Slime Mold: Definition & Protista Characteristics

Lesson Transcript
Instructor: Sarah Phenix
Slime molds are a species of colonial protists. Explore how these protists form colonies, what they look like, the types of slime molds, and why they are not a type of mold despite their name. Updated: 12/15/2021

Introduction to Slime Molds

To be, or not to be, a fungi? So maybe we don't have to get all Shakespearean about it, but the real question we must ask for this lesson is, Are slime molds a type of mold? The answer to that question is actually a loud and resounding NO.

For a long time, slime molds were thought to be a type of mold (hence their name), but more recently we found out that we were wrong. They don't actually belong to the kingdom Fungi but to the kingdom Protista. That's right, slime mold is a type of protist!

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  • 0:00 Introduction to Slime Molds
  • 0:39 Background on Protists
  • 1:39 Definition of Slime Molds
  • 2:36 Why Isn't Slime Mold a Mold?
  • 3:47 Types of Slime Molds
  • 4:27 Lesson Summary
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Background on Protists

You may be asking yourself what a protist is. It's a fair question, because even biologists have a tough time with this one. We've really only been able to narrow down the protist identity to eukaryotic organisms, meaning having membrane-bound organelles within their cell membranes, that can't be classified as belonging to either the animal, plant, fungus, or bacteria kingdoms. Sounds pretty vague, right?

So where does all this confusion come from? It stems from the polyphyletic nature of protists, meaning that, if you were to draw a family tree of their lineage, you would find that not all protists share the same common ancestor. In other words, some organisms that are considered protists may have stemmed from other kingdoms and, while they no longer share the characteristics that define their ancestral group, they have carried over some of their traits and/or behaviors.

Definition of Slime Molds

Slime molds are a type of protist that aggregates into colonies and ingest bacteria, fungal spores, and possibly other protists. Once the slime mold cells group, they do one of two things: Either they fuse their individual cells into one massive multi-nucleated cell, or they fuse their membranes to one another to form a cluster of individual cells. This fusion results in slime mold colonies that can be anywhere from half an inch in diameter to 12 inches long.

What's more is that not all slime molds are alike. Some have flagellated cells with a tail-like flagellum, while others are amoeboid, having an amorphous shape, and because of these features, they can actually move! But you won't see them in high-speed chases any time soon, as their max speed is about 1 millimeter per hour.

Why Isn't Slime Mold a Mold?

To be a true fungus, an organism must share the following characteristics:

  • Cells greater in size than bacteria
  • Has chitin, the same substance that forms an insect's exoskeleton, in its cell walls
  • Be sessile, or immobile, throughout all life stages
  • Lack chlorophyll

While slime mold cells are larger than bacteria and don't have chlorophyll, they lack chitin in their cell walls and they move. What's more is that fungi don't ingest organisms. They release an enzyme that breaks down their food items, which they then absorb through their skin.

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