How Slime Mold Eats

Eating without a mouth might sound impossible, but Physarum polycephalum has been doing it for hundreds of millions of years. This single-celled organism locates food sources at a distance, moves toward them, engulfs them whole, digests them internally, and then distributes the nutrients throughout its sprawling body. Even more remarkably, it selects a nutritionally balanced diet when given the choice. Here is how all of that works.

Step 1: Finding Food Through Chemotaxis

Before slime mold can eat, it needs to find something worth eating. It does this through chemotaxis: the ability to detect and move toward (or away from) chemical signals in the environment.

Food sources release volatile organic compounds as they decompose. Bacteria, fungal spores, and oat flakes (the standard laboratory food) all emit distinct chemical signatures. The slime mold detects these signals at its growth front, where the cell membrane is in direct contact with the substrate.

The mechanism is remarkably sensitive. Slime mold can detect food sources several centimeters away and will redirect its growth toward the strongest signal. When multiple food sources are present, the organism extends toward all of them simultaneously, creating a branching network that connects each food source to the main body.

Chemotaxis in slime mold is not a simple on-off response. The organism integrates multiple signals simultaneously, weighing the strength of different attractants and repellents to make a net directional decision. This integration happens without any central processing unit. It emerges from the collective behavior of the cytoplasm and the vein network.

Step 2: Engulfing Food Through Phagocytosis

Once slime mold reaches a food source, it eats through phagocytosis, literally "cell eating." The growth front extends around and over the food particle, enclosing it within the cytoplasm. A membrane-bound compartment called a food vacuole forms around the engulfed material.

Inside the food vacuole, digestive enzymes break down the food into its component molecules: sugars, amino acids, lipids, and minerals. These nutrients are then absorbed into the surrounding cytoplasm and become available to the entire organism.

The process is fundamentally the same as how an amoeba eats, or how white blood cells in your own immune system engulf bacteria. The difference is scale. While an amoeba engulfs individual bacteria, a slime mold plasmodium can engulf oat flakes, mushroom fragments, and large colonies of bacteria. Its growth front simply flows over the food and keeps going.

The Menu: Natural Diet vs. Laboratory Diet

Step 3: Distributing Nutrients Through the Vein Network

Phagocytosis happens at the growth front, but the entire organism needs to be fed. This is where the vein network and cytoplasmic streaming become essential.

Once nutrients are digested and absorbed into the cytoplasm, they are carried throughout the organism by the rhythmic shuttle flow that characterizes slime mold. The cytoplasm pulses back and forth through the vein network with a period of approximately 90 seconds, distributing nutrients, waste products, and chemical signals to every part of the body.

The vein network itself adapts in response to feeding. When food is found at a particular location:

1. The veins connecting to that food source become thicker, allowing more cytoplasm to flow toward it.
2. Veins leading to areas without food gradually thin and may be reabsorbed.
3. The organism effectively concentrates its transport infrastructure around productive food sources.
4. If food is depleted at one location, the veins thin again, and resources are redirected elsewhere.

This dynamic remodeling is analogous to how blood vessels grow toward tissues that need more oxygen (angiogenesis) or how roads are built where traffic demand is highest. The difference is that slime mold does it without any central planning authority.

The Cafeteria Experiment: Nutritional Intelligence

Perhaps the most remarkable discovery about slime mold feeding behavior came from research by Audrey Dussutour and her collaborators, published in 2010. The experiment, sometimes called the "cafeteria experiment," revealed that slime mold does not just eat whatever it finds. It actively selects a balanced diet.

The Setup

Researchers offered Physarum polycephalum a choice between different food sources with varying ratios of protein to carbohydrate. Some food patches were high in protein and low in carbohydrate. Others were high in carbohydrate and low in protein. Some had balanced ratios.

The Results

When given a choice, slime mold consistently selected food sources that provided a 2:1 ratio of protein to carbohydrates. This was true regardless of how the food sources were arranged spatially. The organism would extend toward high-protein and high-carbohydrate sources in proportions that resulted in an overall balanced intake.

When only imbalanced food was available (for example, all high-protein or all high-carbohydrate), the organism grew more slowly and showed signs of stress. The 2:1 ratio appeared to be optimal for growth and health.

Avoiding Toxins

Slime mold does not just seek out good food. It also avoids harmful substances. When the growth front encounters salt, quinine, heavy metal ions, or other repellent chemicals, it changes direction and grows away from the source.

This avoidance behavior is mediated by the same chemotactic mechanisms that guide food-seeking. The cell membrane contains receptors that respond to both attractant and repellent molecules. The net result of these opposing signals determines the direction of growth.

Interestingly, slime mold can learn to tolerate repellent substances that are not actually harmful. This was the basis of Audrey Dussutour's groundbreaking 2016 habituation experiment. Slime mold initially avoided bridges coated with quinine or caffeine, but after repeated exposure, it learned that these substances were harmless and began crossing them without hesitation. (For the full story, see Slime Mold Memory.)

Energy Budget and Efficiency

Slime mold is remarkably efficient in its use of energy. When food is scarce, it retracts from less productive areas and concentrates its biomass where food is available. When food is completely absent, it converts its entire body into a dormant sclerotium, reducing its metabolic rate to near zero.

The organism also recycles its own material constantly. Veins that are no longer needed are broken down, and their components are redistributed to growing areas. This means that slime mold produces very little waste. Almost everything it builds can be disassembled and reused.

• A well-fed plasmodium can double its area in approximately 24 hours.
• A starving plasmodium can survive for weeks by progressively shrinking and recycling its own veins.
• A sclerotium (dormant form) can survive without food for months or even years.

Feeding Behavior as Problem-Solving

What makes slime mold feeding behavior so interesting to researchers is not just the mechanics of phagocytosis or the chemistry of chemotaxis. It is the way the organism integrates information from its entire body to make feeding decisions that are, by any reasonable standard, intelligent.

When Toshiyuki Nakagaki placed food at the entrance and exit of a maze, the slime mold found the shortest connecting path. When Andrew Adamatzky placed food on maps in positions corresponding to cities, the slime mold created transport networks comparable to engineered highway systems. When Audrey Dussutour offered dietary choices, the organism selected the optimal nutritional ratio.

In each case, the feeding behavior is the driver. The organism is trying to eat as efficiently as possible, and in doing so, it solves optimization problems that are computationally hard.