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Astraeus hygrometricus

[ Basidiomycota > Boletales > Diplocystidiaceae > Astraeus . . . ]

by Michael Kuo

This little thing is in just about every North American field guide ever published (except mine) and it is collected so frequently that its records take up 18 screens-worth of collections in our nation's major herbaria, databased at MyCoPortal. But I have seen it only twice, in more than 20 years of collecting. Perhaps I keep missing it, or maybe I don't collect very often in sandy, disturbed-ground areas (which is where Astraeus hygrometricus is usually found). Or maybe I'm just flipping incompetent. At any rate, I have only collected it in the New Jersey pine barrens and in northern Michigan on the shores of Lake Superior—and I have studied a collection sent to me from North Carolina.

Astraeus hygrometricus looks like "earth stars" in the genus Geastrum, but its rays are "hygroscopic"; they cover the round spore case in dry weather but peel away from it in wet conditions. The inner (or "upper") surfaces of the rays become finely cracked, and the surface of the spore case is matted-fibrillose. The spore powder, at maturity, is chocolate brown—and, under the microscope, the spores are much larger than the spores of Geastrum species.

Geastrum and Astraeus are not, in fact, very closely related; Geastrum is related to the stinkhorns, while Astraeus is related to the boletes. Thus the ball-on-a-star strategy for dispersing spores is an example of convergent evolution, not phylogenetic similarity.

A study by Phosri and collaborators (2013) suggests support for several phylogenetic species within Astraeus hygrometricus, but the paper is riddled with errors, inconsistent species descriptions, and poor documentation; I believe further research is required before accepting the paper's suggestions.

Thanks to Greg Allikas for documenting, collecting, and preserving Astraeus hygrometricus for study; his collection is deposited in The Herbarium of Michael Kuo.


Ecology: Mycorrhizal; growing alone, scattered, or gregariously in sandy soil, especially in disturbed-ground areas, near hardwoods or conifers; summer and fall (over winter in warmer climates); originally described from Europe; widely distributed in Europe, Asia, Oceania, North America, Central America, and South America. The illustrated and described collections are from Michigan, New Jersey and North Carolina.

Fruiting Body: A more or less spherical spore case sitting atop pointed rays that fold over the spore case in dry conditions.

Spore Case: 1–2 cm across; more or less spherical; dry; matted-fibrillose; papery; rupturing at the top with maturity; whitish becoming grayish to brownish.

Interior: White and fleshy when young; becoming chocolate brown and powdery.

Rays: Numbering 6–12; more or less triangular or elongated-triangular; about 1 mm thick; inner/upper surface dark brown to black, becoming finely cracked overall; outer/lower surface brown, matted-fibrillose, often covered with sand.

Odor: Not distinctive.

Spore Print: Brown.

Microscopic Features: Spores 7.5–12 µm including ornamentation; globose; echinate; spines densely crowded, about 1 µm long and 1 µm wide at the base; brownish golden in KOH. Capillitial threads 2.5–7 µm; wide; yellowish to brownish in KOH; roughened; thick-walled.

REFERENCES: (Persoon, 1801) Morgan, 1889. (Saccardo, 1891; Coker & Couch, 1928; Smith, 1951; Smith, 1975; Smith, Smith & Weber, 1981; Arora, 1986; States, 1990; Phillips, 1991/2005; Lincoff, 1992; Metzler & Metzler, 1992; Barron, 1999; Roody, 2003; Bates, 2004; Phosri et al., 2004; McNeil, 2006; Miller & Miller, 2006; Buczacki, 2012; Phosri et al., 2013; Desjardin, Wood & Stevens, 2015; Evenson, 2015; Siegel & Schwarz, 2016; Jeppson in Knudsen & Vesterholdt, 2018; Sturgeon, 2018; Læssøe & Petersen, 2019; Kibby, 2020; Cabrera-Rodríguez et al., 2021; McKnight et al., 2021.) Herb. Kuo 10101509, 01152002, 05252201.

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Astraeus hygrometricus

Astraeus hygrometricus

Astraeus hygrometricus

Astraeus hygrometricus

Astraeus hygrometricus

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Kuo, M. (2022, June). Astraeus hygrometricus. Retrieved from the MushroomExpert.Com Web site: