Tapinella atrotomentosa

Tapinella atrotomentosa was originally described as Agaricus atrotomentosus by German naturalist August Batsch in his 1783 work Elenchus Fungorum,[2] and given its current name by Josef Šutara in 1992.[3] It is commonly known as the "velvet-footed pax",[4] and the "velvet rollrim".[5] Historical synonyms include Paxillus atrotomentosus by Elias Magnus Fries (1833),[6] Rhymovis atrotomentosa by Gottlob Ludwig Rabenhorst (1844),[7] and Sarcopaxillus atrotomentosus by Ivan Zmitrovich (2004).[1][8] The variety bambusinus was described from Trinidad in 1951 by British mycologists Richard Eric Defoe Baker and William Thomas Dale.[9]

The species name is derived from the Latin words atrotomentosus, meaning "black-haired".[10] It is still commonly seen under its old name Paxillus atrotomentosus in guidebooks.[11] Tapinella atrotomentosa and its relative T. panuoides were placed in a separate genus Tapinella on account of their habit of growing on (and rotting) wood, and microscopic differences including much smaller spore size, lack of cystidia, and differing basidia.[3] Their off-centre stipe also distinguished them from other members of the genus Paxillus,[12] and genetic analysis confirmed them as only distantly related.[13]

The fruit body is squat mushroom with a cap up to 28 cm (11 in) across, sepia- or walnut brown in colour with a rolled rim and depressed centre. The stem is covered with dark brown or black velvety fur. The gills are cream-yellow and forked, while the thick stipe is dark brown and juts out sidewards from the mushroom.[14] The flesh is yellowish,[15] and has been described as appetising in appearance, and is little affected by insects;[16] the taste however is acrid.[15] The spore print is yellow and the spores are round to oval and measure 5-6 μm long.[17]

It is a saprobic fungus found growing on tree stumps of conifers in North America, Europe,[18] Central America (Costa Rica),[19] east into Asia where it has been recorded from Pakistan[20] and China.[21] The fruit bodies appear in summer and autumn, even in drier spells when other mushrooms are not evident.[16]

The species contains toxins which may cause gastrointestinal upset.[22] There have been cases of poisoning reported in European literature.[23]

Although Tapinella atrotomentosa mushrooms are not generally considered edible,[10] they have been used as a food source in parts of eastern Europe.[18] Tests on the chemical composition and free amino acid levels of the mushroom suggest that they are not considerably different from other edible gilled mushrooms such as Armillaria mellea.[24] Linus Zeitlmayr reports that young mushrooms are edible, but warns than older ones have a foul bitter or inky flavour and are possibly poisonous.[17] The bitter flavour is allegedly improved by boiling the mushrooms and discarding the water, but is indigestible to many.[16]

Tapinella atrotomentosa has a wound-activated defence mechanism whereby injured fruit bodies convert chemicals known as leucomentins into atromentin, butenolide, and the feeding deterrent osmundalactone.[25] Atromentin had previously been identified as the pigment producing the brown colour of the cap,[26] but was not characterized as a chemical defence compound until 1989.[27] Other compounds produced by the fungus include the orange-yellow flavomentins and violet spiromentin pigments.[28] A novel dimeric lactone, bis-osumundalactone, was isolated from the variety bambusinus.[29]

Several phytoecdysteroids (compounds related to the insect moulting hormone ecdysteroid) have been identified from the fungus, including paxillosterone, 20,22-p-hydroxybenzylidene acetal, atrotosterones A, B, and C, and 25-hydroxyatrotosterones A and B.[30]