The Daily Bucket: Slime Molds - Seriously misunderstood organisms

Pacific Northwest

Whatcom Co., WA

What’s in a name? (apologies to the Bard). In this case a lot is conveyed by the unfortunate connotations of the common label of “ slime mold.” And the name isn’t entirely accurate as they are not molds and not always slimy. Worse yet, they have been dubbed variously as “Dog Vomit,” “Blobs,” “Demon droppings,” “Witches butter” and by the venerable Linnaeus, ”Rotting Mucus.”

In their single cell mode, they are essentially amoebae and who doesn’t love little amoebae? They are really not that different. In fact they are often referred to as “Dirt dwelling amoebas.” They are single celled organisms, both of the Protista Kingdom. A major difference however is that slime molds can be beautiful, intriguing, and even demonstrate intelligence, or at least intelligent-like behavior. (I’ll get to this later.)

Let’s start with a quick look some of these little beauties. The following photos of slime mold are from a recent issue of Scientific American. You are not apt observe these beauties on rotting logs as these photos are composites of many photos taken by a professional photographer, Barry Webb.

*Cribraria aurantiaca Photo* Credit: Barry Webb — Scientific American

Found around the world, there are at least 900 species of slime molds and they have occupied space on earth for a very long time, perhaps as long as a billion years. They are all over the place, even if you don’t see or recognize them. In their amoeboid state they are tiny microscopic cells, often only 1 to 4 mm. As shown below they can coalesce into larger visible forms under certain conditions. They are in your gardens, lawns, forests, parks - anywhere that there is moisture, decaying vegetable matter and bacteria, all of which they feed on.

Slime Molds are one of the more strange life forms as their classification is based largely on what they are not rather than what they are. As one author put it: “For Scientists, classifying slime molds has proved to be as slippery as their namesake.”

Over time they have been variously classed with plants, animals and most recently with fungi as they share attributes of each of these kingdoms. They are now placed in the Kingdom of Protista, largely because they don’t fit in any of the other kingdoms. And even within their individual groupings, some of them share little in common.

*Comatricha Photo* Credit: Barry Webb — Scientific American

Some taxonomists say slime molds fall into one of two classifications while others break them into three and some even into five. Further, these groupings do not share a common evolutionary ancestor. That is they are polyphyletic in that they have each evolved separately. So, they are “slime molds” because they are not something else. They are kind of an“other” category of life, but interesting nonetheless.

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Slime Mold Life Cycle

Before describing some of various species, I will review the slime mold life cycle. Actually each of the major classifications varies a bit from one another but for the most part they follow this pattern, much like that seen in the fungi. Being circular, we need to just start somewhere so let’s begin with spores. You can follow visually with the graphic below.

Spores from mature slime molds are released and on germination, (with water) release little single cell amoebas (mxyamoeba). These are essentially amoebae that are capable of moving with pseudopods and with which they engulf food such as bacteria.

If in a wet environment some form tails/flagella to swim with. If conditions are not supportive, they can form a protective cyst and curl up until conditions improve.

When they meet compatible cells they mate, and eventually if food and other conditions are good, they might aggregate into a large plasmid form called the acellular or plasmodial slime mold. (the two main types differ a bit at this stage but I’ll explain the differences shortly.) This aggregated bunch of cells is the blob we see on a log or the ground. Some are colored, (yellow, orange) while others are colorless or transparent.

The plasmodium matures into a network of interconnected filaments, which slowly moves as a unit as its protoplasm streams along the network. They are indeed slow as it is reported that the fastest speed recorded is 1.35 mm/sec. Nonetheless, they are the fastest microorganism around. These plasmodia can be quite large, many stretching to a square meter and more!

The plasmodium continues to feed and grow until some specific conditions occur such as lack of food or too much light. Then the plasmid forms into a solid layer that produce little shoots called sporinga or fruiting bodies that produce spores. The photos above are examples of multiple colors and forms these sporinga can take. I think they look like blown glass art. Once this fruiting has taken place, the slime molds are done and spores produced are dispersed to start life anew.

Types of Slime Molds

I focus here on the major Phylum of slime mold. : Myxomycota, (myxo means mucus in Greek), also called “true slime molds” or “plasmodial” illustrated above.

Briefly, the other major type of slime mold — Acrasiomycota, is referred to as “cellular.” These spend their life as individual amoeba cells but when a chemical is secreted they can glom on to each other to form a swarm that looks like a slug and this leads to their formation of spore producing fruiting bodies. In contrast to the true slime molds, these retain their individual cell structures even when in their swarm.

There are more slime mold types described but the ones that you are most likely to see are true or plasmodial types.

Most slime molds will take different forms at different times or stages of its life cycle. The large blob or plasmodium that we see slowly moving around one’s garden or on rotting logs in the forest is the adult stage of the life cycle. At this stage, the little single cell amoebae gather into a large undifferentiated mass of protoplasm as a single cell although each of the constituents retains its nucleus rendering the blob a multi-nucleated large large cell. One of the more common plasmodial slime molds is the the “Dog Vomit” (Fuligo septica) shown below.

Two views of the dog vomit (Fuligo septica) also called Scrambled Egg, taken at different times. sIt is rumored that some indigineous peoplke from South America scrape it up and eat it. The second photo might be drying up and entering the sporangium stage getting ready to develop spores.

Another frequently occurring acellular or plasmodial slime mold is Lycogala epidendrum (aka Wolf’s Milk). They are small pinkish orange blobs that if poked, will exude a pink toothpaste-like substance and thus, also called “toothpaste slime molds.”

Ceratiomyxa fruticulosa is one of the more attractive slime molds having the appearance of coral-like features giving it its common name of “Coral Slime.” The lead photo is also an example of this. You will want to Zoom on the one below to get an appreciation of the coral-like patterns.

Ceratiomyxa fruticulosa (Corals slime,) Enlarge to see coral-like features and notice the three yellow blobs at the bottom right, surrounded by coral cells. They appear to be feeding on what ever these are, probably a fungus of some kind. ZOOM for better resolution

I think this is another of the Coral slimes but am not positive. On magnification I think I see the coral-like cells. ZOOM

I am not sure what this organism is. While I believe it to be a slime mold of some kind, it might also be a fungus. Unfortunately neither iNaturalist nor Seek were able to help with identification.

Are Slime Molds capable of learning? And if they can learn, how?

Slime molds are able to do a surprising number of tasks, a fact that has caused great debate among neuroscientists and cognitive scientists. A major problem is less about what they can do, and perhaps more that we don’t know how they do what they do without a hint of a brain or even a rudimentary nervous system. They are essentially single celled blobs of protoplasm.

Physarum polycephalum, www.labroots.com/… One must be patient to study these organisms as this one has been clocked at a speed ? of 5cm / hour.

A considerable amount of research been conducted to understand the types and extent of slime mold’s ability to learn and even more significant is the theorizing on how it can learn and retain information in absence of a nervous system.

Although various forms of learning have been demonstrated in a variety of these organisms, the subject most commonly used is Physarum polycephalum, one of the true molds.

A few of the most well documented accomplishments of these organisms follow with a list of descriptions and reports of the studies for those who want a deeper dive into this protoplasmic pool.

“Sense of Self”: If the plasmodium of P. polycephalum is physically separated (cut up), the various parts can find their way back to its other parts to reunite as a whole.
Habituation: When a new, repetitive stimulus is introduced to us such as the annoying tick of a clock, we initially react to it. However, if it continues over time, we cease or diminish our reaction to it, having “habituated” to it. This is considered the most elementary form of learning. Slime molds too can “learn” to not respond to such annoying stimuli.
Gauge time: If presented with an aversive stimulus such a blast of cold air at one hour intervals, by the third trial, a slime mold will retract its body just before the hour is up. They can acquire a sense of time in that they anticipate when the next blast is due.
Maze learning: put into a maze, with food at one end, the P. polycephalum will send runners to all options until it finds the correct path to food. Then it retracts all cytoplasm that does not go straight to the food goal. Similarly, if placed on a map on which food (oats) are placed at major cities, the P. polycephalum spreads out at first and over time as it finds food at these centers, it pulls in its body to a thin line that is the most efficient way between cities (food). This leaves a network of its body strands that matches the highway system or the rail system on the map, just as was laid out by the transportation planners.

How does this work? Researchers are starting to home in on what happens inside the blob that allows them to remember or learn these thing's but there is much yet to learn. As is the case most such organisms, the learning relates directly to either acquisition of food or avoidance of negative or aversive stimuli. In general the cells pulsate as tubes and when they contact a nutrient source they pulsate faster and when they contact a negative stimulus, the pulsation slows and they back off.

When P. polycephalum contacts a nutrient source, a softening agent is released that in turn fosters an increase in tubes’ diameter. Correspondingly, the portion of the organism that does not receive the softening agent, decreases the tubular diameter. It appears to be the tubular diameter that stores the food related information. For greater detail and video illustrations, see this recent research article from PNAS which is the latest word on this process.

I’ll close with a quotation from Dr. John T. Bonner, aka “The Sultan of Slime,” and Princeton U. professor of ecology who studied slime mold for 70 years. He argues that they are… :

"… no more than a bag of amoebae encased in a thin slime sheath, yet they manage to have various behaviors that are equal to those of animals who possess muscles and nerves with ganglia – that is, simple brains."

If you have read this far, I am sure that most readers will have learned much more about slime mold than they ever thought possible or wanted to. Congratulations on getting this far. I am sure that most of us have known someone who made us wonder how they can survive without a brain. Now we know that it is possible.

What’s oozing around your back yard on these fine moist spring days?