Hygroscopic earthstar

Appearance

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 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.

"This veritable barometer is the most theatrical of all the earthstars. A few minutes immersion in water will open up old, dried-up specimens that seem as tightly closed as clenched fists."

David Arora in "Mushrooms Demystified"

The fruit body is 1–8 cm in diameter from tip to tip when expanded. The exoperidium is thick, and the rays are typically areolate on the upper surface, and are dark grey to black. The spore case is sessile, light gray to tan color and 1 to 3 cm broad with a felt-like or scurfy 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 extend from the base of the fruit body into the substrate. The rhizomorphs are fragile, and often break off after maturity.

The spores are spherical or nearly so, reddish-brown, thick-walled and verrucose. The spores' dimensions are 7–11 µm; the warts are about 1 µm long. The spores are non-amyloid, and will not stain with iodine from Melzer's reagent. The use of scanning electron microscopy has shown that the spines are 0.90–1.45 µm long, rounded at the tip, narrow, tapered, and sometime joined together at the top. The capillitia are branched, 3.5–6.5 µm in diameter, and hyaline. The basidia are four- to eight-spored, with very short sterigmata. The basidia are arranged in long strings of clusters; individual basidia measure 11–15 by 18–24 µm. The threads of the capillitia arise from the inner surface of the peridium, and are thick-walled, long, interwoven, and branched, measuring 3–5.5 µm thick. The exoperidium is made of four distinct layers of tissue: the mycelial layer contains branched hyphae that are 4–6 μm in diameter; the hyphae of the fibrous layer are 6–8 μm diameter and branched; the collenchyma-type layer has branched hyphae of 3–4 μm diameter; the soft layer contains hyphae that are 3–6 μm in diameter.

Naming

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. 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: The treatment of this species by certain taxonomists well illustrates the pitfalls that lie in wait for those who worship at the shrine of ontogenic classification... The only feature of those outlined in which the species differs from others of "Geastrum" is the somewhat primitive hymenium. In the developing plant the glebal cavities are separated by tramal plates so tenuous as to be overlooked by the uncritical worker. Each cavity is filled with basidia somewhat irregularly arranged in clusters and not in the definite palisade of the species which have been studied. This difference disappears as maturity is reached, when plants resemble closely the fructification of any other member of the genus. The taxonomist is then unable to indicate any point of difference by which ""Astraeus"" may be separated from "Geastrum", which indicates that the name should be discarded. 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.; "Geastrum fibrillosum" Schwein.; "Geastrum stellatum" Wettst.; and "Astraeus stellatus" E.Fisch..

"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 accounts for the common name "false earthstar". The specific name is derived from the Greek words ὑγρός "wet" and μέτρον "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". 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 and from the Mediterranean. 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 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.Although "A. hygrometricus" bears a superficial resemblance to member of the "true earthstars" "Geastrum", it may be readily differentiated from most by the hygroscopic nature of its rays. Hygroscopic earthstars include "G. arenarium", "G. corollinum", "G. floriforme", "G. recolligens", and "G. kotlabae". Unlike "Geastrum", the young fruit bodies of "A. hygrometricus" do not have a columella. "Geastrum" tends to have its spore sac opening surrounded by a peristome or a disc, in contrast with the single lacerate slit of "A. hygrometricus". There are also several microscopic differences: in "A. hygrometricus", the basidia are not arranged in parallel columns, the spores are larger, and the threads of the capillitia are branched and continuous with the hyphae of the peridium. Despite these differences, older specimens can be difficult to distinguish from "Geastrum" in the field. One species of "Geastrum", "G. mammosum", does have thick and brittle rays that are moderately hygroscopic, and could be confused with "A. hygrometricus"; however, its spores are smaller than "A. hygrometricus", typically about 4 µm in diameter.

"Astraeus pteridis" is larger, 5 to 15 cm or more when expanded, and often has a more pronounced areolate pattern on the inner surface of the rays. It is found in North America and the Canary Islands. "A. asiaticus" and "A. odoratus" are two similar species known from throughout Asia and Southeast Asia, respectively. "A. odoratus" is distinguished from "A. hygrometricus" by a smooth outer mycelial layer with few adhering soil particles, 3–9 broad rays, and a fresh odor similar to moist soil. The spore ornamentation of "A. odoratus" is also distinct from "A. hygrometricus", with longer and narrower spines that often joined together. "A. asiaticus" has an outer peridial surface covered with small granules, and a gleba that is purplish-chestnut in color, compared to the smooth peridial surface and brownish gleba of "A. hygrometricus". The upper limit of the spore size of "A. asiaticus" is larger than that of its more common relative, ranging from 8.75–15.2 μm. "A. koreanus" differs from the more common form in its smaller size, paler fruit body, and greater number of rays; microscopically, it has smaller spores, and the spines on the spores differ in length and morphology. It is known from Korea and Japan.

Distribution

"Astraeus hygrometricus" is an ectomycorrhizal fungus and grows in association with a broad range of tree species. The mutualistic association between tree roots and the mycelium of the fungus helps the trees extract nutrients from the earth; in exchange, the fungus receives carbohydrates from photosynthesis. In North America, associations with oak and pine are usual, while in India, it has been noted to grow commonly with chir pine and sal. The false earthstar is found on the ground in open fields, often scattered or in groups, especially in nutrient-poor, sandy or loamy soils. It has also been reported to grow on rocks, preferring acid substrates like slate and granite, while avoiding substrates rich in lime. In Nepal, fruit bodies have been collected at elevations of 3,000 m. Fruit bodies typically appear in autumn, although the dry fruit bodies are persistent and may last up to several years. "Gelatinipulvinella astraeicola" is a leotiaceous fungus with minute, gelatinous, pulvinate apothecia, known to grow only on the inner surface of the rays of dead "Astraeus" species, including "A. hygrometricus".

The species has a cosmopolitan distribution except for arctic, alpine and cold temperate regions; it is common in temperate and tropical regions of the world. It has been collected in Africa, Asia, Australia, Europe, North America, and South America.

Habitat

"Astraeus hygrometricus" is an ectomycorrhizal fungus and grows in association with a broad range of tree species. The mutualistic association between tree roots and the mycelium of the fungus helps the trees extract nutrients from the earth; in exchange, the fungus receives carbohydrates from photosynthesis. In North America, associations with oak and pine are usual, while in India, it has been noted to grow commonly with chir pine and sal. The false earthstar is found on the ground in open fields, often scattered or in groups, especially in nutrient-poor, sandy or loamy soils. It has also been reported to grow on rocks, preferring acid substrates like slate and granite, while avoiding substrates rich in lime. In Nepal, fruit bodies have been collected at elevations of 3,000 m. Fruit bodies typically appear in autumn, although the dry fruit bodies are persistent and may last up to several years. "Gelatinipulvinella astraeicola" is a leotiaceous fungus with minute, gelatinous, pulvinate apothecia, known to grow only on the inner surface of the rays of dead "Astraeus" species, including "A. hygrometricus".

The species has a cosmopolitan distribution except for arctic, alpine and cold temperate regions; it is common in temperate and tropical regions of the world. It has been collected in Africa, Asia, Australia, Europe, North America, and South America.