Most fruiting bodies have evolutionarily opted for traits associated with wind dispersal. Gilled euagarics, and pored boletes drop billions of spores that, if all goes according to plan, catch rising air currents that transport them miles away from their parent. There is no doubt that this mechanism works really well, considering its conservation across the eons that these organisms have been on Earth. Though, a common trend seen not only in mycology but riddled throughout nature is that there is always another way to carry out a biological function. Yes, effective strategies evolve, and may cause a widespread adaptive radiation like the original gilled mushrooms, but other mechanisms have evolved since then that too accomplish the fruiting bodies goal of dispersal. Here, I’d like to talk about the ecology of different bird’s nest fungi, as well as their mechanisms of spore dispersal.
The family Nidulariaceae contain five genera of bird’s nest forming fungi, all of which use rain to propel their peridioles away from their parent fungus. Now recently, genetic analysis has been moving some genera of the Nidulariaceae to the massive family Agaricaceae. As always, fungal taxonomy is always being shuffled around. It is something we should be used to by now. Regardless, they all use rain, but the way they use it differs between genera. Using high-speed cameras, Maribeth Hassett and her team for the first time precisely describe the mechanisms and forces at play with this dispersal mechanism.
Surprisingly, this mechanism of splash dispersal in fungi has only been first described in the 1920’s. These bird’s nest fungi must have gone under the radar until then, with mycologists assuming that wind played the only role in their dispersal. Before we get into the mechanism, we must fully understand the sum of their parts. These fungi are definitely on the small side, which at first glance, looks like a miniature bird’s nest-eggs included. We refer to the entire structure as the basidiome, with the largest, funnel shaped ‘nest’ structure known as the peridium. Inside the peridium lies egg like structures called peridioles, each encasing millions of fungal spores. These structures are found in all bird’s nest fungi, though there are more structures found in the genera that launch their peridioles the farthest.
In 2013, Hassett and her team looked into two of the larger, more robust genera in the bird’s nest family, including species of Crucibulum and Cyathus. They found that the experimental raindrops that hit the fungus off center, translated the most force for peridiole expulsion. (Click this link to see this mechanism in action.) Drops that landed dead center still dispersed spore filled peridioles, but significantly shorter distances compared to off-center droplet strikes. These genera also differ in morphology, as peridioles have additional tether-like appendages that adhere to vegetation.
Upon peridiole launch in species of Crucibulum and Cyathus, the tether structure also known as the funicular cord remains condensed beneath the peridiole. At the base of the funicular cord there is an adhesive structure called a hapteron. Once the hapteron comes in contact with vegetation, it sticks to it, and the funicular cord is deployed. The funicular cord initially acts as a break, then the momentum of the launch causes the entire structure to swing around the vegetation like a tetherball, where it becomes safely secured. (Another video showing mechanism.) Now that the mechanism for these genera is described, we can tie it together with their ecology.
Species of Crucibulum and Cyathus are saprobes that digest wood. Although they can utilize wood, many of these species prefer a coprophilous lifestyle, meaning that their most suitable substrate is feces. Secured to vegetation via funicular cord, these species increase their chances of being ingested by an herbivore, where they pass through the digestive tract, ultimately having fresh substrate to themselves. This noteworthy launch mechanism is analogous to other coprophilous fungi, like Sphaerobolus and Pilobolus. All of these genera have the same strategy, attaching to high regions of vegetation, where herbivore consumption is most probable so they can colonize the fecal substrate before excretion. This ultimately enhances their competitive edge, as other individuals that were not consumed with vegetation, must disperse to droppings directly, where their chances of germination are far less especially if there is already a species colonizing the substrate. The rounded cup structures in these bird’s nest fungi have been selected for to launch peridioles high into the air. Large horizontal distances don’t enhance fitness much if the peridiole attaches at the base of vegetation, since most gazing herbivores favor the newly grown, less fibrous plant material located at the top.
Bird’s nest fungi from the genera Nidularia are smaller, less robust species that don’t have funicular cords. These species are less coprophilous, preferring a woody substrate. Much of the time, you will find these species in beds of woodchips. Unlike the species of Crucibulum and Cyathus, Nidularia species have less defined peridiums, and the most effective peridiole dispersal results from direct, center strikes of water droplets. Another study carried out by Hassett and her team the following year looked into this mechanism as well. They found that the launch speed of Nidularia species was one third the velocity recorded from Crucibulum laeve and Cyathus striatus from the previous study. For Nidularia species, fitness is not enhanced by structures that maximize peridiole distance. Instead of investing energy to the production of other structures, Nidularia species allocate their resources to the formation of peridioles. Each one of these tiny fruiting bodies contain up to 100 peridioles, compared to 15 in other Crucibulum species. These wood chip loving fungi are most likely inhabiting an area with plentiful substrate, so traits that maximize distance of dispersal have not been selected for.
An important thing to note is that the fungal spores in all bird’s nest fungi are not just confined to their peridioles. Remember, millions and millions of spores are in here that can be taken away by wind once the peridiole dries and ruptures. In fact, the peridioles of the funicular cord producing species that attach to elevated vegetation that aren’t consumed by an herbivore, still have an edge because over time, the peridiole will rupture. Its spores will have a better chance of wind dispersal with its increased distance from the forest floors still boundary layer.
These species have an elegant evolutionary design that I finally understand. Bird’s nest fungi utilize just a fraction of the kinetic energy provided by raindrops. Some structures including enhanced ‘nest’ structures called peridiums and funicular cords highlight the species coprophilous lifestyle, while smaller, less robust species with huge quantities of peridioles reveal their saprobic ecology. We always are finding that species maximize their fitness by selecting traits that provide their potential offspring with the best chance to grow and reproduce. In a way, these bird’s nest fungi kind of have dual dispersal-peridiole expulsion to selectively disperse to a nearby substrate, or wind dispersal once the peridiole ages after its not eaten, or falls outside of the mulch pile.