Why do they do this? These fungi and plants grow in habitats that offer little of the nutrients they need, especially nitrogen, a necessary element for making proteins. The fungi tend to parasitize or decompose the woody trunks of trees, which are very limited in nitrogen. The plants are usually found in acidic bogs, sphagnum moss, or other nitrogen-poor environments. Most plants take up nitrogen through their roots, often with the help of nitrogen-fixing bacteria, and most fungi absorb nutrients from the soil. But in their nutrient-poor habitats, these meat-eating fungi and plants have evolved various forms of lures and weapons, some of them rivaling the most vicious and brutal devices seen in any medieval torture chamber, to attract and kill their hapless victims.
10 Toilet Bowl Pitcher Plants
The 150 or so species of tropical pitcher plants from the genus Nepenthes are found in Southeast Asia, the Philippines, Borneo, Sumatra, New Guinea, Sri Lanka, and the eastern margin of Madagascar. Some of them are quite large. Most of them trap and digest animals of various kinds, including small vertebrates. Three species from the mountainous rain forests of Borneo can be aptly (if unofficially) called “toilet bowl pitchers”—Nepenthes lowii, N. rajah, and N. macrophylla. In addition to trapping and digesting small animals in smaller pitchers growing along the ground, these species also have modified aerial “toilet pitchers” growing high off the ground on vine-like stems. These aerial toilet bowls are specifically designed to serve as a perch for the mountain treeshrew (Tupaia montana), as it licks up the copious secretions of sugary nectar produced by the pitcher’s lid. In order to reach the nectar, the shrew must perch directly over the funnel-shaped opening to the pitcher, whose rim is not slippery like those of its insect-devouring relatives. As it feeds, the shrew often defecates into the pitcher. The next rain will ensure that the poop is flushed into the bowl, where it will be digested and provide a rich source of nitrogen for the plant.
9 Oyster Mushrooms
Oyster mushrooms of the genus Pleurotus are among the most prized edible mushrooms collected in the wild by human mycophages (a Greek word meaning “fungus eaters”). Oyster mushrooms grow on the trunks of dying and dead trees and break down the wood. The wood contains plenty of cellulose and lignin, but little nitrogen, so these crafty fungi exude chemical lures to attract their microscopic nematode prey. When the worms crawl onto the fungal hyphae (threadlike filaments that make up most of the mushroom’s mass), the mushroom releases toxins from the tips of tiny, matchstick-like glands that paralyze the worms. The fungus then sends digestive hyphae down the victim’s mouth. They penetrate throughout the body and slowly digest the helpless worm from the inside while it’s still alive.
8 Shaggy Mane (AKA Shaggy Ink Cap)
Another choice edible mushroom (if collected while it’s still young and fresh) is the nearly cosmopolitan shaggy mane (Coprinus comatus). One of the ink cap mushrooms, the shaggy mane autodeliquesces (digests its own spore-bearing gills and cap) to produce a slimy, black, liquid mess within four to six hours after it either deposits its spores or is collected by a mushroom hunter. It must be promptly sauteed or placed in a glass of ice water to prevent this from happening. The time-lapse photographic sequence above shows how this occurs. Nematodes feeding on nitrogen-fixing bacteria obtain much more nitrogen than they can use. The worms excrete most of their excess nitrogen as ammonia, which is why they are the primary prey of most carnivorous fungi. The shaggy mane preys on two species of nematodes that attack plants—Panagrellus redivivus and Meloidogyne arenaria. When they contact these mushrooms, the worms are damaged by tiny, mace-like “spiny balls” at the ends of short hyphal branches. The hollow spikes on the spiny balls pierce the worm’s cuticle (skin), and the nematode’s high internal pressure forces its body contents outside. This mechanical injury together with a potent cocktail of poisons released by the spikes kills the worm within a few minutes. Colonizing hyphae then penetrate the victim’s body through the wounds to digest and assimilate its contents.
7 A Fungus That Kills With A Net
Arthrobotrys oligospora is an anamorphic (exclusively asexually reproducing) fungus that does not produce fruiting bodies (mushrooms). It produces a complex three-dimensional adhesive network of sticky, ring-like snares that chemically bind to the surface of a nematode’s cuticle. Lectin (a protein that is highly specific for certain carbohydrates) on the surface of the net combines irreversibly with a specific sugar on the cuticle to form an unbreakable bond. No matter how much the worm struggles, it can’t break these bonds or escape. By far the most widespread and abundant of all nematode-trapping fungi, A. oligospora is found living in soil, animal feces, and even in freshwater and seawater, where it feeds on decaying vegetation. It only produces its sticky nets when nematodes appear, which the fungus can detect by the worms’ smell. The worms secrete a family of chemical pheromones called ascarosides, which they use to communicate with each other, control their numbers, and locate mates. This way, the fungi don’t needlessly waste the energy and resources needed to create their traps. Different nematode-trapping fungi respond to different sets of ascarosides according to their preferred nematode species, but the plot thickens further still. Certain bacteria release large amounts of urea, which diffuses through the soil and is absorbed by the fungus. The fungi then convert it into ammonia, which stimulates the production of their adhesive networks. The urea also attracts nematodes, whose numbers swell as they feast on the bacteria. In response, the bacteria increase their output of urea, which stimulates the fungus to produce more adhesive nets to trap the nematodes and bring their numbers under control. The bacteria thus cleverly enlist the fungi in their own defense against the worms! Ultimately, the ammonia released by the bacteria-eating nematodes provides the nitrogen sought by the fungi.
6 A Fungus That Kills With A Lasso
Some nematode-killing fungi like Dreschlerella anchonia create lassos to catch prey. They’re produced by three cells on a specialized hyphal branch and form a circle and fuse to form a tiny, constricting ring just 0.03 mm in diameter. A nematode entering one of these rings mechanically ruptures a line of weakness along the inner walls of the cells forming the ring. The cells’ internal osmotic pressure causes the water outside to rush in through the break, making them swell and increase their volume threefold in a tenth of a second. The swollen ring constricts the helpless worm in a tight noose from which escape is impossible. The worm’s thrashing often leads to its entrapment in a second noose, as shown above. (Note that in the video above, the fungus is misidentified as Arthrobotrys oligospora.) After the worm is captured, invasive hyphae emerge from the ring cells that penetrate the victim’s body and digest it alive from the inside. A very early version of a nematode-killing fungus that used constricting rings was documented in a piece of 100-million-year-old fossilized amber from southwestern France. It lived during the Middle Cretaceous period, when gigantic dinosaurs still roamed the planet and flying reptiles ruled the skies. But unlike its modern counterparts, these rings were formed by a single cell instead of three cells and were even tinier (just 0.015 mm) across.
5 Bladderworts
Over 200 species of the genus Utricularia are found in freshwater habitats (like ponds and bogs) and wet, low-nutrient soil on every continent except Antarctica. All are carnivorous. Although they are very generalized plants whose tissues, except for their flowers, are not differentiated into stems, roots, and leaves, they all trap their small animal prey by means of highly sophisticated bladder-trap devices. These unique bladder traps are found only in this genus of plants. The bladder creates a partial internal vacuum by actively pumping the water inside the bladder to the outside, collapsing the sides of the bladder together. The mouth-like opening is then effectively sealed by a combination of specialized flexible tissues and a sticky mucilage, which keeps the water out. Near the highly sensitive trigger hairs, the mucilage is enriched with sugary carbohydrates, which attract prey. When a tiny copepod, cladoceran (“water flea”), or other suitable-sized prey brushes against the trigger hairs, the seal is mechanically broken, the side walls spring back into place, and water rushes back inside through the open mouth, carrying the unfortunate prey with it. This all happens in less than 0.001 seconds. The trap is immediately resealed, the water is quickly pumped outside again, and the trap is reset. The trapped prey is then digested by enzymes released inside the bladder.
4 Butterworts
The butterworts of the genus Pinguicula belong to the same family (Lentibulariaceae) as the bladderworts. However, they use “flypaper traps,” which consist of very fine, hair-like pedunculate (stalked) glands on the upper surfaces of their leaves, which secrete shiny droplets of sticky mucilage. This glistening mucilage is thought to attract insects looking for water. Insects venturing onto the mucilage become stuck. The insect’s struggles cause the edges of the leaf to slowly curl over, partially enclosing the prey and releasing more sticky mucilage. Sessile glands lying beneath the pedunculate glands then secrete enzymes that digest the trapped prey. The products of digestion are absorbed by the leaf through holes in the protective waxy cuticle, called cuticular gaps. Such holes are very unusual in plants and make butterworts susceptible to dehydration. Their brightly colored flowers, with their sweet nectar, are perched atop long central stalks to attract pollinators without running the risk of killing them. Their musty-smelling flypaper leaf traps are arranged much closer to the ground to lure water-seeking midges, gnats, and other insects.
3 Sundews
Sundews use much more elaborate flypaper traps than butterworts, and the over 180 species of the genus Drosera belong to a different family (Drosseraceae). Their glistening glandular leaf hairs (which give the sundews their common name) are much larger and more conspicuous than those of butterworts, but they work exactly the same way. The glands exude a nectar to attract insects, as well as a sticky mucilage and digestive enzymes. Flies and other insects landing on the leaves to drink the nectar are trapped and held fast by the glue. Other glandular leaf hairs are brought into play, and some or all of the leaf may also curl over to enclose the prey. These actions occur in slow motion and can take several hours, but the prey, stuck to the adhesive, isn’t going anywhere! Enzymes are then released by the leaf hairs, which slowly digest the victim.
2 Insect-Eating Pitcher Plants
Pitcher plants form their leaves into pitfall traps—tall, hollow, trumpet-like basins containing a mixture of acidic water and a detergent-like wetting agent (surfactant). These pitcher-shaped leaves also resemble insect-attracting flowers that gradually turn purple-red from the accumulation of pigments known as anthocyanins, the same pigments responsible for the brilliant colors we see when the leaves turn in the fall. Near the opening to the trap, the leaves also produce a sweet nectar to attract flies, ants, beetles, and other insects. The vertical inner walls of the upper part of the pitcher are coated with a slippery wax that causes a fly or other insect to slide into the pool of water waiting below. If the prey manages to escape from the water trap, it flies headlong into the steep walls of the trumpet-like tube and is knocked back into the water. The wetting agent quickly prevents the insect from escaping and makes it sink to the bottom, where it is slowly digested by the acidic liquid. This process is assisted by bacteria living in this soup that contribute their digestive enzymes. About a dozen species of a single genus (Sarracenia) are found in the acidic bogs of eastern North America, and perhaps twice that many belonging to a different genus (Heliamphora) live in South America. A single species from a third genus, Nepenthes, occurs in northern California and Oregon.
1 Carnivorous Bromeliads
Bromeliads are a family consisting of roughly 3,000 species of primitive plants related to grasses and sedges that are found almost exclusively in the American tropics and subtropics. A single species occurs in Africa. This family includes the pineapple, Spanish moss, and countless species of epiphytes in the tropical cloud and rain forests of Central and South America. Many of these aerial plants live high up in the trees, where they absorb the carbon dioxide they need for photosynthesis directly from the air. The overlapping bases of the leaves of the so-called “tank bromeliads” typically store pools of water that provide nurseries in which tropical tree frogs can lay their eggs and hatch out their tadpoles. Several bromeliads are also common ground-living succulents in Southwestern US deserts. These plants would seem to be perfectly preadapted for developing a carnivorous lifestyle, especially since insects often fall into the water pools and drown. However, just three species in two genera (Brocchinia and Catopsis) are actually carnivorous. The upright leaves of these three species are specialized to store permanent pools of water, and their leaves are coated with a crumbly powder that reflects ultraviolet radiation and attracts UV-sensitive beetles and other insects. The attraction is enhanced by nectar-like secretions upon which the insects feed. Insects landing on the powdery surface lose their footing and fall into the water, where they are digested by a combination of enzymes produced by the plant as well as bacteria living in the pools. A retired naturalist, author, and educator, I am the author of numerous research and popular articles about biology and nature, as well as seven academic books in several disciplines, including one currently in press about mushrooms. It is due to be published next year.
