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Last week, a few days after the rain, I was walking across my yard when I noticed what looked like some sort of disease on the artemisia.
I looked more closely. Could it be insect eggs?
Nearby I saw something similar on the mulch.
Could an insect lay that many eggs? Or was it some kind of plant? Seeds? Two days later I saw a yellow version growing up from the base of one of my bottlebrush trees.
Holy cow. It was even coming out of the bricks below the tree. Yellow algae? Bubbly fungus?
It turns out to be “insect egg slime mold”, also known as Leocarpus fragilis. Surprisingly, a slime mold is not a fungus, nor is it a plant. Rather, it is a protist, which means, among other things, that it has only one cell. A single cell that is easily visible to the eye. It took me a while to wrap my head around that.
They don’t start off so big. Their life begins with a tiny microscopic spore. (That is one reason why scientists considered slime molds to be fungi for a while.) When the spore germinates, a single-celled amoeba emerges, sometimes with two tails (flagellae) for swimming if the environment is wet. These tiny cells eat decayed matter and can live in perpetuity in dark and damp areas. In harsh environments, they transition to microcysts, a dormant phase that can better withstand variations in humidity, temperature, and pH, even for decades. When conditions improve, they go back to being an amoeba.1
If two compatible amoebas find each other, they can fuse their bodies and nuclei to form a zygote. As that new cell eats and grows, it does not do typical cell division. The nucleus divides, but no new cell walls are formed. So it becomes a very large multi-nucleated, single-celled plasmodium. This veiny structure creeps very slowly over damp wood and leaves looking for food and staying away from light. Some of these can be as large as 2-3 feet in diameter. This phase can also go dormant, forming a dried out sclerotium in adverse conditions. (This form is sometimes conveniently used for shipping specimens.)2
In the picture below you can see some of the wet plasmodium on a branch of artemisia as the mold prepares to transition to producing the fruiting bodies (sporangia) shown in the earlier photos.
Here you can see the veiny outline of the plasmodium as it starts to form sporangia.
This is a pretty interesting lifecycle, no? When the slime mold shifts from the plasmodial stage to producing sporangia, it often seeks a lighter and drier location where the spores have a better chance of dispersing. That must be why it crept up the trunk of my bottlebrush and peeked out from between the bricks.
Scientists have been pretty intrigued by this organism. It is only a single cell but it can navigate a complex environment, head towards food and away from light, and fight off predators (some bacteria, fungi, beetles, etc). Researchers have explored its anti-microbial properties for potential drug use.
A plasmodium has no brain or other center of control, but it seems to move with purpose. You can find many videos of them solving mazes and finding efficient routes on mini maps of countries or cities that are similar to the highway or transit systems humans have built. Slime mold movement has even inspired a computational model for how dark matter is distributed throughout space.3 More generally, these very simple organisms show evidence of communication, memory, and intelligence that mimic behaviors of much more complex organisms, and researchers are working to understand that.
After just a few days in my yard, the sporangia of the Leocarpus fragilis turned darker and hardened as they dried out. Compare the photo below with the one above, taken just one day earlier.
Here you can see traces of desiccated plasmodium in the middle of the hardening sporangia.
The slime mold at the base of the bottlebrush now looks like this.
You may be wondering what’s inside the little knobs. When the sporangia are newly formed they exude a milky substance if they are broken apart.
Over time the inside darkens and hardens. When the outer shell cracks, dry spores are released. The spores look a bit like pepper in the photo below. They are typically dispersed through the air, but sometimes insects or other small animals pick them up as they are walking by and distribute them to new places.
Over 1000 species of slime molds have been identified. Some different fruiting bodies found locally are shown below, taken by Palo Alto resident Robert Siegel. (More of his slime mold photos can be found here.) You cann also find some beautiful pictures on Instagram at #slimemoldsunday.
Next week, after the rains finish, will be a great time to look for slime molds in wooded areas, under logs, in nooks and crannies on old bark. Some of them are easy to spot — bright yellow and large, as in my yard — while others are smaller. If you see people with big cameras peering at fallen trees in a forest, they may be looking for slime molds. This terrific video gives you a sense of the scale of some of these sporangia.
I loved finding these in my yard. My gardener suspects they may have come in via some mulch we put down last spring. That mulch was supposed to have been composted, but I wonder if some microcysts survived the process, or if the composting was incomplete. Either way, I am thrilled to have them.
If you are fortunate to have a yard or garden of some sort, take a moment to think about what role you want it to play. It should look nice. You should be able to relax in and enjoy it. Beyond that, though, you might also consider some simple ways to nurture biodiversity in your garden, for example by using arbor mulch, leaving downed tree branches, allowing leaf litter to stay on the ground. Taking such steps helps to provide good habitat for all of these tiny and not-so-tiny creatures. In turn, they will feed a food chain, make our soil richer, and grow healthier plants.
I would love to hear any questions you have about these slime molds, or see any photos you have taken if you find some!
Current Climate Data
Global impacts (December 2023), US impacts (January 2024), CO2 metric, Climate dashboard
In just the five years since I have been writing this blog, the CO2 level in our atmosphere has climbed from 410 ppm to 423 ppm. In pre-industrial times, it was 280 ppm. As you can see in this chart, the rate of growth is accelerating.
Notes and References
- These organisms are very resilient, in part because of the many forms they can take. In 2019 researchers published a report in Nature describing their find of 100-million-year-old slime mold sporangia that were extremely well preserved in amber. Slime molds are found all around the planet, though more in temperate areas like ours. ↩︎
- This blog post describes the lifecycle of one type of slime mode, the plasmodial slime molds. There are other kinds that have different stages of development. ↩︎
- People have gotten pretty silly with slime mold applications as slime molds have entered the popular culture. Some have made music from them, or animated expressions on sculptural heads. One artist frustrated with navigating Ikea stores asked a slime mold to find the exit. ↩︎