Mushroom Hunters
Summer 1992 is a good one for mushrooms in this part of Alaska's interior. Boletus of several kinds line local dirt roads, going from small and succulent to enormous and wormy apparently overnight. Puffballs erupt in the unshorn edges of lawns. Fly agarics, the toxic, big red amanitas with white freckles, reach Frisbee size at the verge of woodlots. Mushroom hunters, whether searching for food or photographic subjects, are doing well hereabouts.
But, as I learned recently, the term "mushroom hunter" has more than one possible meaning. Improbable as it seems, some mushrooms are hunters. They are active predators with an array of weapons weird enough to suit a science fiction writer's nightmares.
The lowly, but edible, puffball is one of the predatory kinds. Puffballs are the fruiting bodies of a saprophytic fungus---that is, one that lives on dead organic material. Each fruiting body is supported by an array of rootlike filaments called hyphae. Bundled together, a thousand hyphae would be about as thick as a human hair. Strung end to end, the hyphae supporting a single puffball would extend for miles.
Hyphae secrete enzymes that attack woody debris in soils, reducing the complex carbohydrates in plant detritus to sugars that nourish the mushroom. Recent work at the University of Guelph in Ontario showed that dead plant tissue is not the only thing some hyphae attack. The filaments beneath some kinds of mushrooms grow toward colonies of bacteria. When the hyphae touch the bacteria, they secrete enzymes that destroy the bacterial cells, whereupon the hyphae absorb the dissolved product. Puffballs are among these more-than-saprophytic fungi; so are shaggymanes, another edible mushroom found in Alaska.
It is surprising enough that some common mushrooms actively seek out, kill, and eat living cells, but other fungi go after larger living prey. One fungus found in ponds and ditches grows branches with thickened ends. The swollen ends attract tiny aquatic animals known as rotifers, which try to swallow these ends, but become stuck instead. Once in a rotifer's mouth, the branch ends sprout hyphae that invade and colonize the animal.
Predatory fungi apply their most vicious weapons to nematodes, little soil dwelling worms without segments. Nematodes have no sense of sight; they depend on chemical clues to track down food and avoid danger. Carnivorous fungi take advantage of that by exuding chemicals that attract nematodes, then snaring them. The net fungus produces a protein that combines on contact with a sugar in nematode surface cuticle to produce instant superglue. The chemical bond cannot be broken by physical force; no matter how violently the worm wiggles, it can't get loose after just a few seconds contact. Hyphae strands invade its body, first releasing paralyzing toxins, then digesting it from the inside out.
Yeeccch!
Less repulsive, but more dramatic, is the killing technique of another family of fungi. A typical member of this group also baits a trap with nematode-attracting chemical, but the trap is set of hoops made of hyphal branches bent back upon themselves. When a chemically lured worm wriggles into one of the hoops, the cells of which it is composed expand massively and instantaneously. The hoop becomes a noose, crushing the nematode's body.
Mushrooms, it seems, go to some lengths to balance their diets, The carbon-rich carbohydrate of decaying plant matter making up the bulk of their nutrition gets supplemented by nitrogen- rich protein of bacteria or animals. Well, fair's fair: I've often seen worm-eaten mushrooms, but now I know worm-eating mushrooms are busy evening out the score.
