Blob vs. mushroom: why they are not the same thing
The blob looks like a mold, but it is NOT a fungus. Here are 7 fundamental differences between Physarum polycephalum and true fungi (mycelium, yeasts, molds).
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When you discover the blob, the first instinct is to think of it as a fungus. It’s understandable: same habitat, same appearance. Yet, they are two very distinct branches of life that function in radically different ways. Here are 7 real differences — and one very widespread myth to correct along the way.
1. The cell wall
Fungi: rigid wall composed of chitin (the same substance as the shell of insects).
Blob: simple, flexible plasma membrane, like an animal cell. This is what allows it to change shape constantly and crawl.
2. Mode of nutrition
Fungi: they digest externally — they secrete enzymes into their environment, then absorb the dissolved nutrients.
Blob: it performs phagocytosis — it physically engulfs bacteria, yeast, and spores inside its cell, like a giant amoeba. It also absorbs dissolved nutrients, which explains why it is perfectly happy with rolled oats.
We often read that "the blob is a predator, fungi are decomposers." That is a school-level simplification: the blob is not strictly a predator, and fungi are not only decomposers (there are parasites, mycorrhizal symbionts, and even nematode-trapping fungi). The real difference is where the digestion happens: inside for the blob, outside for the fungus.
3. Mobility
Fungi: the mycelium extends through growth, but the organism does not move.
Blob: it actually moves, transporting its entire cytoplasm, from a few millimeters to a few centimeters per hour. It can abandon an entire area to go elsewhere.
"A fungus grows toward its food. A blob goes to it, then moves on."
4. Structure
Fungi: multicellular, with partitions between cells.
Blob: a single cell, containing millions of nuclei in a common cytoplasm — a syncytium. No partitions anywhere.
5. Reproduction — watch out for the myth
Blob: 720 mating types (matA 16 alleles × matB 15 × matC 3).
6. Classification
Fungi: Fungi, in the Opisthokonta group — the one that also contains animals.
Blob: Amoebozoa → Myxogastria (myxomycetes) → Physarales → Physaraceae.
Amoebozoa and Opisthokonta belong to the same grand group, Amorphea. In other words, the blob is about as close to animals as it is to fungi — and very far from plants. Therefore, there is no evolutionary chasm between it and a fungus: they are two neighboring branches that have invented completely different solutions.
Source: Adl, S.M. et al. (2019). Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes. Journal of Eukaryotic Microbiology, 66(1), 4-119.
7. Evolutionary age
Both lineages are very ancient, and both dates are debated: about 1 billion years for fungi according to molecular clocks, and a comparable order of magnitude for Amoebozoa. Neither one leaves usable fossils — be wary of figures presented as settled facts.
Quick recap
| Criterion | Fungus | Blob |
|---|---|---|
| Wall | Chitin | Flexible membrane |
| Digestion | Outside the cell | Inside the cell |
| Mobility | Grows, does not move | Moves (mm to cm/h) |
| Structure | Multicellular | One cell, millions of nuclei |
| Mating types | 2 to over 23,000 depending on species | 720 |
| Group | Fungi (Opisthokonta) | Myxogastria (Amoebozoa) |
So why do we confuse them?
- Appearance — a soft, colored mass on dead wood looks like mold
- Habitat — exactly the same damp undergrowth
- History of science — they were long classified among fungi. It was Anton de Bary who, in 1859, linked them to animals under the name Mycetozoa — the complete separation from fungi would take another century and a half.
