Raspberry Slime Mold Spotted!

Baker Creek Preserve in Knoxville, TN

Walking in the forest is always a treat. The crunch of leaf litter, the snap of small twigs, the flutter of leaves and bird wings, and of course, good exercise in good company.

But there’s absolutely nothing like walking in the forest, surrounded by every imaginable shade of green, and turning the corner to find a bright pink slime mold.

Raspberry Slime Mold, Tubifera ferruginosa, joined by moss on a rotting log

My husband and I really couldn’t believe our eyes! It looked like someone had dropped some cotton candy on a log. The main patch was only about three inches across, but it was as vibrant as a neon sign yelling, “SLIME MOLD HERE!”

Although slime molds have historically been studied by mycologists, they are not fungi. As of the early 1990s, slime molds are classified in a kingdom called Protozoa, which also includes amoebas. There are two major types of slime molds: dictyostelids (dik-tee-oh-steel-ids) and myxomycetes (mix-oh-my-see-tees). Dictyostelids (or “social amoebas”) are microscopic, feeding mostly on bacteria. They live as single cells (“amoebas”) until food runs out, at which point they all join together into a big lump called a “slug” to reproduce (hence, “social”). The myxomycetes are the slime molds visible to the naked eye, and you can see their taxonomic history in their name: myxa is Greek for “slime”, while “mycete” usually refers to a fungus.

Myxomycetes occur worldwide on soil and various types of decaying plant matter (1). They have two vegetative (non-reproductive) life stages, the first of which is the microscopic “amoeba” stage. The second growth phase and the reproductive phase are both visible to the naked eye in many species, including this one. The second growth phase is called a plasmodium (plaz-mow-dee-um), and I believe we can see the plasmodium here as the goopy, undifferentiated mass in the foreground of this picture:

This plasmodium is not a collection of single-celled amoebas, as it would be in a dictyostelid slug. Instead, the amoeba life phase ends as amoebas merge into the large mass, creating one huge multinucleate goop. This is basically one cell with hundreds or thousands of nuclei that continue feeding and moving along, helping each other, until it’s time to reproduce.

It’s hard to confirm that my picture shows the plasmodium because all the pictures I could find of the “Raspberry Slime Mold”, Tubifera ferruginosa, were of their spectacular fruiting bodies. The myxomycota were originally considered fungi because, like fungi, they reproduce by releasing spores from specialized fruiting bodies. The fruiting bodies are also often the most easily identifiable part of a slime mold’s life cycle, and in this case, they are the source of the slime mold’s common name. See how the surface of the structures in the next picture looks like the bumps on a raspberry? The vibrant color of the slime mold is also conveyed in its Latin name: ferrug- means “rusty” (2), a common Latin word used to describe red organisms.

Tubifera ferruginosa showing well-defined cylindrical fruiting bodies clustered together in a pillow-like structure (pseudoaethalia). The white slime visible at the base of each pseudoaethalium is the hypothallus.

In Tubifera ferruginosa, the spores are stored in vertical tubes stacked together like pencils or cigars. The clumps are called pseudoaethalia (soo-dough-eh-thall-ee-ah), which means “structures similar to aethalia”. An aethalium (eh-thall-ee-um) is a pillow-shaped structure made up of plasmodium. So a pseudoathalium is a pillow-like structure (like an aethalium) but it is made up of fruiting bodies (unlike an aethalium). Easy enough…?

Since there are lots of these well defined, pillow-shaped clusters of well-defined spore tubes, and there is a slimy mass of the same color that is not a well defined shape, I am assuming that that slimy mass is the plasmodium. I am not an expert on myxomycetes and it is possible that the slime is a different structure getting ready to firm up into a pseudoaethalium.

At the base of each pseudoaethalium, we can see the white mass of the hypothallus (hi-po-thall-us) supporting the fruiting structures off the log.

More pseudoaethalia of Tubifera ferruginosa. The white hypothallus has been indicated with an arrow.

As T. ferruginosa ages, the vibrant red/pink color fades to purple, then to grey or brown as the spores mature and become ready to disperse. We unfortunately didn’t make it back to the park to watch the slime mold mature through its color changes, but I’m very grateful that we saw it at its most vibrant!

I hope you’re able to enjoy a walk outside this summer. Remember, no matter your biome – forest, prairie, park, or desert – there are fungi (and myxomycetes) out there waiting for you to discover!

Cited Sources

  1. Martin, G. W., and C. J. Alexopoulos. 1969. The Myxomycetes. Iowa City: University of Iowa Press.
  2. Emberger, G. (2008). Tubifera ferruginosa. Fungi Growing on Wood. Retrieved October 10, 2021, from https://www.messiah.edu/Oakes/fungi_on_wood/club%20and%20coral/species%20pages/Tubifera%20ferruginosa.htm.


    • I’m glad you enjoyed the slime molds! The question of the plural is an interesting one. The spores in the little tubes are definitely individuals, but the plasmodium is basically a giant sac with lots of nuclei. Is each nucleus its own entity? Or are they all one being together? It’s hard to know.

      I also have updated the post to clarify that at the point I photographed this species, it was no longer in the amoeba stage. Thank you very much for your comment!


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