The woolly oyster (Hohenbuehelia mastrucata) is a rare saprotroph found in the eastern United States. It has also been observed throughout Europe and Siberia, decomposing broadleaf trees like beech, hazel, birch and maple. It has a special taste for breaking down and consuming maples, but like other species within its family, it has another strategy for nutrient acquisition.
Hohenbuehelia mastrucata illustration.
Like several other species in the Pleurotaceae, the woolly oyster is nematophagous. Unsuspecting nematodes squirming around in dead broadleaf trees come in contact with sticky traps made out of fungal conidia. There’s a diverse array of fungal structures that evolved to trap nematodes, including hyphal nets, constricting lassos, and the sticky knobs H. mastrucata creates. Once the nematode is stuck in place, mycelial strands penetrate the organism, and exude enzymes that break down nematode tissue from the inside-out. The nitrogen rich slurry then becomes absorbed by the fungus.
A nematophagous fungi utilizing a constricting lasso.
This is an advantageous adaptation for saprotrophic fungi to utilize. Their woody substrate is extremely nutrient poor. On some the broadleaf trees the woolly oyster breaks down, carbon to nitrogen ratios (C:N) can exceed 1,200:1! That is, for every one molecule of nitrogen, there are 1,200 molecules of carbon. This staggering nutritional unevenness is balanced by a nematophagous ecology.
H. mastrucata and its sticky conidia (arrows and inset image) adhering to nematode. Thorn and Barron 1984.
Nematode trapping fungi are analogous to carnivorous plants growing in nutrient poor habitats. These fitness enhancing adaptations allow species to inhabit places with high C:N ratios. Not only does woody debris have a high C:N ratio, but the enzymes needed to break down lignin and cellulose are increasingly expensive to synthesize. An adequate evolutionary response to living in a nutrient poor substrate is the acquisition of a nematophagous ecology. The woolly oyster, along with other species within the Pleurotaceae have done just that. There have been over 150 nematophagous fungi found throughout Earth’s ecosystems, and that number is likely to increase. This is without a doubt a successful strategy to make a living from the forest floor.
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