2025 began with a burst of color in our yard. Along the wall in our backyard, I spotted a patch of brilliance, the astonishing yellow-gold of a slime mold. It looked a bit like a two-dimensional broccoli, with radiating stems topped by little balls, each about 2 millimeters wide. The growth on fir needles, cones, and sticks was less diverse, basically rows of lemon-colored balls. I have seen slime molds many times before on hikes but never in our yard; I took it for a good omen for the year.
Overlooked, often unloved, sometimes derided, and once an inspiration for an evil alien that ate teenagers, and tried to eat a young Steve McQueen (more about this later), slime molds sound like they might be disgusting. After all, their name combines two substances that most people abhor. And, to make matters worse, the species I saw is often called dog vomit slime mold. But, of course, slime molds didn’t name themselves, and these fascinating organisms merit far more appreciation than their name implies.
Slime molds have confounded scientists since at least the 17th century when a German botanist described them as fungi. Subsequently a French botanist thought they were a sponge, which would confer upon slime molds animal status. Others placed them in the plant kingdom. None of these early writers were correct. Scientists now put slime molds in the kingdom, Protista, labeling them as myxomycetes (myxa, meaning slime and mycetes, referring to fungi), mycetozoa, or myxogastria. (By the way, molds actually are fungi so slime mold is terribly misleading as a name.)
About a 1,000 species exist, primarily in temperate and tropical forests but also in more extreme environments, including perhaps an aquatic variety. They are neither dangerous nor bad for you or the environment; in fact, they help maintain a healthy ecosystem. At least one species, F. septica (most likely the one in our yard), is eaten; in Veracruz, Mexico, it is scrambled like eggs and known as caca de luna, or excrement of the moon, arguably one of the world’s finest colloquial names.
Slime molds attract our attention in two forms. The form I encountered in our yard is called a plasmodium (plural: plasmodia). These naked (lacking a cell wall) blobs of protoplasm are often yellow or white and spread amoeba-like in a quest for food, which they engulf and digest. Their preferred meal is bacteria often found in decaying plant matter, such as the Douglas fir detritus in our yard. Plasmodia can move up to an inch per hour.
The secord form of slime mold can be even more beautiful than the plasmodial mode. These fruiting bodies of spores, called sporangia, often have a fungi-like shape of a stalk topped balls, columns, or tubes. They can be extremely colorful and gorgeous. Unfortunately for people with aging eyes like mine, sporagania are typically no more than a few millimeters tall so you may want to bring along a hand lens to assist your viewing pleasures. (The fruiting body of F. septica, which resembles dog vomit, is what gives this slime mold, its unappealing name.)
More astoundingly, plasmodia have been found to possess “a unique, coupled-oscillator based sensorimotor system that may be the key to its highly developed problem-solving abilities.” In other words, they have the capacity to make complex decisions. Slime molds, or at least the Einstein of the group, Physarum polycephalum, can find the shortest route through a maze and construct a highly efficient network of nutrient-feeding tubes in structure comparable to the Tokyo subway system. Researchers have begun to analyze the slime mold for insights into malleable network systems, including road, trains, and computers.
These problem-solving techniques of slime mold take us back to Steve McQueen, star of the slime mold inspired, 1958 sci-fi schlock of The Blob. After a meteor crash lands (how else do they land?), it unleashes a malevolent red-brown blob that begins to subsume people, “assimilating flesh at a remarkable speed,” says the soon-to-be assimilated town doctor. Fortunately, Steve (also his name in the movie) realizes what’s happening and warns his fellow citizens of the danger. He also saves the day by figuring out that the Blob cannot handle freezing. In the end, the Blob is airlifted to the Arctic, where the ice will prevent the Blob from rampaging again. Everyone in town lives happily ever after. And, you wonder why Steve McQueen was a legend!
Clearly, modern myxomycetes scientists need to be aware of the capacities of the beings they are studying. All’s well if the slime molds are merely passing the time by navigating mazes or working on more efficient subway systems. What happens if these slime molds go rogue or worse, like the Blob? We all know how easy it is to flip from simply driving a car to road rage. We can only hope that slime molds never learn how to drive cars!
P.S. Within a week the slime mold in our backyard had changed to a collection of little dark brown blobs. If no newsletter arrives next week, you’ll know that our slime mold went rogue.
Upcoming Event: February 19, 2025 - 7pm - Elliott Bay Bookstore - Book Launch of Seattle Walks: 2nd Edition. I will be in conversation with Taha Ebrahimi, author of Street Trees of Seattle. Free, ticketed. Register here.
Sources: C. Reid et al, “Decision-making with a brain: how an amoeboid organism solves the two-armed bandit,” Journal of the Royal Society: Interface 13 (19), June 2016.
Raphael Key et al, “Stepwise slime mould growth as a template for urban design, Nature,” Scientific Reports 12, 1322 (2022).
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