Slime Molds: The Weirdos of the Forest
By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
Some strange things live in the forest at Tryon Creek State Natural Area (TCSNA) but for my money, none is stranger than the organisms known as slime molds. Taxonomists, folks who specialize in classifying organisms, haven’t all agreed on how to classify slime molds. They do, however, all agree that slime molds are clearly neither plants, animals, fungi nor bacteria. Slime molds are fascinating creatures because they have a very strange life cycle, and a highly unusual “body”. This note focuses only on the plasmodial slime molds, which are the type you will probably see at TCSNA. [Important: The slime mold names used in this note represent my best efforts at identifying these creatures, based on their strong similarity to photos on the internet.]
Where do slime molds grow?
The best place to find slime molds at TCSNA is on rotting wood. Old logs, tree stumps, or dead standing trees are prime candidates. This is because the primary foods for slime molds are bacteria and fungi. These are abundant in dead wood. Experts say the best times to find these slime molds is either spring or fall, when the forest is fairly damp. The slime molds pictured in this note were found at TCSNA in April, July, September and November.
What’s so weird about slime molds?
The weirdest thing about slime molds is their dramatic changes in shape over the course of their life cycle. Let’s start with the point in the life cycle which gives these organisms their name. The most active adult stage of the slime mold is when it looks like (surprise, surprise) slime! This nearly formless stage is called the plasmodial stage. At the risk of being indelicate, the adult slime mold in this stage looks like someone with serious nasal congestion blew their nose onto a log. This stage looks a little “blob-y” and has a distinct “wet” appearance. The plasmodium stage of life is the diploid stage where the slime mold has chromosomes from both parents, just like you. The example pictured below was on a decaying tree trunk that was lying on the ground near the West Horse Loop Trail.
These blobs of life are unusual in that they are giant cells with many thousands of nuclei in each cell. For most life forms, one nucleus per cell is the rule. Also, at this stage, there is a thin cell membrane, but no rigid cell wall. The big advantage to having giant wall free cells is that these plasmodia can move by streaming the cell contents (cytoplasm) from one end of the plasmodium to the other end. The plasmodium will move in the direction that the streaming cytoplasm is heading. Laboratory studies have observed slime molds moving at approximately 1 inch per day towards concentrations of food.
When food starts to become scarce, the slime mold moves into the next stage of life. This stage is called a sporangium. The sporangium, as you might guess, is the stage that produces the spores. The forms of the sporangium differ greatly, depending on the species of slime mold.
How does the sporangium develop?
There are many different sizes, shapes and colors of sporangia, depending on the species of slime mold. Examples I’ve found at TCSNA are included below.
The series of photos below shows the development of a single sporangium found on a standing dead tree along the Trillium Trail is shown. Unfortunately, I found the sporangium when it was completely developed. This was formed by a plasmodial mass similar to the one pictured above. A tough shell develops to protect the developing spores on the inside. This sporangium is the species of slime mold called “false puffball”. Its scientific name is Enteridium lycoperdon. The most striking thing about this sporangium is that in my entire life I have never seen a natural object that has looked so much like plastic. Measured vertically along the trunk of the tree, it is about 3 inches long.
Just one day later, the surface of the sporangium has started to crack apart. The interior of the sporangium is filled with small brown spores. This particular sporangium was growing very close to the trail. I suspect the yellowish area which is oozing just a little yellow fluid is in fact a wound inflicted by a curious visitor to the park!
After three additional days, the surface of the sporangium is starting to seriously deteriorate, exposing even more brown spores.
In just an additional 3 days, the surface of the sporangium is almost completely gone, and many of the spores have been washed or blown away. Now the spores will germinate and produce single celled amoeba-like cells that crawl around. These cells are the functional equivalent of human egg and sperm cells. These amoeba-like cells will find and fuse with a compatible amoeba-like cell. Then this fused cell will grow to become a new plasmodium, restarting the cycle.
The photo below gives you an idea of what the interior of this slime mold sporangia contains.
While observing the above slime mold, I noticed some insects on its surface. As I approached quite close to take photos, the insects boldly maintained their positions. I sent this picture to Josh Vlach, an entomologist with the Oregon Department of Agriculture. He indicated this insect “looks like a Mycetophilidae possibly a species of Mycetophila”. Mycetophilidae is a family of insects, while the Mycetophila is a genus within that family. The common name for this group of insects is “fungus gnats.” This type of insects oftentimes lay their eggs in either mushrooms or slime molds. The developing larvae eat the mushroom or slime mold. One of these insects appears in the picture below.
Are there other kinds of slime mold at TCSNA?
Yes, I’ve spotted several other kinds of slime molds at Tryon Creek. Below is an example of a slime mold in an advanced stage of spore production. It was on the side of a downed log just off the Old Main Trail. The cluster of spore producing bodies seem to be resting on a thin sheet of shiny material that looks like dried slug slime. The entire cluster is 9 inches horizontally, and 6-1/2 inches vertically. The thickness of these spore clusters is less than 1 inch. When touched, they easily broke into a dark brown powder. These appear to be the species Tubifera ferruginosa, the red raspberry slime mold. In a younger stage, which I clearly missed, they are bright red.
In the close-up below, you can see more detail of the structure of this slime mold.
Next is the dog vomit slime mold. (I don’t name ‘em, I just report ‘em!) For once, you might like the Latin name better – Fuligo septica. Below is the sporangium of this colorful slime mold, which I found on a fallen log next to the Middle Creek Trail. The outer covering of the sporangium is just starting to break apart, revealing the brown spore bearing parts of the slime mold. On the moss just below the sporangium, you can see a few remnants of plasmodial strands that didn’t quite make it into the sporangium.
Below is a close-up of the surface of the dog vomit slime mold. It is substantially different in both color and texture from the first slime mold pictured in this note.
Slime molds are an amazingly diverse group of organisms, and the next species testifies to that. The photo below appears to be a slime mold in the genus Trichia. The plasmodium, the white slimy part, and the sporangia, the orange balls on a stalk, coexist. The orange blobs bear the spores for this slime mold.
Not only are the sporangia of this species dramatically different in appearance, they also differ in size. The next photo compares the sporangia to my thumbnail.
So what’s the lesson here?
The slime molds really are the weirdos of the forest, and trust me, this note only scratches the surface of that weirdness. They remind us that there are many ways to be successful. The slime molds eat the bacteria, and the larvae of the gnat fly eat the slime molds, and many things eat the gnat files. Every creature in creation is linked together, and we would be wise to remember that.