Slime Mold: Brainless Wonder & Evolutionary Link
- Sylvia Rose
- Mar 26
- 4 min read
Slime molds dwell in dark, damp environments of decaying matter, like forest floors or compost heaps. They morph between fungi and single-celled amoebas and are lauded for their intelligence.
Slime molds begin as several single celled individuals. When conditions are right they unite to create either multicellular structures or one big cell. They often appear rain or high humidity.
Slime molds are not plant, animal or fungus, but belong to the kingdom Protista, like amoebae. They're renowned for showing what humans perceive as intelligence, even though slime molds don't have a brain.
Instead they have multiple nuclei, a natural phenomenon shared with heterokaryotic fungi. A nucleus is the cell's control center, holding genetic material (DNA).
The nucleus regulates essential cellular activities like growth, protein synthesis, and reproduction. In animals, the skeletal muscle cells, liver cells and osteoclasts have multiple nuclei. Nuclei communicate with each other.
They can coordinate functions and process local signals. They may show regional specialization, a function shared by non-motile yeast colonies.
Slime molds can pulsate. This is caused by chemicals moving inside it. Water waves carry calcium molecules from back and forth.
The "grainy" appearance of some slime molds is due to minute calcium crystals on the peridium (the outer layer). Slime molds often make round or blobbish formations which can be in close proximity or adjoining, as below.
Slime molds are classed into two main groups.
Cellular Slime Molds
Cellular slime molds live as individual amoebae, foraging for food like bacteria and organic particles in soil, leaf litter or dung. When food gets scarce, individual cells aggregate, drawn together by chemical signals.
Amoebas move using pseudopodia or false feet, temporary extensions of cytoplasm. These allow the protists to crawl or glide along a surface and forms the basis of slime mold movement. Pseudopodia also capture food.
Cells communicate with each other through signaling molecules interpreted by nuclei. The molecules bind to receptors on other cells, triggering intracellular reactions to coordinate functions.
The amoeba cells form a sluggish mass or pseudoplasmodium. This coordinated group migrates towards light and air. It produces stalk-like fruiting bodies, which are the spore-producing organs of a fungus.
Spores are formed at the end of the stalk in the sporangiophore, where each haploid nucleus is enclosed in a tough outer membrane. The stalk elevates sporangia and releases spores to find new food sources and begin the cycle again.
The individual cells "sacrifice" themselves to form the stalks. Echinosteliales may produce just one stalk. Seemingly altruistic behavior also appears in yeast (fungi) and certain bacteria.
For example they create biofilms from their own bodies and microbial glue. A yeast cell may sacrifice itself to create spores within it, made from the nuclei of nearby sister cells.
Plasmodial Slime Molds
Plasmodial slime molds, such as Physarum polycephalum, begin as spores. The spores germinate into single cells which fuse to form a single giant cell, the plasmodium, with thousands of nuclei.
The plasmodium creeps along surfaces, engulfing bacteria and other organic matter. It's usually brightly colored yellow, orange, or red, and can cover an obvious area.
When conditions go bad, as in lack of food or moisture, the plasmodium retracts to form fruiting bodies or stalks. At the ends of the stalks nuclei undergo meiosis.
Chromosomes are reshuffled and nuclei divide making spores which are haploid, having just one set of chromosomes. Upon release the spores disperse to start the life cycle again as haploid amoebae.
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Slime molds are famous for certain traits:
Decentralized Problem Solving: Slime molds solve complex problems like finding the shortest path through a maze to reach food. They avoid areas they've already explored.
This is accomplished by detecting minute quantities of their own secretions. It manifests as "memory".
Network Optimization: Slime molds construct efficient transportation networks, mimicking layout of railway systems in urban areas. They can connect food sources.
Self-Organization and Collective Behavior: Slime molds are organized, with individual cells coordinating actions to achieve a common goal.
Bio-Inspired Design: Slime mold behavior inspires new approaches in computer science, robotics, and architecture. Their ability to optimize paths and solve problems is used to develop algorithms and designs.
Slime molds are important to ecosystems. Breaking down organic matter, they recycle nutrients back into the soil to supporting plants and other organisms. This promotes soil health and biodiversity.
They're indicators of environmental health. Their presence and abundance signals plentiful organic material and moisture, and a thriving ecosystem. It's a lucky sign when they show up in compost.
Slime molds have been on Earth for 600 million+ years. They predate both plants, which appear 500 million ya, and animals with brains or nervous systems.
Amoebae, from which slime molds originate, are known to exist for at least 750 million years. Comparatively, bacteria, a favorite slime mold food source, are among the earliest forms of life.
They appear 3.5 billion years ago, in the early Precambrian period. Fungi have been on Earth for about a billion years. The earliest humans emerge a mere 2 million ya.
Slime molds are a major step evolutionary step beyond lichen for example, which forms from a fungus and alga or cyanobacteria, and microbe colonies which exhibit collective behavior.
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