Astraeus hygrometricus

Astraeus hygrometricus, commonly known as the hygroscopic earthstar, the barometer earthstar, or the false earthstar, is a species of fungus in the family Diplocystaceae. Young specimens resemble a puffball when unopened. In maturity, the mushroom displays the characteristic earthstar shape that is a result of the outer layer of fruit body tissue splitting open in a star-like manner. The false earthstar is an ectomycorrhizal species that grows in association with various trees, especially in sandy soils. A. hygrometricus was previously thought to have a cosmopolitan distribution, though it is now thought to be restricted to Southern Europe, and Astraeus are common in temperate and tropical regions. Its common names refer to the fact that it is hygroscopic (water-absorbing), and can open up its rays to expose the spore sac in response to increased humidity, and close them up again in drier conditions. The rays have an irregularly cracked surface, while the spore case is pale brown and smooth with an irregular slit or tear at the top. The gleba is white initially, but turns brown and powdery when the spores mature. The spores are reddish-brown, roughly spherical with minute warts, measuring 7.5–11 micrometers in diameter. Despite a similar overall appearance, A. hygrometricus is not related to the true earthstars of genus Geastrum, although historically, they have been taxonomically confused. The species was first described by Christiaan Hendrik Persoon in 1801 as Geastrum hygrometricus. In 1885, Andrew P. Morgan proposed that differences in microscopic characteristics warranted the creation of a new genus Astraeus distinct from Geastrum; this opinion was not universally accepted by later authorities. Several Asian populations formerly thought to be A. hygrometricus were renamed in the 2000s once phylogenetic analyses revealed they were unique Astraeus species, including A. asiaticus and A. odoratus. Similarly, in 2013, North American populations were divided into A. pteridis, A. morganii, and A. smithii on the basis of molecular phylogentics. This research suggests that the type specimen of Astraeus hygrometricus originates in a population restricted to Europe between Southern France and Turkey, with A. telleriae found nearby in Spain and Greece. Research has revealed the presence of several bioactive chemical compounds in Astraeus fruit bodies. North American field guides typically rate A. hygrometricus as inedible. Because this species resembles the earthstar fungi of Geastrum, it was placed in that genus by early authors, starting with Christian Hendrik Persoon in 1801 (as Geaster, an alternate spelling of Geastrum). According to the American botanist Andrew P. Morgan, however, the species differed from those of Geastrum in not having open chambers in the young gleba, having larger and branched capillitium threads, not having a true hymenium, and having larger spores. Accordingly, Morgan set Persoon's Geaster hygrometricum as the type species of his new genus Astraeus in 1889. Despite Morgan's publication, some authorities in the following decades continued to classify the species in Geastrum. The New-Zealand based mycologist Gordon Herriot Cunningham explicitly transferred the species back to the genus Geastrum in 1944, explaining: Cunningham's treatment was not followed by later authorities, who largely considered Astraeus a distinct genus. According to the taxonomical authority MycoBank, synonyms of Astraeus hygrometricus include Lycoperdon stellatus Scop. (1772); Geastrum fibrillosum Schwein. (1822); Geastrum stellatum (Scop.) Wettst. (1885); and Astraeus stellatus E.Fisch. (1900). Astraeus hygrometricus has been given a number of colloquial names that allude to its hygroscopic behavior, including the 'hygrometer earthstar', the 'hygroscopic earthstar', the 'barometer earthstar', and the 'water-measure earthstar'. The resemblance to Geastrum species (also known as true earthstars) accounts for the common name 'false earthstar'. The specific name is derived from the Greek words ὑγρός (hygros) 'wet' and μέτρον (metron) 'measure'. The German Mycological Society selected the species as their 'Mushroom of the Year' in 2005. Studies in the 2000s showed that several species from Asian collection sites labelled under the specific epithet hygrometricus were actually considerably variable in a number of macroscopic and microscopic characteristics. Molecular studies of the DNA sequences of the ITS region of the ribosomal DNA from a number of Astraeus specimens from around the world have helped to clarify phylogenetic relationships within the genus. Based on these results, two Asian 'hygrometricus' populations have been described as new species: A. asiaticus and A. odoratus (synonymous with Petcharat's A. thailandicus described in 2003). Preliminary DNA analyses suggests that the European A. hygrometricus described by Persoon is a different species than the North American version described by Morgan, and that the European population may be divided into two distinct phylotypes, from France (A. hygrometricus) and from the Mediterranean (A. telleriae). A follow-up analysis from 2013 named two new North American species: A. morganii from the Southern US and Mexico and A. smithii from the Central and Northern United States, and grouped western US specimens in A. pteridis. A 2010 study identified a Japanese species, previously identified as A. hygrometricus, as genetically distinct; it has yet to be officially named. A form of the species found in Korea and Japan, A. hygrometricus var. koreanus, was named by V.J. Stanĕk in 1958; it was later (1976) published as a distinct species—A. koreanus—by Hanns Kreisel. As pointed out by Fangfuk and colleagues, clarification of the proper name for this taxon must await analysis of A. hygrometricus var. koreanus specimens from the type locality in North Korea. Young specimens of A. hygrometricus have roughly spherical fruit bodies that typically start their development partially embedded in the substrate. A smooth whitish mycelial layer covers the fruit body, and may be partially encrusted with debris. As the fruit body matures, the mycelial layer tears away, and the outer tissue layer, the exoperidium, breaks open in a star-shaped (stellate) pattern to form 4–20 irregular 'rays'. This simultaneously pushes the fruit body above ground to reveal a round spore case enclosed in a thin papery endoperidium. The rays open and close in response to levels of moisture in the environment, opening up in high humidity, and closing when the air is dry. This is possible because the exoperidium is made of several different layers of tissue; the innermost, fibrous layer is hygroscopic, and curls or uncurls the entire ray as it loses or gains moisture from its surroundings. This adaptation enables the fruit body to disperse spores at times of optimum moisture, and reduce evaporation during dry periods. Further, dry fruit bodies with the rays curled up may be readily blown about by the wind, allowing them to scatter spores from the pore as they roll. The fruit body is 1–8 cm (0.4–3.1 in) in diameter from tip to tip when expanded. The exoperidium is thick, and the rays are typically areolate (divided into small areas by cracks and crevices) on the upper surface, and are dark grey to black. The spore case is sessile (lacking a stalk), light gray to tan color and 1 to 3 cm (0.4 to 1.2 in) broad with a felt-like or scurfy (coated with loose scaly crust) surface; the top of the spore case is opened by an irregular slit, tear or pore. The interior of the spore case, the gleba, is white and solid when young, and divided into oval locules—a characteristic that helps to distinguish it from Geastrum. The gleba becomes brown and powdery as the specimen matures. Small dark hairlike threads (rhizomorphs) extend from the base of the fruit body into the substrate. The rhizomorphs are fragile, and often break off after maturity.