Cannonball Fungus

Image: Malcolm Storey, bioimages.org.uk

It's the KABOOM mushroom!

Get those safety goggles on - it's all fun and games until someone gets shot in the eye by a mushroom. We're carefully looking at a couple of species usually known as Cannonball Fungi. They also go by other names, like Artillery Fungi, Shotgun Fungi or, more whimsically, Shooting Stars. Even their genus name, Sphaerobolus, comes from the Greek words for "sphere" and "to throw".

You HAVE been warned! Repeatedly.

Cannonball Fungi can be found across the world, growing on moist, fragrant and edible things like dung and rotting wood. Fragrant and edible in their point of view. I would respectfully disagree.

Image: Kathie Hodge
15 Cannonball Fungi in the hand are worth... a bunch of others... somewhere else

They're absolutely tiny! A little more or a little less than 2 millimetres across. How could such a puny pixie-cannon possibly trouble the cleanliness of my house windows?

Let's see shall we?

The fruiting bodies of Cannon Fungi start off looking like little spheres. If you could see inside, you'd see that it's really a kind of spherical barrel lined with an inner membrane and half full of fluid. Resting in this fluid is the peridiole, a capsule (or cannonball) containing the spores.

Image: Malcolm Storey, bioimages.org.uk

The fluid ensures the peridiole doesn't stick to the inner lining, and it's the spores that will hopefully grow into new Cannonball Fungi. The plan here is to fling that peridiole as far as possible to give the spores a good chance of landing somewhere nice, like on dung or rotting wood.

The inner lining isn't firmly attached to the barrel. There's a thin space almost all the way around. The only place where the lining really does stick to the barrel is at the top, what you might call the lid.

Eventually, the sphere starts to crack open in the zigzag pattern that will eventually make up the star shape. The inner lining is now attached only at the tips of each of the 8 or so points. The cells that make up the lining begin to absorb water...

Image: Malcolm Storey, bioimages.org.uk

They throb and they bulge, they swell and pulsate; they threaten to break through every constraint.

Eventually, she cannae take any more....

Video: Myxomycetes Italiani

KABOOM!

Image: W. G. Smith

The inner lining suddenly everts. It goes from an innie to an outie.

Image: Malcolm Storey, bioimages.org.uk

What was once a bowl becomes a kind of crystal ball.

This eversion takes about one thousandth of a second, and the little peridiole that had been quietly bathing in what was once a bowl is thrown into the air at speeds of 3 to 5 metres per second (up to 11 mph). It lands some 6 metres (20 feet) away, which ain't bad for a 2 mm cannon!

The Cannonball Fungus is also attracted to light so it grows in such a way as to aim for the sun when it fires. It's unlikely that they'll ever hit it (though I like their ambition), but at least they won't fire their precious load straight into a tree that's right next to them.

Image: Malcolm Storey, bioimages.org.uk
Sticky peridioles

When the peridiole finally lands, it does so with a *splat* rather than the bone-crunch and wall-smash of a normal cannonball. It sticks fast to whatever surface it lands on, be it a leaf, a freshly-painted house or the eye protection you're so glad you put on.

The peridiole soon dries up and serves as a protective shield for the spores within. They may not need it; if they landed on something tasty they can just germinate immediately. If not, then hopefully they're on a leaf that'll get eaten by a herbivore. When they survive a trip through the gut and come out the other end, they'll definitely be embedded deep within a fresh serving of something moist, fragrant and edible. In their opinion.

Image: Malcolm Storey, bioimages.org.uk

The spores aren't in any real hurry, though. They can stay in their dried up peridiole for years if they need to. Some have been seen to germinate a whole decade after they were first fired from the mother-cannon. They might stay stuck to your house for that long, too. They're extremely difficult to remove, especially once they dry up. People talk about carving them off with a razor blade! They're also very dark in colour and Cannonball Fungi tend to emerge in clusters rather than solitary individuals, so they result in a whole peppering of dark blobs.

I think it's amazing that a tiny fungus of all things can reach up from a patch of mulch and spray a house with unsightly and almost irremovable spots. I'm inspired! It's also really cool that the Cannonball Fungus is so concerned with their placement. It takes a lot of preparation for that 1/1000th of a second to go right.