Mighty Mites and Nifty Mushrooms

© Else C. Vellinga
Original publication: Mycena News, February 2006

Those plants with seeds that stick to our cat’s fur have really come up with a nice way to get their offspring to new places. It works well for all fur bearing mammals and those naked ones who wear clothes. Some mushrooms have come up with the same brilliant idea: make spores that stick to hairs, and let an animal disperse them.

Consider Tomentella sublilacina. This fungus forms fuzzy grey-purple crusts on the underside of pine branches lying on the forest floor. It is extremely common in the coastal Bishop pine forests of Point Reyes and Salt Point where it is one of the dominant ectomycorrhizal species. After the 1995 Mount Vision fire it immediately colonized the newly germinated Bishop pine seedlings. However, it is not restricted to coastal California. It can be found on a wide range of host trees, and has been recorded all through North America, and in Eurasia from the Atlantic Ocean to Kamchatka and from the Caucasus to the northern Ural.

There is not much breeze close to the forest floor, so relying on wind to disperse its spores wouldn’t get Tomentella far. How then is it possible for such a species to be so widely distributed?

The answer depends on the little creeping, crawling, racing animals, of which forests and their soils are full. To prepare for their ride the spores of this and all other Tomentellas have developed irregular shapes with knobs and spines. For protection on the way, they are thick-walled and pigmented.

These spores adhere particularly well to the hairs of Oribatid mites (a group of mites especially species-rich in woody settings). The mites walk over the Tomentella and, acting like little lawn mowers, eat spores and hyphae as they go. Inside and out they become covered in spores. The mites being small do not get far; they certainly do not disperse the spores from one continent to another. However, they get eaten, and other critters like millipedes, centipedes, salamanders, and beetles acquire the spores at second hand. The nice touch is that the Tomentella spores are pretty good at surviving the passage through two digestive systems and a reasonable percentage remains viable. An experiment with millipede frass on young pine seedlings resulted in ectomycorhization of the growing roots, though it developed slowly. This showed that these spores are good at surviving hostile environments, like stomachs, and soils on fire.

That mites are good at transporting propagules has been shown in lichens as well. Lichens of course demand more because they consist of two organisms, a fungus, and an alga (or a blue-green alga, or both). Mites which graze on the yellow wall lichen, Xanthoria parietina, digest bits of the fungus and of the alga, both of which are still able to germinate after passage throughout the mite’s intestinal tract. This of course is of great advantage to the lichen, as the two constituents of the organism are close when they are ready to commence a new life together.

Other studies have focused on the oribatid mites themselves and what they do. They are everywhere in the soil. So many species, with so many individuals, exist in one habitat, that there must be some kind of differentiation in food, microhabitat, or life cycle. In one German forest 120 species were encountered, with densities up to 400,000 individuals per square meter! Some mites are litter feeders, but others specialize on fungi. Do these species indiscriminately eat any kind of fungus, or do they have preferences? To figure this out, several different mites were presented with a choice of fungi served up as little disks of mycelium (the vegetative part of the mushroom, which is underground and not visible to the naked eye). Only a few species of mites were used in this experiment, but they had strikingly different tastes; Boletus badius was eaten more than the other species offered, but one species of mite preferred the ericoid mycorrhizal species Hymenoscyphus ericae while Agrocybe species, Paxillus involutus and Amanita muscaria were shunned completely by another of the mite species. However, problems with equalizing mite motivation and appetite for the fungal feasts, cloud these results.

In the case of Tomentella, the fungivores serve the fungus by dispersing its spores. But in general, fungi not unreasonably put up defenses against being eaten. Fruitbodies (the mushrooms) are ephemeral. The long-lived part of a fungus which must be preserved consists of its underground hyphae, which can be covered in calcium oxalate crystals, the way walls are covered in barbed wire and broken glass. In the fruitbodies themselves, crystals on cystidia may just be a remnant or by-product of this or a similar hyphal defense mechanism. Thick cell walls, though desirable for resisting digestion, can decrease palatability as can acrid or bitter substances within cells.

So far we have taken a fungal perspective. Our examples have indicated the influence these little animals have on fungi, on fungal composition and their functioning in nature. But the opposite is also true – the composition of the soil fauna is determined by the presence of fungi. So far, only a few of the intricate mutual relationships between mushrooms and soil dwelling creatures have been unraveled, but it is clear that we have only got a glimpse into terra firma, the black box on which we all stand.

For further reading and some nice pictures check out the following papers: