Spore sac on a star-like base [earthstars]

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The mature fruitbody consists of a roughly spherical spore sac atop the centre of a star-like base and the fruitbodies are commonly called earthstars. The arms of the star are very wide near the centre of the star and each tapers outwards to a point. The wall of the spore sac is thin and flexible. In the immature fruitbody the spore sac is enclosed within a thick casing. At maturity that casing splits radially along several lines and the segments then fold out to create the star-like base.

In the genus Astraeus the arms are tough, almost woody when dry, and polygonally cracked (like ‘crazy paving’) on the upper surfaces. The arms are hygroscopic, folding in when dry and opening again when moist. Initially at the top of the mature spore sac there is rough apical hole or tear and this may enlarge.

Most species in this group belong to the genus Geastrum. In this genus the spore sac (from a few millimetres to 3 centimetres in diameter) has a well-defined apical hole of constant size. The spore sac holds the powdery spores within a bundle of entangled filaments (much like a wad of cotton-wool). The wall of the spore sac is thin and flexible with an apical hole through which the spores can puff out when the sac is compressed (e.g. by a raindrop, a falling twig, a finger). When the pressure eases, the compressed ‘cotton-wool’ expands, restoring the sac to its pre-puff shape and so allowing future puffs. Without the ‘cotton wool’, the spore sac would stay compressed and puff no more.

The surface of a Geastrum spore sac is smooth to velvety, the spore sac may be raised on a short stem or be stemless, the arms may be hygroscopic or not, the apical opening may be just a simple hole or a more complex structure, the undersides of the arms may be smooth and clear of soil grains/debris or be coated with a strongly glued layer of soil grains/debris. These are some of the macroscopic features that help identify species.

Warning

If you think you have an earthstar with a spore sac about a millimetre in diameter, check the slime mould Diderma subasteroides.

** Myrisotoma: This looks like an earthstar with a number of holes over the upper half of the spore sac (which is held up by a number of sort, wiry stems). In Australia it has been found only in the Sydney-Newcastle area and in Queensland at Oakey, north-west of Toowoomba. Given those widely separated locations it must occur elsewhere as well. Should you find it in the local region, it would be a very noteworthy sighting! Here is a photo of a Dutch specimen: http://www.anbg.gov.au/fungi/images-captions/myriostoma-coliforme-0213.html.

References

Lepp, H. (2008). Myriostoma coliforme in Queensland, Australasian Mycological Society Newsletter, No. 4, 7.

Rees, B. J., Taeker, F., & Coveny, R. G. (2005). Myriostoma coliforme in Australia, Australasian Mycologist, 24, 25-28.

Sousa, J.O., Baseia, I.G. & Martín, M.P. (2019). Strengthening Myriostoma (Geastraceae, Basidiomycota) diversity: Myriostoma australianum sp. nov., Mycoscience, 60, 25-30.

Spore sac on a star-like base [earthstars]

Featured sightings

Geastrum sp. (genus) (An earthstar) at Paddys River, ACT - 21 Jun 2020 by AaronClausen

Geastrum tenuipes (An earthstar) at Burra, NSW - 8 Jul 2017 by Woowoolahra

Geastrum lageniforme s.l. at Cotter River, ACT - 25 May 2017 by KenT

Latest sightings

Geastrum sp. (genus) (An earthstar) at Paddys River, ACT - 1 May 2025 by TimL

Geastrum sp. (genus) (An earthstar) at Karabar, NSW - 4 May 2025 by Clarel

Geastrum sp. (genus) (An earthstar) at Yarralumla, ACT - 1 May 2025 by Jennybach

Geastrum sp. (genus) (An earthstar) at Kambah, ACT - 23 Apr 2025 by debhart

Astraeus hygrometricus (Barometer Earthstar) at Reid, ACT - 22 Apr 2025 by Pirom

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Discussion

Heino1 wrote:

28 Oct 2024

These are photos of dried herbarium specimens. However, these fruitbodies looked virtually the same when they were collected. In the first photo you see several mature fruitbodies, one upside down showing the densely encrusted soil/debris that is bound to the underside. The second shows a close view of the fibrillose peristome. In the third you see the spore sac atop a short stalk and also the dense layer of pale calcium oxalate crystals over the surface of the spore sac. The fruitbodies in the fourth photo are in the process of opening out to create the star-like bases. The fruitbodies formed beneath a layer of debris below some Casuarinas.

Geastrum austrominimum

Heino1 wrote:

1 Oct 2024

The photos show a dried, herbarium collection with the fruitbodies viewed from both the upper sides and the lower sides.

Geastrum lageniforme s.l.

Heino1 wrote:

1 Oct 2024

The first photo shows a dried, herbarium collection. In the top central fruitbody part of the brown fleshy layer has shrunk and split to reveal some of the middle, whiteish fibrous layer. At the lower left you see the white, radial streaks on the underside of the arms, bits of the fibrous layer that show through the radially cracked mycelial layer (as explained at: https://canberra.naturemapr.org/species/21877- and there’s a closer view in the second photo).

Geastrum lageniforme s.l.

Heino1 wrote:

1 Oct 2024

The photo shows a dried, herbarium collection consisting of four unopened fruitbodies and one that’s mature and open. Two of the unopened fruitbodies have pronounced ‘beaks’ and, overall, a distinctly lageniform shape. Three of the unopened ones still have some white, thread-like rhizomorphs attached at their bases. The inset photos show some of the crystals found on the rhizomorphs (and these are briefly mentioned in my comments about this species: Geastrum lageniforme s.l.).

Geastrum lageniforme s.l.

LisaH wrote:

13 Jul 2024

Thank you - this is the largest one I’ve seen, and it had an other-worldly look about it!

Astraeus hygrometricus