Note: The two fungal divisions Myxomycota (slime molds) and Oomycota (water molds) have been
placed in the kingdom Protista (Protoctista) by some authorities. These divisions all produce motile
cells (including swarm cells and zoospores) during some stage of their life cycles. Another division
called Acrasiomycota (cellular slime molds) is also placed in the Protista. True fungi typically do not
have eukaryotic 9 + 2 flagella (called undulipodia) at any stage in their life cycles..
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Slime molds were once placed in the kingdom Fungi, although some authorities now place them in the kingdom Protoctista (Protista). Most fungi are composed of threadlike filaments of eukaryotic cells called hyphae, collectively referred to as a mycelium. Slime molds are composed of an acellular mass of naked protoplasm with no cell walls in its vegetative state. They are typically found in damp, shady areas with abundant organic matter, although they may move to bright areas to "fruit."
The vegetative stage is essentially a multinucleate mass of protoplasm (called a plasmodium) composed of numerous diploid nuclei. Depending on the species, the plasmodium may be only a few millimeters in diameter to large pizza-sized slime molds over 12 inches (30 cm) across. The plasmodium moves like a giant amoeba, flowing over the surface as it ingests organic matter, such as dead leaves and wood. It may be be brightly colored, such as the yellow Physarum polycephalum commonly studied in general botany laboratories. Some tropical slime molds are bioluminescent and glow in the dark. Slime molds are important heterotrophs in the decomposition of dead organic matter in temperate and tropical forests.
The transformation of the plasmodium into spore-bearing "fruiting bodies" or fructifications can occur within hours. Depending on the species, these fructifications can be in the form of sporangia, aethalia or plasmodiocarps. Sporangia may be stalked and have unique shapes (e.g. Stemonitis and Dictydium). Aethalia are large, thickened, cushion-shaped structures containing numerous spores (e.g. Fuligo). Plasmodiocarps often retain the branching habit of the original plasmodium (e.g. Hemitrichia). Plasmodiocarps are the most primitive type of fructification, while sporangia are the most advanced.
The life cycle of a slime mold begins with a haploid spore which is produced inside the fructification by meiosis. The germinated spore can transform into either an amoeba-like myxamoeba cell or a flagellated swarm cell. Both myxamoeba and swarm cells divide into large populations. This stage composed of amoeboid and flagellated cells is why some authorities place the slime molds in the kingdom Protoctista. Two myxamoeba cells or two swarm cells may join together in a cellular fusion (plasmogamy) followed by nuclear fusion (karyogamy). The combining of their cell contents (protoplasts) represents a very primitive form of sexual reproduction. It also provides a source of genetic variability; however, swarm cells always fuse with swarm cells and myxamoeba cells always fuse with myxamoeba cells. The fusion of two haploid cells results in a diploid zygote which transforms into a developing plasmodium. As the diploid nuclei divide, the plasmodium grows larger and larger, slowly moving (creeping) along the forest floor and feeding like a giant amoeba. The plasmodium of some slime fungi (Fuligo) can grow to the size of a large pizza.
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Slime Mold: Physarum polycephalum
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A slime mold (Physarum polycephalum), showing a creeping mass of yellowish protoplasm called a plasmodium. Photo courtesy of Dr. George Zabka, Palomar College.
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The yellowish plasmodia of these slime molds (Physarum polycephalum) have crawled out of their Petri dishes. On one occasion they nearly escaped from their container. The slime molds were fed oats in a nutrient agar.
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The plasmodium of this Physarum polycephalum has produced dark "fruiting bodies" (fructifications) that contain spores. Note the traces of the former plasmodium in the background.
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Slime Mold In A Fish Aquarium
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The plasmodium of this slime mold is on the glass of a freshwater tropical fish aquarium. According to Dr. George Zabka, the plasmodium is similar to Physarum. The plasmodium crawled up onto the glass from its previous position on the bottom. It also spent time on a section of bogwood, perhaps where it was feeding (see next image). Photo courtesy of Tony Batley.
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The plasmodium of a slime mold on a section of bogwood in a freshwater tropical aquarium. It appears to be feeding on the wood substrate. Photo courtesy of Tony Batley.
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Vomit Slime Mold: Fuligo septica
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Fuligo septica is a large slime mold in the order Physarales. In urbanized areas it is usually found growing on bark chips which it digests for a carbon source. Like Physarum, the plasmodium consists of a multinucleate mass of protoplasm that is not differentiated into cells. It moves like an amoeba, the protoplasm streaming internally in one direction and then in another. The yellowish, bile-colored plasmodium is the origin of the uncomplimentary name "dog vomit slime mold." The slimy plasmodium develops into a thicker, lime encrusted mass which is essentially the spore-bearing stage (fructification) called the aethalium. At the San Diego Wild Animal Park, the yellowish plasmodium transformed into a pinkish-tinged aethalium within one day. The aethalium is analogous to the spore-bearing "fruiting body" of a mushroom or toadstool, except it is much more simple and primitive in structure. It has the texture of porous bread or a thick pancake. Unlike more advanced slime fungi, the spores are not produced in distinct spore cases called sporangia. As it ages, the aethalium breaks down into dark red liquified areas which resemble blood. The dried aethalium contains literally millions of spores which are dispersed into the air.
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Early stage in the development of Fuligo septica. At the San Diego Wild Animal Park, this interesting fungus feeds on wood chips. Some plasmodia develop a yellowish coloration at this stage.
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Early stage in the development of Fuligo septica. At the San Diego Wild Animal Park, this interesting fungus feeds on wood chips. In less than one day, this granular, yellowish mass transformed into a thick, spore-bearing structure called an aethalium
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Slimy plasmodium of Fuligo septica the color of peanut butter. This creeping fungus moves very slowly in amoeboid fashion.
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Slimy plasmodium of Fuligo septica the color of peanut butter. A number of descriptive terms have been applied to this fungus, including "vomit slime mold" and "dog vomit slime mold."
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Aethalium of Fuligo septica. The aethalium is a spore-bearing mass analogous to the "fruiting body" (fructification) of a mushroom or toadstool. Unlike more advanced slime fungi, it is not composed of individual sporangia. The aethalium develops from a slimy, yellowish, plasmodial mass. It has red, blood-like spots from the liquified breakdown of fungal tissue.
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Close-up view of the aethalium of Fuligo septica. Note the porous, "bread-like" texture and the deep red liquified areas. To one astute observer, it superficially resembles a cherry cobler; however, it certainly does not taste or smell like a cherry cobler.
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Mature aethalium of Fuligo septica with crusty, powdery surface resembling cement. Just below the suface are masses of spores resembling fine brown dust.
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Mature aethalium of Fuligo septica with crusty, powdery surface resembling cement. Just below the surface are masses of spores resembling fine brown dust.
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The crusty, powdery surface of this aethalium has been gently scraped away to reveal a spore mass resembling fine brown dust.
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The slime fungus (Fuligo septica) at the San Diego Wild Animal Park: A. Yellowish (bile-colored) plasmodial stage growing on wood chips. B. One day later, the plasmodium has developed into a thick, spore-bearing mass called an aethalium.
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Go To The Wayne's Word Fungus Article