Slime Mold: Not a Mold, and Its Body Can Be a Single Cell

Bench card

Name

Physarum polycephalum Schwein., 1822, a plasmodial slime mold; the feeding stage, the plasmodium, is one cell with many nucleiSource: https://www.gbif.org/species/3214606; Gerber et al. 2022, https://doi.org/10.7554/eLife.69745

Group

Amoebozoa (Schaap et al.); in GBIF: kingdom Protozoa, phylum Mycetozoa, class Myxomycetes, order Physarales; not a fungusSource: https://www.gbif.org/species/3214606; Schaap et al. 2016, https://doi.org/10.1093/gbe/evv237

Actual size

plasmodium 0.5–1.0 cm in small lab networks to tens of centimetres or more [10⁻³–10⁻¹ m]; as large as 900 cm² in area under ideal conditions in one review, square metres in another account; nuclei 5.65 µm on average; veins 10–140 µm wide in one imaging studySource: Gerber et al. 2022, https://doi.org/10.7554/eLife.69745; Reid 2023, https://doi.org/10.1007/s10071-023-01782-1; Marbach et al. 2023, https://doi.org/10.7554/eLife.78100

Size in this image

not measurable from this image

Lives in

decaying plant material; its prey includes bacteria and yeastsSource: Schaap et al. 2016, https://doi.org/10.1093/gbe/evv237; Reid 2023, https://doi.org/10.1007/s10071-023-01782-1

Seen with

naked eye for the plasmodium and its wider veins; light microscope for the nuclei and the flowSource: Schaap et al. 2016 (macroscopic plasmodial cells), https://doi.org/10.1093/gbe/evv237; Gerber et al. 2022 (nuclei imaged by microscope), https://doi.org/10.7554/eLife.69745

Color in images

native in the three photographs (no stain stated); drawn colours in the diagramsSource: photo captions on this page

Key source

Gerber et al. 2022, eLife 11:e69745 (one cell, nuclei); Reid 2023, Animal Cognition 26:1783-1797 (biology, maze)

slime-mold-bench.csv (CSV) · where each value comes from, and its license (JSON)

Data credits for this card

Data: GBIF Secretariat, GBIF Backbone Taxonomy, checklist dataset https://doi.org/10.15468/39omei, CC BY 4.0, Physarum polycephalum (3214606), retrieved 2026-10-25; Gerber T et al. 2022, eLife 11:e69745, CC BY 4.0, https://doi.org/10.7554/eLife.69745, retrieved 2026-10-25; Schaap P et al. 2016, Genome Biology and Evolution 8(1):109-125, CC BY 4.0, https://doi.org/10.1093/gbe/evv237, retrieved 2026-10-25; Reid CR 2023, Animal Cognition 26(6):1783-1797, CC BY 4.0, https://doi.org/10.1007/s10071-023-01782-1, retrieved 2026-10-25; Marbach S, Ziethen N, Bastin L, Bäuerle FK, Alim K 2023, eLife 12:e78100, CC BY 4.0, https://doi.org/10.7554/eLife.78100, retrieved 2026-10-25

Is a slime mold a mold, or any kind of fungus?

No, although it was long filed as one. Myxomycetes, the plasmodial slime molds that include Physarum, were first thought to be fungi, Stephenson writes in a 2023 review: their fruiting bodies look like those of some small fungi, and both groups turn up in many of the same places. Up close the fruiting bodies are built very differently, and the two life cycles share little beyond making resistant spores. Myxomycetes were not universally recognized as a separate group until well into the twentieth century.

Genome work puts P. polycephalum in the Amoebozoa. Schaap and colleagues describe the Amoebozoa as the sister group of the opisthokonts, the branch that holds both fungi and animals (2016). The cellular slime mold Dictyostelium discoideum, the closest model organism with a genome sequenced before then, is an amoebozoan too. As of October 2026 the GBIF Backbone Taxonomy files P. polycephalum under kingdom Protozoa, phylum Mycetozoa, class Myxomycetes, and files baker's yeast under kingdom Fungi.

The slime is real. A plasmodium has no cell wall, but a thin layer of slime usually covers it and protects it against injury and drying out, by Stephenson's account; plasmodia range from colorless to black, and white and yellow are common. P. polycephalum grows on decaying plant material (Schaap and colleagues) and takes in bacteria, yeasts and small particles as food (Reid, 2023).

How can one cell hold millions of nuclei?

A plasmodium grows by dividing its nuclei without dividing the cell around them. The result is a syncytium, one mass of cytoplasm shared by many nuclei, Gerber and colleagues explain in a 2022 study. Citing earlier studies, they put the count at tens to millions of nuclei depending on the plasmodium's size, and in fixed, stained plasmodia grown on lab dishes they saw tens of thousands. Reid's review gives millions.

In Gerber's images the nuclei averaged an estimated 5.65 µm across and sat 6.63 µm apart on average, and 99.9% had a neighbour within 12 µm. They were packed more densely at the growing front than in the veins. Some sat trapped in the ectoplasm, which is made of cross-linked contractile fibres, while others rode the flow. By Schaap and colleagues' account, the nuclei of a plasmodium divide in step, keeping their nuclear envelopes intact, and a piece cut from it keeps that timing.

Slide the diagram sideways to read its labels, or open it full size.

That makes the plasmodium an odd single cell. A Paramecium is also one cell, and P. caudatum is 140–311 µm long across the studies one database compiles (Velho Rodrigues and colleagues, 2021). Volvox goes the other way: a V. carteri sphere several hundred micrometres across is built from about 2,000 small cells and 12–16 large ones (Umen, 2020).

A plasmodium a few centimetres across would sit in the 0.2–10 cm macrozooplankton class, beside krill and comb jellies, by the size classes that one global ocean study uses for zooplankton, part of the plankton (Hatton and colleagues, 2021). Reid's review gives up to 900 cm² under ideal conditions, and Gerber's team writes that a plasmodium can grow to cover square metres, near the top of the size range of cells.

Why does its inside flow back and forth?

Behind the fan, the plasmodium is a network of veins. In networks that Marbach and colleagues imaged in 2023, the veins had radii of 5–70 µm, so the widest, at 140 µm, were wider than a human hair, typically 50–100 µm (Li and colleagues, 2023). The vein walls contract in a rhythm, once every 1–2 minutes in those measurements. The flow inside reverses within each cycle.

Other sources also give minutes. Reid's review says around 1–5 minutes, depending on the plasmodium's internal state, and Kishimoto's 1958 recordings (abstract) give 60 to 180 seconds for an electrical rhythm that matched the streaming.

For a cell, the flow is fast. Typical speeds reach about 1 mm/s, Marbach's team writes, citing earlier work, and Gerber's team gives up to 1.3 mm/s for nuclei carried along, among the fastest cytoplasmic flows ever measured. The whole plasmodium crawls at up to 5 cm an hour by Reid's account, so the fastest stated flow is roughly 90 times that, computed for this page.

Viscosity still wins in these veins. The Reynolds number — a ratio of inertia to stickiness — for 1.3 mm/s in a 140 µm vein is about 0.18 even if the cytoplasm were as thin as water at 20 °C, computed for this page. With the endoplasm density and viscosity that Oettmeier and Döbereiner use, 1,120 kg/m³ and 0.275 Pa·s, it drops to about 0.00074.

Slide the diagram sideways to read its labels, or open it full size.

Flow covers distance far faster than diffusion would. At 1 mm/s the flow carries material 1 cm in 10 seconds, while oxygen diffusing through still water at 25 °C would need about 24,000 seconds, some 6.6 hours, to spread that far along one axis, computed for this page. Reid's review says the flow moves biomass, nutrients and chemical signals around the cell. The same arithmetic of diffusion and distance comes up again in how insects breathe.

Can a slime mold solve a maze?

In the sense one experiment measured, yes: as a 2023 review describes it, a plasmodium spread through a maze ended up as one tube along the shortest route. The experiment appeared in Nature in 2000 under the title "Maze-solving by an amoeboid organism" (Nakagaki and colleagues, bibliographic record). In Reid's account, plasmodia were spread through a labyrinth linking two food sources, so that a single cell filled the maze. That maze had four possible solutions, and over time the cell pulled back from the dead ends and the longer paths until one tube remained.

The shortening, in Reid's account, runs through the flow itself. Tubes lined up with the waves of contraction get more of the flow and thicken, while tubes that lie across them get less and decay. The tubes that can carry the most flow are those on the shortest path, so feedback trims the network down to it. Reid also points out that these maze tests measured navigation, not learning: no plasmodium was given the same maze twice.

Slide the diagram sideways to read its labels, or open it full size.

A larger test followed. Tero and colleagues placed food on an agar plate shaped like the Tokyo area, at the positions of railway stations, and the plasmodium pruned its network to link them over about 26 hours, by Reid's account. The networks it built had efficiency, fault tolerance and cost comparable to the real Tokyo rail system, the study's abstract reports (2010).

How does a plasmodium turn into spores?

When food runs out, a plasmodium can turn into fruiting bodies. After several days without food, a P. polycephalum plasmodium becomes ready to sporulate, and in the lab a pulse of blue or far-red light, or a heat shock, sets it off, Barrantes and colleagues write (2010). The protoplasmic mass then forms multiple fruiting bodies, and each sporangium holds hundreds of spores made by meiosis, each with one nucleus (Schaap and colleagues). Among myxomycetes, Stephenson counts a fruiting body of about 1 mm as moderately sized.

One amoeba with a single nucleus hatches from each spore. In free water most such cells take a flagellated form, while in drier conditions most stay amoeboid, Stephenson writes of myxomycetes in general. Two amoebae of different mating types can fuse into a zygote, which grows into a new plasmodium by dividing its nucleus again and again (Schaap and colleagues; Gerber and colleagues).

So a new plasmodium starts from a cell with one nucleus. Gerber's team saw tens of thousands in plasmodia grown on lab dishes, and Reid's review gives millions, but none of the sources cited here counts the nuclei in a plasmodium of known size near the 900 cm² that Reid gives as possible.

Sources

1. Gerber T, Loureiro C, Schramma N, Chen S, Jain A, Weber A, Weigert A, Santel M, Alim K, Treutlein B, Camp JG. "Spatial transcriptomic and single-nucleus analysis reveals heterogeneity in a gigantic single-celled syncytium." eLife 11:e69745 (2022). https://doi.org/10.7554/eLife.69745 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
2. Reid CR. "Thoughts from the forest floor: a review of cognition in the slime mould Physarum polycephalum." Animal Cognition 26(6):1783–1797 (2023). https://doi.org/10.1007/s10071-023-01782-1 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
3. Schaap P, Barrantes I, Minx P, Sasaki N, Anderson RW, Bénard M, Biggar KK, Buchler NE, Bundschuh R, Chen X, Fronick C, Fulton L, Golderer G, Jahn N, Knoop V, Landweber LF, Maric C, Miller D, Noegel AA, Peace R, Pierron G, Sasaki T, Schallenberg-Rüdinger M, Schleicher M, Singh R, Spaller T, Storey KB, Suzuki T, Tomlinson C, Tyson JJ, Warren WC, Werner ER, Werner-Felmayer G, Wilson RK, Winckler T, Gott JM, Glöckner G, Marwan W. "The Physarum polycephalum Genome Reveals Extensive Use of Prokaryotic Two-Component and Metazoan-Type Tyrosine Kinase Signaling." Genome Biology and Evolution 8(1):109–125 (2016). https://doi.org/10.1093/gbe/evv237 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
4. Stephenson SL. "Past and Ongoing Field-Based Studies of Myxomycetes." Microorganisms 11(9):2283 (2023). https://doi.org/10.3390/microorganisms11092283 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
5. GBIF Secretariat. GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei, accessed 2026-10-25; Physarum polycephalum Schwein., 1822, usage key 3214606; Saccharomyces cerevisiae (Desm.) Meyen, usage key 7440012. Read: dataset record (species match and species records). Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
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9. Marbach S, Ziethen N, Bastin L, Bäuerle FK, Alim K. "Vein fate determined by flow-based but time-delayed integration of network architecture." eLife 12:e78100 (2023). https://doi.org/10.7554/eLife.78100 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
10. Li C, Du Y, Chen H, Han X, Wu W, Kong X, Zhang C, Yuan X. "Transmission infrared micro-spectroscopic study of individual human hair." Journal of Biomedical Optics 28(11):116501 (2023). https://doi.org/10.1117/1.JBO.28.11.116501 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
11. Kishimoto U. "Rhythmicity in the protoplasmic streaming of a slime mold, Physarum polycephalum. I. A statistical analysis of the electric potential rhythm." The Journal of General Physiology 41(6):1205–1222 (1958). https://doi.org/10.1085/jgp.41.6.1205 Read: abstract.
12. Oettmeier C, Döbereiner HG. "A lumped parameter model of endoplasm flow in Physarum polycephalum explains migration and polarization-induced asymmetry during the onset of locomotion." PLOS ONE 14(4):e0215622 (2019). https://doi.org/10.1371/journal.pone.0215622 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
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14. International Association for the Properties of Water and Steam. IAPWS R12-08, Release on the IAPWS Formulation 2008 for the Viscosity of Ordinary Water Substance (2008). https://iapws.org/technical-guidance/release/viscosity Read: values as computed from the release equations in this site's water-properties table. Text license: IAPWS: publication in whole or in part allowed with attribution (https://iapws.org/technical-guidance/release/viscosity).
15. Somerville GA, Proctor RA. "Cultivation conditions and the diffusion of oxygen into culture media: the rationale for the flask-to-medium ratio in microbiology." BMC Microbiology 13:9 (2013). https://doi.org/10.1186/1471-2180-13-9 Read: full text. Text license: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/).
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17. Nakagaki T, Yamada H, Tóth Á. "Maze-solving by an amoeboid organism." Nature 407(6803):470 (2000). https://doi.org/10.1038/35035159 Read: bibliographic record only.
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19. Barrantes I, Glöckner G, Meyer S, Marwan W. "Transcriptomic changes arising during light-induced sporulation in Physarum polycephalum." BMC Genomics 11:115 (2010). https://doi.org/10.1186/1471-2164-11-115 Read: full text. Text license: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/).

Cite this page

Ants, Cells & Atoms. “Slime Mold: Not a Mold, and Its Body Can Be a Single Cell.” Published 2026-10-25. https://antscellsandatoms.com/microscopic-life/slime-mold/. Accessed (add the date you read it).

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