Barometer Earthstar (Astraeus smithii)
- Aubrey
- Mar 25
- 4 min read
Good evening, friends,
I took advantage of the rain yesterday and got out to look for mushrooms in the Frances A. Crane Wildlife Management Area in Falmouth, MA. I sought out this preserve in particular because the state conducted a prescribed burn there last spring. After all the fires the northeast experienced last fall, I’m especially interested in the fungi that can be found after fires. I’m going to hit a lot of burn sites over the coming year and this was just the start.
These barometer earthstars (Astraeus smithii) were popping out of the scorched, sandy soil but certainly resembled something a bit more extraterrestrial.
Fun Facts
These mushrooms are not your typical toadstool which leads to a unique method of spore dispersal. When dry, those arm-like projections (the rays, the exoperidium) hug the spore sac (the endoperidium) while the mushroom waits for rain. When it finally rains, the mushroom hydrates and the rays unfurl to raise the spore sac off the ground, promoting spore distribution. When the mushroom dries out, the rays shrivel up to hug the spore sac again. This weather-sensitive movement leads to the name “barometer earthstar”.
The cycle of curling and unfurling repeats until all the spores are distributed — you’ll frequently find the outer rays laying around in the sand without any trace of the spore sac inside.
Astraeus is derived from the Greek word for “star” and Astraeus is also the Titan of astronomy and astrology in Greek mythology. All this celestial nomenclature is a reference to the star-like rays of the mushroom. The species epithet Smithii is named for the mycologist Alexander H. Smith; he was a mycologist and curator of the herbarium at the University of Michigan in the mid-1900’s.
An Asian species of Astraeus is cultivated and eaten in Southeast Asia. They harvest the mushrooms in their infancy when the spores haven’t developed and the interior is firm, not powdery. You can buy them pickled in a can. One study even found that the fungus inhibited the bacteria responsible for causing Leishmaniasis, a parasitic disease transmitted by horseflies (Reference 4).
Ecology
The mushroom is ectomycorrhizal and forms a nutrient exchange with plants where the fungus wraps around the plant’s root tips. The most predominant plants in this area were pitch pines (Pinus rigida) and some sort of Andropogon or Schizachyrium bunch grass (grass that grows in clumps rather than spreading horizontally like turf).
They grow summer through fall in sandy soils, and overwinter in more temperate locales. I believe the mushrooms I found grew in the fall, some were rather haggard and had lost their spore sacs entirely.
All barometer earthstars used to be called Astraeus hygrometricus before a few papers (Phosri et al, 2007; Phosri et al, 2013) parsed out unique species based on geographic distribution. In North America there are theoretically three: A. smithii in the northern part of the continent, A. morganii in the southern part of the continent, and A. pteridis west of the Rockies.
Of course, the authenticity of these species is disputed and the legitimacy of the papers describing them is contentious (Michael Kuo of mushroomexpert.com is not a fan). It’s all above my pay grade, what the fungi are doing in the environment is more interesting to me.
Astraeus mushrooms look like “true earthstars” (Geastrum) but they’re not at all related to them. The big differences are that true earthstars are saprobic (decomposers) and their outer rays only open once. Hygroscopic earth stars are mycorrhizal and can open and close their rays based on moisture.
The fact that the two are similar in shape and structure, but not remotely related, is an example of convergent evolution — two unrelated organisms developing similar characteristics based on similar environmental pressures. Interestingly, Astraeus mushrooms are closely related to Boletes (your porcini/ceps).
Speaking of true earth stars and Geastrum, the t-shirts have arrived and will start going out to paid subscribers. I’ll work on figuring out individual t-shirt sales at some point, give you an option to buy one on mushroommonday.com, but don’t hold your breath because I can barely send an email. Right now these are just a treat for the people who’ve thrown me a few bones.
A month from today I will be doing a workshop and walk at Wave Hill in the Bronx as part of their Earth Day weekend festivities. It’s on a Friday which is great because it gives you a reason to take work off and start the long weekend you just made with a little mushroom walk and cultivation workshop. I don’t want to speak for the Earth here, but I imagine that she’d be pretty stoked if that’s how you decide to celebrate her big day.
Speaking of stars, there’s a new moon and partial solar eclipse this Saturday. The eclipse is early in the morning, peaking right after sunrise, and it’s supposed to be rainy so we’ll see,
Aubrey
References:
https://blog.mycology.cornell.edu/2014/06/03/twinkly-earthstars/
Kuo, M. (2024, February). Astraeus hygrometricus. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/astraeus_hygrometricus.html
Lai TK, Biswas G, Chatterjee S, Dutta A, Pal C, Banerji J, Bhuvanesh N, Reibenspies JH, Acharya K. Leishmanicidal and anticandidal activity of constituents of Indian edible mushroom Astraeus hygrometricus. Chem Biodivers. 2012 Aug;9(8):1517-24. doi: 10.1002/cbdv.201100272. PMID: 22899612.