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Discovering Slime Molds


Wolf’s milk growing at Indian Cave State Park on Brueggemann’s prized slime mold log. Photo by Gerry Steinauer.

Story and photos by Gerry Steinauer

Two years ago, while morel mushroom hunting in a creekside woodland near Aurora, I saw a pink, dime-sized “ball” sprouting from a log. Baffled, I concluded it was a strange puffball mushroom. I snapped a photo and texted it to my go-to guy for mushroom identification, Chance Brueggemann, woodland ecologist at Indian Cave State Park.

His response: “It’s wolf’s milk, a slime mold.” I wasn’t sure what a slime mold was. I assumed they were slimy, mold-like fungi that grew on basement walls, long-forgotten food in a refrigerator and other dark, damp places. A little research showed how wrong I was.

Slime molds are like no other organism on our planet and these single-celled creatures have confounded naturalists for centuries. In their feeding stage, they are called a “plasmodium” and slowly creep under and about rotting logs, soil and leaf litter. Amazingly, experiments have shown that plasmodia display simple forms of intelligence and memory. Late in life, plasmodia transform into stationary, spore-producing, fruiting bodies that often appear as tiny, colorful mushrooms.

Coral slime. Photo by Gerry Steinauer.

Recently, scientists, with the aid of DNA analysis, have placed slime molds (Myxomycetes) into the kingdom protista, a catchall group of mostly single-celled organisms not fitting neatly into the other four kingdoms of life: animals, plants, fungi and bacteria. The slime molds’ DNA suggests they are ancient creatures, one to two billion years old, slithering over the Earth’s barren rock surface at a time when the only other land creatures were bacteria. Today, they mostly inhabit moist forests.

I would be remiss not to mention the slime molds’ intricate beauty.

Red Lollipop

Last June, Brueggemann and I spent two days searching for and photographing slime molds in the steep Missouri River bluff forests at Indian Cave. The first morning, we veered off a hiking trail and entered a deep ravine bottom with a trickling stream. After a short hike, parting low-hanging ironwood and elm branches, our target came into sight — Brueggemann’s favorite spot to find slime molds: a hefty, decaying log.

Red lollipop fruiting heads are young and lacking color. Photo by Gerry Steinauer.

The log did not disappoint. First, Brueggemann showed me a colony of fruiting wolf’s milk (Lycogala epidendrum) —pink globes growing from the rain-soaked wood. He pricked one with a twig and out oozed a bubblegum-colored paste containing the developing spores. It’s not clear why pink ooze would lead to the name of wolf’s milk, but such is the privilege of naming.

Within days, the fruiting bodies’ pink color will fade, and then they will disintegrate into a brown powdery smudge. The powder is millions of microscopic, ripe spores.

He next pointed to a cluster of tiny red dots that he calls red lollipops (Hemitrichia calyculata). Only through my camera’s macro lens could I make out their narrow stalks topped by round, red fruiting heads calling to mind the childhood delicacy of cherry Tootsie Pops, but only a fraction of an inch tall. Seeing the stalked heads, I grasped how slime molds could be mistaken for tiny, toadstool mushrooms.

Last, the log treated us to a patch of coral slime (Ceratiomyxa fruticulosa). Composed of white, translucent columns that often branch, its fruiting bodies indeed resemble coral, or perhaps more relatable for Nebraskans, a glazing of hoar-frost. When large, colonies can fuse, covering entire logs and tree stumps in white. Unique among slime molds, coral slime spores occur singly at the tips of incredibly thin stalks that project outward from the columns. All other slime molds bear their spores within fruiting bodies.

Red lollipop slime with color. Photo by Gerry Steinauer.

Though other slime molds probably grow on Brueggemann’s prized log, they were not fruiting and so not visible. No one knows how many slime mold species are found in Nebraska. Because our state has a relatively dry climate and few forests, it may be only a few dozen.World-wide, over a thousand known species of slime molds exist. Their wind-blown spores disperse far and wide, many species occurring on several continents.

Chocolate Tube Slime

Later in the day while cruising along in a UTV, Brueggeman driving, I spotted a small, white blob in the leaf litter. I shouted, Brueggemann hit the brakes, and we were treated to a slime mold, covered in low, round bumps. The round bumps were fruiting heads, just beginning to develop. Since immature slime molds are difficult to identify, we returned the next morning.

The fruiting heads had matured overnight, turning brownish in color, revealing itself to be a chocolate tube slime (Stenomitis spp.). Chocolate tubes, also called pipe cleaner slimes, have bunched, columnar, chocolate brown fruiting heads supported by short, thin stalks. The erect columns of this specimen had already withered and collapsed into a twisted mass, preparing to release its spores in hopes of landing on moist habitat suitable for germination.

A chocolate tube slime at Indian Cave with young fruiting heads. Photo by Gerry Steinauer.

A slime mold’s life begins as a single-celled amoeba, emerging from a germinated, cracked-open spore. The amoeba prowls about, engulfing and digesting bacteria, algae, fungal spores and odd bits of organic matter. To move through wet environments, many species grow a whip-like tail to propel themselves. When the environment dries up, the tail retracts.

When an amoeba is lucky enough to meet another, their bodies and nuclei fuse in a form of primitive sex. Now a plasmodium, the organism continues to feed while the nuclei inside divide until there are millions. Spread microscopically thin, a single-celled plasmodium can encompass several square yards.

By the next morning, the heads had matured. Photo by Gerry Steinauer.

Eventually, the plasmodium migrates from its feeding area and perches on a high spot, such as a log, tree trunk or the top of leaf litter, where its spores can catch a breeze. Now stationary, the plasmodium transforms into fruiting bodies, inside of which the nuclei develop into spores and the life cycle begins again.

Slime molds have a clever adaptation that might explain their eons-long survival on this sometimes harsh planet. When exposed to dry, cold or foodless conditions, they can dehydrate and enter into a state of dormancy lasting weeks, centuries and possibly millennia. When favorable conditions return, they re-hydrate and dormancy is broken. This strategy is especially advantageous for slime molds living in desert soils, on mountain tops and other extreme environments.

Many-headed Slime

During our photo expedition, Brueggemann was keen to show me a many-headed slime (Physarum spp.). Previously, he had seen its beautiful, yellow plasmodia growing vein-like on logs, leaf litter and even large mushrooms. Though we did not find the slime in the field, its story is worth telling.

A many-headed slime plasmodium growing on a log at Indian Cave. Photo by Chance Brueggemann.

Unique among slime molds, many-headed slimes have large plasmodia, easily visible to the naked eye. Because they are easy to grow in petri dishes, they are the world’s most-studied slime mold. One revelation from the lab is that the plasmodia of slime molds propel themselves by rhythmic contractions that push cell fluids back and forth in waves from their center to their extremities. The many-headed slime has been clocked at a tire-squealing speed of over an inch per minute. Having precise control of the contractions, they can change speed, reverse direction, steer around distasteful items (think bright light or salt) and send tendrils out to explore for food and suitable habitat.

Experiments show that Physarum can solve mazes in search of food. It extends tendrils on all paths until it finds the food, then retracts all except the feeding tendril. Later, when placed in the same maze, the slime mold can recall the correct path to its meal. In another experiment, many-headed slimes were hit with shots of cold air at regular intervals. Their response was to retract. Amazingly, they learned to expect the blast and retracted in anticipation.

Contrary to traditional beliefs about primitive organisms, these behaviors suggest that slime molds have forms of intelligence and memory comparable to simple animals that possess nerves and elementary brains. Scientists are unsure how intelligence and memory work in slime molds, but assume it is through chemical messaging in the cell fluids. For now, it remains a mystery.

Dog Vomit

Last August, while tussling with our English Cocker pup in the front yard, I spotted several white blobs clinging to the grass. The blobs were dog vomit — not from our pup, but the so-named slime mold (Fuligo septica).

Dog vomit growing in the author’s front yard. Photo by Gerry Steinauer.

This grossly, but aptly-named species indeed looks like dog vomit, especially the foamy kind they expel after eating too much grass. The yellow-colored form of this species is often called scrambled eggs slime mold.

Dog vomit is widespread, possibly the most readily observed of the slime molds. It inhabits forest floors and logs, urban mulch piles, gardens and lawns. Its appearance in our yard was abrupt, as I had just mowed a day or two prior. A few days after I first observed the dog vomit, its white fruiting heads faded to tan in color and hardened with calcium. Soon after, it disintegrated, releasing its black spores.

Although the dog vomit patches in my yard were at most a few inches across, infestations can be quite large. In 1973 in a Dallas suburb, a particularly large outbreak of dog vomit spread across lawns. Panicking, residents called firefighters to hose the mysterious blobs into oblivion. The jets of water only broke the slime mold into pieces that continued to slither and grow, sending suburban imaginations further out of control.

A damaged dog vomit seeps the amber liquid in which its spores develop. Photo by Gerry Steinauer.

Possibly stoked by the 1958 sci-fi movie The Blob, they now feared that indestructible alien creatures had invaded their neighborhood, even as scientists reassured them that the invaders were harmless, earthly slime molds. Eventually, the “aliens” dissipated on their own, without the need for a nuclear strike.

The Dallas scare testifies to how little we, as a society, perceive and understand nature. Botanists use the phrase “plant blindness” to refer to the general public’s inability to see or notice plants in their environment. This is likely because plants are green, stationary and less eye-catching than mobile and more colorful animals.

As a botanist, I must now confess to “slime mold blindness.” All the years I have hiked Nebraska woodlands, head cranked down searching for plants, I have been oblivious to these beautiful and fascinating organisms. What I have missed!  ■

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