Wednesday, September 26, 2012
The elusive Earthstars
Species name: Geastrum fimbriatum
Common name: fringed earthstar
Since very little is known about the actual abundance of fungal species in the wild, assigning a rarity level or species status to a species of fungi is incredibly difficult. To date there are officially only three globally recognized fungal species as endangered, one of which is located in North America. This is not it, but in general earthstars are deemed to be "not common". This doesn't, however, mean that they're not actually incredibly common! I challenge you to look at the first picture and find it right away. Unless you are of very keen eye and are really looking out for a fungus, you see a tree stump and a bunch of pine needles on the ground and some dirt. Perhaps you notice the sap of the pine tree that used to be there caked around the outside of the stump, but that just would be one more tiny characteristic that would make you think you're looking at a pine stump and nothing else. Such is the case for most species of fungi; they are so non-descript, that unless you're specifically looking for them and documenting every time you find them, they are determined to be "rare" or "uncommon". This is a central theme of my thesis, in fact, since globally the group I work with are considered "rare," but I firmly believe it's only because they are so small (~0.1-0.5 mm across) and grow in a location not many people would be willing to look (underside of well-rotted logs). You know what else grows where my fungi grow? Spiders. Snakes. Salamanders. Frogs. Bugs. And in the tropics, most of these things will kill you if you let them get close enough. So I can see why one might be a little less than excited to go looking for them...
The earthstars in general are very difficult to tell apart, but there are some characteristics to look for in particular. If you find them fresh (I unfortunately didn't) and the body of the fungus is white and the "arms" are pure black, you've actually found a remarkable species called Astraeus hygrometricus. For those of you that are proficient in Latin, you should be able to discern that the second word of this species name implies that it responds to barometric pressure. Pretty silly, right? What fungus can do that?! Well, this one can. It responds to the amount of moisture in the air, opening the arms of the star under times of high humidity (or lots of ground moisture), and closing the arms to different degrees depending on dryness. The clue that I had that what I found was definitely not the barometric false earthstar is that even though these are bone-dry, the arms of the star are still open. If this was truly Astraeus hygrometricus, the arms would be completely closed around the rest of the fruiting body. So this narrows it down to having to be a true earthstar, or in the genus Geastrum.
Within Geastrum, there are only about 5 known species in North America (more in Europe and the tropics; there are about 50 known species in the genus). The key is in the opening of the pore, from which the spores escape (a mechanism like a puffball). I looked closely at the pore (but unfortunately didn't think to take a picture of just the pore opening), and saw that it had the fringe around the opening, meaning that it has to be the fringed earthstar. Easy as pie! I'm sure there are actually significantly more species of Geastrum in North America (and maybe even the genus should be divided into many parts) than just five, but five makes it easy to figure out which is which! Once someone starts to take on the daunting task of sequencing the DNA of every Geastrum ever collected around the world, we might start to get a better picture of how many species there are, and just how they're distributed around the world.