Slime moulds are unusual organisms that defy easy classification, often appearing as a slimy mass in damp, shady environments. They captivate scientists and the public due to their unique life cycles and surprising capabilities. Found globally in diverse habitats, these organisms exhibit characteristics that blur the lines between different biological kingdoms.
Unpacking Slime Mould Identity
Slime moulds are not fungi, plants, or animals, but belong to the Kingdom Protista, a diverse group of eukaryotic organisms. They were historically miscategorized due to superficial resemblances to fungi.
There are two primary types of slime moulds: plasmodial (acellular) and cellular. Plasmodial slime moulds, such as Physarum polycephalum, exist as a single, large cell containing many nuclei without internal cell walls, often forming visible, brightly colored masses that can span several meters. Cellular slime moulds, like Dictyostelium discoideum, spend most of their lives as individual, microscopic amoeboid cells. When conditions change, these individual cells can aggregate to form a temporary multicellular structure.
Remarkable Life Strategies
Slime moulds employ distinct strategies for movement, feeding, and reproduction. Plasmodial slime moulds move through cytoplasmic streaming, where their internal protoplasm rhythmically flows back and forth within vein-like tubes. This movement allows them to explore their environment and engulf food particles like bacteria, yeast, and decaying organic matter through phagocytosis.
Cellular slime moulds move as individual amoeboid cells by extending pseudopods. Both types exhibit complex life cycles involving transitions between forms. Under favorable conditions, plasmodial slime moulds grow as a feeding plasmodium. When conditions become harsh, they transform into spore-producing fruiting bodies, releasing haploid spores that germinate into new amoeboid or flagellated cells.
Cellular slime moulds primarily exist as solitary amoebae, feeding on bacteria. When food becomes scarce, these individual cells aggregate in response to chemical signals, forming a multicellular “slug” that can migrate in search of a suitable location. This slug then differentiates into a fruiting body, which produces spores for dispersal. This aggregation allows them to collectively survive adverse conditions and disperse to new areas.
Collective Intelligence and Problem Solving
Despite lacking a brain or nervous system, certain slime moulds exhibit behaviors that suggest a form of decentralized “intelligence.” Physarum polycephalum, a plasmodial slime mould, is particularly known for its ability to navigate mazes. In laboratory experiments, if food sources are placed at the entry and exit points of a maze, Physarum explores all paths but eventually concentrates its protoplasmic network along the shortest route between the food sources. This ability is attributed to a feedback mechanism where the flow of cytoplasm and signaling molecules reinforces efficient pathways and prunes less efficient ones.
Slime moulds can also form efficient networks that resemble human-designed systems, such as railway networks. When presented with multiple food sources, Physarum will connect them with a network of protoplasmic tubes that are optimized for efficiency, cost, and resilience. This network formation is a result of the organism’s inherent ability to balance resource distribution and connectivity. The internal rhythmic contractions and fluid flow within the plasmodium play a role in this self-organization.
Beyond pathfinding, slime moulds demonstrate rudimentary memory and learning. They can anticipate periodic events, such as the arrival of food, and adapt their foraging behavior. Their ability to modify their network structure based on past experiences and environmental cues suggests a simple form of external memory, where information is encoded in their physical morphology. This capacity for complex behaviors without a centralized brain makes them important for studying distributed intelligence.
Ecological Contributions and Research Insights
Slime moulds play an important ecological role as decomposers in various ecosystems, contributing to nutrient cycling. They consume bacteria, fungi, and decaying organic matter, breaking down complex compounds and returning essential nutrients to the soil. This activity supports soil health and the overall balance of forest ecosystems.
The unique properties of slime moulds have also made them valuable subjects in scientific research. Their ability to solve complex problems, such as finding shortest paths and forming optimized networks, inspires biomimicry. Researchers study slime mould behavior to develop algorithms for designing efficient transportation networks, communication systems, and even urban planning. Their simple yet effective decision-making processes offer insights into the principles of self-organization and adaptive systems, providing models for understanding complexity in biology and engineering.