#043: Mushroom Morphology: Bird’s Nest Fungi
These small (< 1.5cm) fungi are easily recognizable by their striking resemblance to a bird’s nest with eggs inside. The “eggs” are actually spore-containing sacks called peridioles. As in the other gasteromycetes, the bird’s nest fungi produce spores internally and have lost the ability to forcibly discharge their spores. To overcome this obstacle, the bird’s nest fungi developed their unique morphology to act as a splash cup. The basic splash cup mechanism works as follows: a raindrop falls into the cup, which propels one of the peridioles (“eggs”) out of the cup. The next challenge for the bird’s nest fungi is to stick to whatever they land on. These fungi have evolved two different mechanisms in order to accomplish this. Fungi in the genera Mycocalia, Nidula, and Nidularia have peridioles that are covered in sticky mucilage, which allows them to adhere to surfaces on contact. Fungi in the genera Crucibulum and Cyathus, however, have dry peridioles with specialized structures attached to the bottom of each peridiole. The first of these structures is a long cord of hyphae called the funiculus that is coiled up inside a thin membrane called the purse. At the end of the funiculus (cord) is a counterweight called the hapteron. The hapteron is attached to the internal membrane of the nest by hyphae that pass through the purse. Exactly how all these structures allow the peridiole to attach to a surface is not quite clear. There are three ways in which this could be accomplished. The first possibility is that the purse ruptures when the peridiole is splashed out, which causes the counterweight to extend during flight. When the peridiole hits something the counterweight then wraps the cord around the substrate, much like what happens when you throw a pair of tennis shoes at a power line. The second possiblility once again starts with the counterweight being extended during flight. This time, however, the counterweight is sticky and becomes glued to a substrate. This causes the peridiole to wrap the cord around the substrate. The third hypothesis is that the sticky counterweight remains in the purse until it contacts a substrate during flight. The counterweight becomes glued to the substrate, causing the purse to rupture. Once a peridiole from any bird’s nest fungus becomes attached to a substrate, it degrades to release the spores. The tops of immature bird’s nest fungi fruiting bodies are covered by a protective membrane known as an epiphragm. This makes the immature mushrooms look more like bright dots than nests. At maturity, the epiphragm breaks down either through a mechanical process or by microbial degradation to reveal the cup filled with peridioles.
Another mushroom worth discussing here is Sphaerobolus stellatus (literally “star-shaped sphere thrower”), the “Cannonball Fungus.” This unique fungus produces tiny (< 0.2cm), star-shaped fruiting bodies. Each fruiting body contains exactly one peridiole, which is expelled using a mechanical catapult process (unlike the bird’s nest fungi, which use a splash cup mechanism). The peridiole is held in a depression at the center of the fruiting body. At maturity, the depression suddenly pops up and launches the peridiole up to 18 feet away. This catapult action is somewhat similar to the way the button on the lid of a sealed jar pops up when you open the jar. You can trigger the Cannonball Fungus to fire by shining a bright light directly at the fruiting body. The peridiole is sticky and easily becomes stuck to whatever surface it lands on. People mostly notice these fungi when they pepper cars or siding on houses with their tiny, black “cannonballs.”
The bird’s nest fungi form a monophyletic group that is usually placed in the family Agaricaceae (along with the gilled mushroom Agaricus bisporus and the puffballs) or in their own family Nidulariaceae. Both of these families are placed in phylum Basidiomycota, class Agaricomycetes, and order Agaricales. This means that their closest relatives are gilled mushrooms and puffballs. All of the bird’s nest fungi are saprobic and can be found decomposing various types of dead wood. The splash cup mechanism works best in open areas, so you are most likely to encounter them growing in mulch. Although the fruiting bodies themselves are small, they are often found growing in large clusters, sometimes completely covering surfaces. When growing in mulch, they are frequently observed fruiting in fairy rings.
The genus Sphaerobolus only contains two or three species. These few species are the only fungi with the cannonball morphology. These fungi are placed in the phylum Basidiomycota, class Agaricomycetes, order Geastrales, and family Geastraceae. This indicates that their closest relatives are the earthstars. All species of cannonball fungi are saprobic and can be found decomposing all kinds of wood. Despite being discussed in the same post, the cannonball fungi are only distantly related to the bird’s nest fungi.