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Physarum polycephalum, or the 'many-headed slime', is a type of slime mold that thrives in dark, cool, and damp environments, such as decaying organic matter like leaves and logs. Image courtesy of Bernard Spragg/Flickr.For many years, slime molds were a mystery. While we still don’t fully understand them, we now know that these jelly-like organisms, found on rotting logs, aren't fungi. In fact, they share a closer relationship with amoebas. Despite lacking any neurons, slime molds are capable of solving complex problems.
Slime molds (Myxomycetes) come in over 900 species, and they inhabit various environments like soil, leaf litter, and decaying logs. One remarkable discovery is a slime mold preserved in amber, dating back at least 100 million years, showing no significant difference from modern slime molds. These organisms have likely been around for almost a billion years and may have been among the earliest multicellular forms, originating from single cells joining together.
Slime Molds: A Fascinatingly Varied Group
Slime molds encompass a striking variety of organisms. Certain types, known as cellular slime molds, exist as individual cells for much of their existence, yet come together in large groups when responding to chemical signals like, "Food is scarce!" or "Time to reproduce!" In contrast, plasmodial slime molds spend their entire lives as one enormous organism wrapped in a single membrane, containing thousands of nuclei. These arise when countless individual, flagellated cells merge into one.
What unites all slime molds is their shared life cycle, which bears a resemblance to that of fungi—this similarity led taxonomists to classify them within the fungi kingdom for a long time. After depleting the available food in their environment, slime molds transform into spore clusters, often brightly colored and sitting on stalks, called sporangia. These spore-producing bodies release clouds of spores into the air, which sprout new life wherever they land. The single-celled organisms that emerge from these spores start the slime mold cycle anew.
The sporangia, or fruiting bodies, of carnival candy slime mold (Arcyria denudata) consume bacteria, yeast, and other fungi commonly found in decaying plant matter, thus playing a vital role in breaking down dead vegetation.
Ed Reschke/Getty Images"We still know very little about the ecology of 'wild' slime molds," says Tanya Latty, a researcher at the School of Life and Environmental Sciences at the University of Sydney, in an email. "For example, the ways they interact with other organisms and their role within ecosystems remain largely unknown."
Latty investigates cognitive processes in both insects and slime molds. Although we tend to underestimate the intelligence of insects, when it comes to slime molds, the already complex concept of cognition becomes even more puzzling.
"Slime molds and social insects both function as 'decentralized' systems, where no single entity dictates decisions," says Latty. "However, while insects act at both the individual level — having brains — and the collective level, defining what constitutes an individual in slime molds is far more complicated."
The fruiting bodies of one of the most prevalent slime molds, Ceratiomyxa fruticulosa, are just a few millimeters tall and thrive on decaying logs in humid environments, a typical habitat for slime molds.
Ed Reschke/Getty ImagesHow Slime Molds, Octopuses and Humans Acquire Knowledge
Humans rely on their brains for thinking, but many other creatures possess the ability to reason, learn, plan, and solve complex problems without having a brain as large as ours. Take the octopus, for example — a cephalopod related to clams and snails. While it has a brain, most of its neurons are scattered across its body, especially in its arms. Despite this, octopuses exhibit remarkable intelligence: they can distinguish between identically-dressed humans or even escape from their tanks, slipping through drainpipes and returning to the ocean. This fascinating cognitive ability is entirely unrelated to ours, as the neural systems of octopuses evolved separately from ours more than 460 million years ago.
On the other hand, slime molds don't have brains or even anything resembling a neuron. Nevertheless, researchers have been able to train plasmodial slime molds to navigate mazes. So, while the mechanisms of learning are vastly different for slime molds, octopuses, and humans, the end result — the ability to learn — is strikingly similar.
Slime molds are capable of a kind of learning known as habituation. You might recognize this in yourself: over time, you get used to the chill of a cold lake or the irritating buzz of a fluorescent light overhead. Your brain helps you tune out these bothersome sensations. Similarly, the unicellular slime mold Physarum polycephalum can become accustomed to harsh conditions, such as dry, salty, or acidic environments, or chemicals like caffeine or quinine — provided they are rewarded for enduring them.
Not only can slime molds adapt to unfavorable conditions if there's a reward in it for them, but they also appear to possess memory. The slime mold species Physarum polycephalum, which was used in the habituation experiment, can apparently retain memories. In one study, slime molds were habituated to salt, a known repellent, and then allowed to enter a dormant state. After a year of dormancy, they were able to recall how to cope with a salty environment. Additionally, they seem capable of navigating based on prior encounters with food in particular directions.
Just wait — in a few years, slime molds will score a 1,200 on the SAT, and scientists will have to really start explaining themselves.
Depending on the species, slime molds can be sensitive to things like bright light, caffeine, or salt.
