Myxarium nucleatum

The species was originally described by Karl Friedrich Wilhelm Wallroth in 1833, who found it growing on hawthorn in Germany. He placed it in his new genus Myxarium based on its visible white inclusions which he interpreted as spores.[7] Wallroth noted its similarity to Tremella nucleata, but nonetheless described Myxarium nucleatum as a new and separate species, using the same epithet.[1]

Lewis David von Schweinitz had previously described Tremella nucleata from the United States, a species that was later transferred to the genus Exidia by Burt. Many subsequent publications, summarized by Donk (1966) and Reid (1970), discussed whether the European Myxarium nucleatum is conspecific with the American Exidia nucleata.[1][3] The consensus, based on morphological similarity, was that they are the same.[2]

Molecular research, based on cladistic analysis of DNA sequences, has indicated that Myxarium nucleatum belongs in a clade that is quite separate from the Exidia clade.[8] Furthermore, at least three similar species of Myxarium occur in northern Europe, with a single American collection appearing distinct.[6] No DNA-based research has yet been published on the status of "Myxarium nucleatum" worldwide.

The basidia are ellipsoid, longitudinally septate, and measure 10.5–13 × 8–10.5 µm, with a stalk cell up to 30 µm long that becomes enucleate on maturity. The spores are allantoid (sausage-shaped), measuring 8–14 × 3.5–5.5 µm. Hyphae are produced in a gelatinous matrix and are clamped, hyaline, and 1 to 3.5 µm wide.[6]

Exidia thuretiana is a lookalike species.

In Europe, Myxarium hyalinum (formerly considered synonymous with M. nucleatum) is very similar. Its fruit bodies tend to be more ochraceous or brownish with mineral inclusions less evident, except in older, coalesced specimens. Microscopically, it has slightly larger basidiospores, 9-16.5 x 4-6 µm.[6] Myxarium populinum is known from poplar and willow in northern Europe, and lacks mineral inclusions visible with the naked eye.[6] Myxarium cinnamomescens is another, similar northern species, also lacking mineral inclusions.[6]

Fruit bodies of Exidia thuretiana are similarly coloured, but are typically more opaque and often appear pleated. They do not (or very rarely) contain white, granular inclusions. Microscopically, E. thuretiana can easily be distinguished by its sessile (not stalked) basidia. Fruit bodies of several gelatinous Tremella species may also appear similar, but never contain granular inclusions. All can be distinguished microscopically by their non-allantoid spores.[3]