Slime molds are fascinating organisms that challenge traditional biological definitions, often appearing as colorful, amorphous blobs on decaying wood and leaf litter. These creatures are not true fungi, plants, or animals, yet they possess characteristics that seem to borrow from all three kingdoms. Their name comes from their gelatinous or slimy appearance, with macroscopic species frequently observed as bright yellow, orange, or creamy masses that can appear almost overnight on a forest floor or in mulch. This unusual appearance and ability to change form contributed to scientific confusion regarding their proper biological classification.
Understanding the Classification Confusion
Slime molds were historically grouped with fungi because they reproduce by forming spore-producing structures. However, modern molecular and cellular analysis established that they do not belong to the Kingdom Fungi. Slime molds are now categorized under the Kingdom Protista, a diverse group of eukaryotic organisms that are not plants, animals, or fungi.
A key difference is cell wall composition; true fungi have cell walls made of chitin, while slime mold spores possess cellulose. Furthermore, fungi obtain nutrients by external digestion and absorption. Slime molds, conversely, feed like protozoans, engulfing food through phagocytosis. This ability to exhibit amoeboid movement and ingest particles distinguishes them from fungi.
The Two Major Types
Slime molds are broadly separated into two main categories: Plasmodial (Acellular) and Cellular slime molds. Plasmodial slime molds, such as Physarum polycephalum, are the ones most often seen, growing into a massive, visible blob. This organism exists as a single, giant cell called a plasmodium, which can grow to several feet in diameter. It contains millions of nuclei that are not separated by individual cell membranes, allowing this multinucleated mass to move and feed as one cohesive unit.
Cellular slime molds, in contrast, spend the majority of their feeding stage as individual, microscopic amoeboid cells called myxamoebae. These solitary cells feed on bacteria and yeast, multiplying when food is plentiful. When the food supply becomes scarce, these individual cells aggregate into a multicellular structure known as a “slug” or pseudoplasmodium. The cells in this slug retain their individual plasma membranes and nuclei, meaning they have merely clumped together rather than fusing into a single supercell.
Life Cycle Stages
The life cycle of a slime mold involves a transition between a motile, feeding phase and a stationary, reproductive phase, often triggered by environmental stress. For the Plasmodial type, the diploid plasmodium is the feeding stage, creeping across surfaces and engulfing food particles. When conditions become unfavorable, such as lack of food or dryness, the plasmodium stops moving and transforms.
This transformation leads to the reproductive stage, where the plasmodium develops into stalked fruiting bodies called sporangia. Meiosis occurs inside these structures to produce haploid spores, which are then released for dispersal. When a spore lands in a moist environment, it germinates to release a haploid amoeboid or flagellated cell. This cell can then fuse with another compatible cell to form a diploid zygote, restarting the growth of a new plasmodium.
Cellular slime molds follow a similar sequence but with a distinct multicellular aggregation stage. After the individual myxamoebae are signaled by chemical attractants due to starvation, they stream together to form the slug. This pseudoplasmodium migrates to a well-lit location before differentiating into a fruiting body. Some cells form the dead stalk, while others become the resistant spores at the top, ensuring the next generation is elevated for better dispersal.
Surprising Abilities and Behaviors
Despite lacking a brain or nervous system, certain slime molds, particularly Physarum polycephalum, demonstrate sophisticated behaviors that resemble intelligence. They exhibit a form of biological computation that allows them to solve complex spatial problems. Researchers have demonstrated their ability to solve mazes by spreading out until they find all food sources and then retracting inefficient connections.
This biological optimization is a self-organized process that results in the most efficient pathway between multiple food sources. In one famous experiment, oat flakes were placed to mimic the major cities around Tokyo. The slime mold successfully grew a network of protoplasmic tubes nearly identical to the highly efficient Tokyo rail system. This ability to find optimal network solutions has made the organism a model for studying decentralized computing and the design of transportation infrastructure.