Jewels from slime

“They possess … a certain curious elegance that makes them very attractive to everyone who has the slightest sense of artistic delicacy and beauty”1 – T.H. Macbride

Lepidoderma tigrinum on moss

2 mm tall Lepidoderma tigrinum on moss

Slime molds are diamonds in the rough, and the closer you look, the more they shine. You may have encountered them as amorphous blobs on a lawn or a walk in the woods. They are on leaves, in the soil of Antarctica, and in deserts, to name a few habitats. Perhaps you sidestepped a slime mold to avoid getting it on your shoe. If so, you also sidestepped a fascinating organism.

10-hour timelapse of flower-like, fruiting bodies of Ceratiomyxa

To be fair, they have an unappealing name that might discourage the neophyte from looking further. Ironically, slime molds are not molds, although scientists initially classified them as fungi.2 These days biologists shuffle them around into different categories. Broadly speaking, they are mycetozoans,3 but for simplicity, I will stick with the term slime molds.

Eureka moment

My first up-close-and-personal encounter with slime molds under a microscope was at an Eagle Hill Institute course on mushrooms. I found a one-centimeter-tall cluster of chocolate tube slime mold (Stemonitis species) which released spores as puffs of brown, smoke-like dust when touched.

Stemonitis spore dispersal by a passerby (Courtesy of Zoe Weil)

Under the microscope, I could see the spores and, amazingly, the minuscule, amoeba-like organisms that emerged swimming from the spores. I felt like Antonie van Leeuwenhoek discovering microscopic life.4 Unlike Leeuwenhoek, I sometimes refer to slime molds as “little buddies.” I have a soft spot for them.

Stemonitis amoeba swimming after emerging from a spore

The Blob

The amoeba phase is typical of slime mold life cycles. The amoebas fuse into a massive, single-celled plasmodium – the “blob” phase – which can move with pulsating, rhythmic motion reminiscent of a heart beating.5 The plasmodium actively crawls around and hunts for food, such as bacteria. However, unlike “The Blob” of Hollywood fame, slime molds are harmless to humans. Plasmodia can be found as they slowly move through moist environments, for example, under leaves or bark.

Timelapse of a slime mold plasmodium forming fruiting bodies

Slime mold plasmodia on the hunt for food
Slime mold plasmodia on the hunt for food

Under certain conditions, such as exhaustion of available food, the plasmodium blob ceases to forage, coalesces into fruiting bodies, and moves into its spore-production phase called sporulation. The fruiting bodies can be beautiful and gemlike like Lepidoderma; whimsical like the chocolate tube Stemonitis seen above; or amorphous like the appropriately-named dog-vomit, Fuligo septica. They often sit atop a stalk that elevates them above the damp substrate and into position for spore dispersal by the wind or by a curious passerby.

Fruiting body of Lepidoderma tigrinum slime mold
1 mm fruiting body of Lepidoderma tigrinum viewed from above. Who wouldn't want this gem on a ring?

… the passers-by see only a wisp of smoke - Vincent van Gogh

When mature, the wet, glistening fruiting bodies transform into dry, spore-filled structures ready to release the enclosed spores. The mature fruiting bodies are the easiest slime mold phase for species identification. They have certain anatomical features that characterize specific mycetozoans, and most slime molds cannot be reliably identified from a plasmodium or early fruiting body.

Overnight timelapse of Stemonitis forming fruiting bodies

The columnar fruiting bodies of Stemonitis become a tangled network of fibers called a capillitium, which may serve as an extended-release mechanism to disperse spores gradually over a prolonged time. The presence or absence of a capillitium or stalk, for example, can help distinguish one species from another.

Slime mold Stemonitis capillitia
Capillitia of Stemonitis with a network of fibers shown in the inset

Wolf’s milk slime mold, Lycogala epidendrum, has neither stalk nor capillitium. It is a common slime mold often found on decaying wood with coral-colored, immature fruiting bodies that turn brown before spore release. At maturity, wolf’s milk slime mold resembles small, less than 1 cm wide, puffball mushrooms. Unlike dog-vomit slime mold, the name of wolf’s milk slime mold seems to be a stretch as I can’t imagine a healthy wolf producing pink or brown milk, nor is there anything wolf-like about these small pink balls.

Wolf's milk slime mold
Immature and mature (inset) Wolf's milk fruiting bodies
Dog-vomit slime mold
Dog-vomit slime mold (courtesy of Zoe Weil)

Wear your heart on your sleeve

Beetle feeding on Ceratiomyxa
Beetle feeding on Ceratiomyxa fruiting bodies

Most slime molds shown so far have spores inside a fruiting body. The common slime mold Ceratiomyxa wears its spores externally like microscopic fuzz, and it is in a different group of slime molds from those with internal spores. The fruiting bodies of Ceratiomyxa can resemble white columns or florets and are a delicacy to certain beetles.

Ceratiomyxa fruiting bodies and spores
Individual spores on Ceratiomyxa fruiting bodies look like microscopic “lollipops”

When the going gets tough, become a sclerotium

Slime mold sclerotium
Slime mold sclerotium on a rotting log

A moist, well-fed slime mold is a happy slime mold. A hungry slime mold may sporulate, hoping to disperse its spores to a more favorable habitat. A cold, hungry, and dry slime mold can become a sclerotium, a dormant and hardened structure that can survive adverse conditions for a year or more and reanimate when its circumstances improve. Specific slime molds cannot be identified based on sclerotia. 

These dormant forms may hide under the bark of decaying logs.

I haven’t got a brain… only straw – Scarecrow in The Wizard of Oz

Their primitive appearance notwithstanding – and they have been around for at least 500 million years – slime molds show evidence of “intelligence,” despite lacking the brain that Scarecrow coveted. When placed on a map with food at the locations of Tokyo train stations, Physarum polycephalum “solved” the problem of creating an efficient network of connections that closely resembled the actual Tokyo rail network.


Physarum can also be trained to modify their behavior based on previous events. Experimenters found that if they decreased the temperature of Physarum’s feeding chamber at regular intervals, the slime mold crawled more slowly during those periods. After several sessions, the researchers stopped changing the temperature, yet Physarum continued to slow down at the previous regular intervals. In other words, the slime mold had a memory of an earlier event – the temperature change – and an internal clock to regulate its behavior. Not bad for a single cell!

What’s in a name? – Shakespeare in Romeo and Juliet

Slime molds have an image problem – neither “slime” nor “mold” has the appeal of names like the Scarlet Tanager or the Resplendent Quetzal. Yet, anyone with a magnifying glass and the willingness to leave no leaf or old log unexamined can experience their mystery and beauty. You may have a Leeuwenhoek moment!

Notes

1 I can’t be sure, but I think Macbride is saying, “You’re an uncouth ignoramus if you don’t appreciate slime molds!”

2 Hence the name slime mold, since molds are fungi. Most go through a slimy phase, so their name is half-right.

3 Mycetozoa [my-see-toe-ZO-an] means “fungus-animal.” Adding to the confusion, myxomycetes [mix-oh-MY-seets, meaning “mucus-fungus”] are a subgroup of these “fungus-animals.”

4 Antonie van Leeuwenhoek (1632-1723) revolutionized microscopy and discovered microscopic life, including bacteria.

5 Some slime molds form a “fake” plasmodium or pseudoplasmodium. Unlike the “real,” single-celled plasmodium, the pseudoplasmodium is composed of many cells.

References

Thanks

Special thanks to Zoe Weil for editorial expertise and select media.

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Rae
Rae
1 year ago

Holy slime Batman! This one blew me away. I would be so happy if you would do a Ted talk on this so you could go into more detail. I have so many questions. Like, is there really a dog-vomit slime mold? Absolutely LOVE the Physarum intelligence studies. And, I would love to know how you set up the time-lapse videos…were you able to set it up and leave it for hours…or did you stay there with the camera the whole time? These posts are really my new fave thing to see in my inbox! Thanks for the joy of your observations in a chaotic and disturbing time on earth with these darn humans. I have my regular meditation and then I have the meditation of reading your blogs!

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