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Eight smart things slime molds can do without a brain

From remembering where they’ve been to recreating the Tokyo rail network, these “slimy aliens” are capable of way more than we give them credit for.

ByAlissa GreenbergNOVA NextNOVA Next
Physarum_Hero.jpg

Slime molds are enormously diverse, coming in all sorts of colors, shapes, and textures. Image Credit: Melissa McMasters, Flickr

Once you’ve seen a slime mold—its gooey, delicately branching structure oozing in a vaguely unsettling way along a log or leaf—you’re unlikely to forget it. They’re unmistakable because there’s nothing else quite like them. 

Slime molds branched off from the evolutionary tree before animals split from plants and fungi. And they don’t quite fit into any other group: Some live as individual cells but come together and work as a group when conditions are right. Others are huge single-celled organisms that can grow to be several feet across and contain thousands (sometimes millions!) of nuclei with no membrane between them.

Physarum polycephalum is perhaps the best-known slime mold, thanks to its distinctive bright yellow color and the fact that it’s easy to grow in a lab—and much of what we know about slime molds comes from research with Physarum. “I think they’re quite beautiful, really,” says ecologist and entomologist Tanya Latty, who has worked with slime molds extensively in her Sydney, Australia, laboratory, of Physarum and its compatriots. “They have these lovely patterns of veins, a fractal look to them.” She even likes the way they smell—vaguely fruity. 

Latty refers to them affectionately as “little slimy aliens.” And those aliens are everywhere. There are at least 900 species of slime mold in the world, some with colorful nicknames, like ‘dog vomit’ and ‘witch’s butter.' They love wet, temperate forests but live just about anywhere, including in huge numbers in soil. 

Much of slime mold research has its roots in Japan, in part because Emperor Hirohito himself was a biologist who loved slime molds and even discovered a new species. While we still know little about how slime molds behave in the wild, lab research has turned up some surprising things about their capacity to do, well, anything at all. Physarum and other so-called “acellular slime molds” (named for their many free-floating nuclei) are super gross, super cool organisms with no brain or nervous system—yet seem somehow capable of learning and making choices. Read on for a list of eight of their slime mold superpowers.

1. They can smell food

We humans have receptors in our noses that detect chemicals wafting off of food into the air. Slime molds have almost the same thing: receptors all over their cell body that detect chemical cues that tell them food is nearby.

And it doesn’t stop there. A slime mold actually has many different types of receptors, each attuned to a different cue in its environment, such as moisture or pH. They can even detect light using photoreceptors similar to those in our eyes. That means that even though it is a single cell, a slime mold has something akin to eyes and a nose. 

2. They can pulsate their way to that food

Here’s where things get gross—and cool—thanks to a transportation system not really found anywhere else in nature.

Within the blob of a slime mold sits a network of veins. Long proteins encircle those veins, and crowds of nuclei stream through them in a thick goo called “protoplasm.” The proteins squeeze and relax, pushing the protoplasm through the veins to the “growth front” of the slime mold, or the part that’s moving forward.

The sensory receptors on a slime mold’s surface are constantly pulsing in response to the signals they’re receiving, and this rhythm creates that flowing ooze. Once a receptor detects food, it starts pulsing faster. And since the inside of a slime mold is essentially one big fluid dynamics experiment, when the pulses quicken, the protoplasm starts to flow in the direction of the food—and the slime mold follows. That means a slime mold is totally decentralized: no brain, no problem.

Edge of a yellow slime mold

Close-up view of a growth front of a Physarum polycephalum slime mold. Image Credit: Norbert Hülsmann, Flickr

3. And they can devise the best route to their meals 

“That you can do experiments on something that looks like a brainless ball of mucus is incredible to me,” Latty says of slime mold science. “I find it fascinating that it can do anything at all.”

But, as it turns out, it can! If you put a slime mold in a maze and put oat flakes—one of its favorite foods—at the entrance and exit, it will slowly search the labyrinth until it finds the shortest path from one end to the other, allowing it to munch on both snacks at once. It can perform much more complicated versions of this task, as well, in one case connecting 37 different points. (Note that the number of possible ways to connect 37 points is somewhere in the neighborhood of an eight, followed by 54 zeros. That is quite the oozy calculator.)

In a famous experiment, researcher Toshiyuki Nakagaki and his team gave their slime mold a real-world problem: Find the most efficient transportation network in the greater Tokyo area. Arranging oat flakes to represent key towns and cities, they put their slime mold in the center where Tokyo would be. The result looked an awful lot like the actual Tokyo rail system.

4. They can re-form if they get torn apart

One of the genius elements of a slime mold is its protoplasm. Each tiny bit is interchangeable, meaning that every individual protoplasm unit can become a vein or a pseudopod—the temporary, limb-like projections that extend out in the direction the slime mold is moving—or any other part of a slime mold. That is, except for its organelles, which seem to be fixed as one thing, even as they slosh around inside the slime mold.

That’s why it was possible for a team of researchers in Germany to shred slime molds into thousands of tiny fragments—only to watch those bits slowly join back together. Compared to other single-celled organisms, slime molds barely have membranes at all, Latty says. “It’s just this flowing gooey stuff that happens to stick together. So if you cut one into two bits and put them near each other, they just flow together again.”

So can you kill a slime mold? It’s hard to say. There’s a beetle that eats slime molds, and a handful of other slime mold predators, but “we don’t know if they eat enough of the body to make a difference,” she says. “You could lose half of the biomass, and it wouldn’t matter. It would just reorganize itself and be like, ‘I’m fine!’”

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5. They can choose a healthy diet

In her research, Latty found that slime molds will move toward a substance like sugar and will also move away from substances like salt. But was this just a response to stimuli, or something else?

Answering this question proved complicated. “When I design an experiment for a bee, I know that it can smell, what range of colors it can see, and to work with a particular behavior,” she says. But slime molds’ worlds are profoundly different than ours. “Something that’s amoeboid, can be cut up into little bits, and you have no idea what its perceptual reality is—that’s difficult to design for.”

To learn more about slime mold nutrition, Latty helped her colleague Audrey Dussutour presented Physarum with 35 recipes made of different ratios of the elements it needs to survive, protein and sugar, creating a kind of slime mold creme brulee. They found that slime molds will avoid food sources that will harm them and prioritize food that will help them.

And not only that, but slime molds can also stretch out tendrils to feed on more than one food source at a time—and do so in the right ratio, so that they receive optimum nutrition. “It’s not just getting protein or carbohydrates, they try to get a particular balance,” Latty says. While animals might balance their nutrition by switching up what they’re eating, slime molds divide their biomass over the food so they’re taking what they need from each.

Reddish slime mold beads on log

Carnival candy slime mold (Arcyria denudata). Image Credit: Mary Keim, Flickr

In another of Latty’s favorite studies, she looked at how slime molds make tradeoffs between quality and risk when it comes to their meals. In that experiment, the slime mold had to choose between high-quality food in bright light (which slime molds don’t like) and lower-quality food in darkness. Then she and her colleagues changed up the choices—in some cases having the slime mold choose between very similar foods in light and dark, and in others creating more contrast. 

“If you’re a basic system, you’d expect you always choose one,” she says. “You have a simple rule that always works. If you’re sophisticated, you get some information about quality of food and intensity of light and do some calculations to figure out if it’s worth it.” Latty’s slime molds fell definitively in the latter category: They were much more likely to venture into the light if the food quality was five times better than the one in the dark. “That implies slime molds are able to process information between two different attributes of a food source, which seems like a pretty sophisticated thing for, well, mucus,” she says.

6. They can “remember” where they’ve been, by leaving themselves gross, slimy breadcrumbs

Another study that Latty helped her colleague Chris Reid complete found that, in searching for food, slime molds rarely retrace their steps. Could they remember where they’d been? The answer was written in slime—like the stuff that slugs leave behind. Just as ants leave trails of pheromones to show where they’ve found food, this slime is a form of “external memory” that tells the slime mold: search elsewhere. 

Realizing that slime molds could functionally “remember” things without a brain changed Latty’s perspective on many life-forms’ capacities to interact with their environment. “It opened my eyes to the fact that brains are not the be-all-end-all of behavior,” she says.

And that has big implications for lots of small organisms. “Slime molds are good representatives because they’re big enough and easy enough to work with. We don’t know if they’re the oddballs. I suspect not.”

7. They can “get used to” situations and remember things even without leaving a trail

Many more advanced organisms are capable of habituation—that is, they can get used to difficult things in their environment, especially if they’re rewarded for it. But what about slime molds? Latty helped Dussutour set up an experiment in slime mold habituation for a slime that they christened “Blob.” They would put Blob on one side of a little bridge, food on the other, and coat the bridge with salt, which is unpleasant for Blob and its slimy compatriots, but not harmful.

Under normal circumstances, it would take an hour for a “control” slime mold to cross an unsalted bridge and claim its prize. The first day of Dussutour’s experiment, it took 10 hours for Blob to cross the salty version. The next day it took 8 hours. And the day after that it completed the journey in even less time—until eventually Blob could cross the salted bridge as fast as the control bridge. Blob had “gotten used to” salt! Dussutour repeated the experiment thousands of times and got the same results.

8. They can teach their friends what they’ve learned

After that, Dussutour started wondering whether a Blob that had learned to deal with salt could teach other Blobs the same. One key, gross fact here is that Blobs stick together—literally. If you take two slime molds and put them next to each other, they’ll combine. Their membranes fuse, they open up their insides, and their veins intertwine to create a new single organism.

Over time, the team found that if they let slime molds that had learned to tolerate salt socialize with slime molds that hadn’t for three hours, they would wind up with all slime molds that like salt. “It makes you think: What even is learning? Does this count?” Latty says. “It sits right there in that grey zone. It’s thought provoking.”

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