In tropical places on Earth, there is an outrageous amount of diversity. For an ecologist, mycologist, botanist, or any naturalist, stepping into a rainforest is an overwhelming experience to say the least. If you take on a long hike, you have to kind of put the blinders on if you plan on making it to your destination. If you don’t, you will literally stop every 6 steps to admire some species remarkable adaptations, or relationships with its surrounding biota. When the long and dreary winter season grips my soul here in Western NY, I yearn to be in the shadows of a warm and humid tropical rainforest. With the dense canopy overhead, one gets the feeling of being truly isolated; disconnected from the concrete jungle of the cities and suburbs that I live in. Once submersed in these fantastic rainforests, I feel like I’m on the cusp of discovery. Though I may ‘discover’ several already described species, they are without a doubt new to me. One of the species I pretty much always encounter on my rainforest adventures is the species featured in today’s edition of Fungi Friday. Let us both become better acquainted with Marasmius crinis-equi, more commonly known as the horsehair fungus. 

I commonly stepped over, maybe even stepped on this fungus while hiking in Central America. They are exceedingly tiny little mushrooms with long and thin black rhizomorphs. These rhizomorphs are key players in their ecology which I will describe later. Before I get into the specifics of this mushroom, I have to tackle a common misconception about rainforests. These are biodiversity hotspots, so competition between species is tremendously fierce. The three main factors that promote biodiversity are the longer annual photoperiods near the equator, the optimal temperatures for a wide array of species, and the large amounts of precipitation these ecosystems receive. Not the available nutrients!! The soils you find in tropical rainforests, contrary to popular belief, are severally depleted of nutrients. 

In these places, plants and fungi absorb nitrogen and phosphorus as soon as it becomes available. There’s so much life competing for nutrients, and decomposition rates are so rapid in these warm and wet environments that fallen animal and plant debris is quickly broken down and assimilated into living structures. Nutrients that aren’t taken up right away are soon washed away with the constant intense rains. This becomes evident as soon as you first step foot in a tropical rainforest. The roots that grow in the temperate systems I’m native too remain hidden in the soils depths. Roots in these rainforest slither along the forest floor’s surface, since that’s where the nutrients are found. There’s also no reason to roots to burrow for water either because of its availability. 

With this in mind, let’s get into this species ecology. It is a saprotroph, so it feeds on dead organic plant matter. These resources are a hot commodity, and because of the intense competition for the nutrients held within the fallen leaves and twigs, this species has had some interesting adaptations that have greatly enhanced its fitness. Unlike the rhizomorphs of the honey mushrooms, (Armillaria sp.) that radiate outward beneath the forest floor to infect another tree, Marasmius crinis-equi extends its rhizomorphs above the forest floor and actually catches leaves and twigs before the hit the ground. 

The horsehair fungus outcompetes other species by taking resources out of thin air. By doing so, other hungry species waiting on the rainforest’s floor never even get a chance to compete. Marasmius crinis-equi and its net-like rhizomorphs are extremely successful because of this adaptation. This trait evolved in this species because of the nutrient poor soil and high level of competition associated with these biodiversity hotspots. Because this mushroom is so abundant in tropical rainforests and has such a tiny body size it is usually overlooked. If you live in a tropical place, or are planning a trip, keep on the lookout for these voracious, debris catching fungus or its similar looking relatives.