easily spotted species and probably one of the few that most of us will look at and feel confident in identifying. There is, however, a cautionary 'but..'. There is another species of Tremella, T. aurantia, that is virtually indistinguishable without a microscope. Both species are bright yellow or orange and are usually found in broadleaved situations. The text books tell us that T. aurantia is often more chunky and less greasy looking with a rather mat texture. Without both species side by side this can be difficult to assess (especially if it has been raining!). If however, your fungus is growing near the common and often very visible wood rotting species, Stereum hirsutum (see 'Habitat' below), then it is well worth taking a closer look.

Peter Roberts wrote an excellent piece in Mycologist 9.3 1995, (for those of you with access to this publication) describing both species. Microscopically he suggests that T. aurantia can be distinguished by its smaller more subglobose spores (5.5-9 x 4.5-7 compared with 10-16 x 6-9.5), smaller basidia (9-13 across compared with 15-21 across) and also by the presence of unclamped host (S. hirsutum) hyphae in the subhymenium and context. Peter also suggests that T. aurantia has been much confused in the past and this probably explains the relative lack of records in the UK and continental Europe pre 1990.

The distribution map shows a scatter of records in England and Wales but nothing in Scotland. Since Stereum hirsutum loves to fruit on dead oak wood and many of our T. mesenterica records are listed as being on oak, I wonder whether we might just have been overlooking T. aurantia in Scotland?

Members of the genus Tremella are heterobasidiomycetes; their basidia are septate and produce long and often sinuous sterigmata. (more information about some of these terms and fungal lifestyles).

Fruiting:

T. mesenterica has been found in every month of the year.

Habitat:

The genus Tremella is understood to live parasitically either on other wood rotting fungi, or on lichenized fungi. T. mesenterica parasites the encrusting genus Peniophora. T. aurantia parasitises Stereum hirsutum and any T. mesenterica-like structures found on or near this latter fungus should be carefully checked.

Most records of T. mesenterica in the databases give the associated species as the tree host of the fungus that is being parasitized. It is not always possible to see the host fungus as the interface between the two is often away from the fruit body and between mycelia hidden in the woody substrate. Thus recording the associated organism as 'gorse', 'hazel' or 'oak' is not technically correct although still useful.

In general, searches will be amongst the dead wood of broadleaved trees (particularly oak and hazel) and gorse as the host fungi colonise these trees.

Distribution:

Tremella mesenterica is given as common throughout whereas T. aurantia is said to be occasional in England and Wales in CBIB. An old Tayside record (as T. frondosa) has been dismissed as a 'rejected name' by Roy Watling in Mycologia Scotica.

Please remember to submit your records to your local recording group or via the Scottish Fungi online recording form.

Liz Holden, 2013