4.1: Slime Molds

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Slime molds encompass organisms from several lineages. Here, we look at three main groups. Cellular slime molds (dictyostelids) are groups of unicellular amoebae that collaborate to form fruiting structures to disperse spores. Protostelids make small fruiting bodies that have cellular stalks. Plasmodial slime molds (classified under Myxogastria or Myxomycetes) form a large, multinucleate amoeba with no cell wall that will eventually wall off individual nuclei to form spores.

Cellular Slime Molds

The life cycle of a cellular slime mold

Fruiting bodies with distinguishable stalks and sporangia — Figure \(\PageIndex{2}\): These images show a species of *Dictyostelium* developing fruiting bodies. In the image on the left, the culmination stages are visible. These appear as teardrop or worm-like shapes. On the right, several sporangia are in the process of forming, with individuals still migrating up the stalk to the apex. Only the individuals who end up in the sporangium will survive to "reproduce" as spores. Photos by Jerry Cooper CC BY 4.0.

Video \(\PageIndex{1}\) : A video showing the cellular slime mold Dictyostelium, both as individual amoebae and collaborating to form fruiting structures. Link to the YouTube video.

Protostelids

The examples shown here are Ceratiomyxa fruticulosa. This organism may not actually be a protostelid, but the small, stalked fruiting bodies with a single sporangium produced on the external surfaces are similar to what would be seen in a protostelid.

A cellular slime mold growing on a log — Figure \(\PageIndex{3}\): This image shows the slime mold *Ceratiomyxa fruticulosa*, which looks a bit like an organism you'd find under the sea. This slime mold (likely) belongs to the protostelid group because it makes its spores externally. Each coral-like extension of this slime mold is covered with tiny spores. Photo by Maria Morrow, CC BY-NC.

Projections of Ceratiomyxa covered in tiny fruiting structures with thin stalks and globose sporangia. — Figure \(\PageIndex{4}\): A close-up of *Ceratiomyxa fruticulosa*, showing the fruiting structures covering the outside of the strange, coralloid projections. Photo by Damon Tighe, CC BY-NC. See another great example here.

Plasmodial Slime Molds

Plasmodial slime molds represent a vast diversity of morphologies. While still a plasmodium (see Figure \(\PageIndex{5}\)), they can be difficult to distinguish. However, once they have formed into a fruiting structure, they can form distinct, varied, and amazing shapes (see Figure \(\PageIndex{6-9}\))!

The Plasmodium

Sporocarp Diversity

A plasmodiocarp. The veins of the plasmodium have formed into a network of spore-filled tubes. — Figure \(\PageIndex{6}\): Hemitrichia serpula forms an uncommon fruiting body called a **plasmodiocarp**. The feeding stage accumulates its protoplasm into the veins of the plasmodium, forming strange linear, intertwining shapes. Photo by Roman Providukhin, CC-BY-NC.

Lycogala oozing pink goo after being poked — Figure \(\PageIndex{7}\): Fruiting bodies of the plasmodial slime mold Lycogala epidendrum form into cushion-like structures called **aethalia**. The plasmodium has formed into pink ball-like structures on the surface of a rotten log. One of these structures has been popped and is oozing a pink slime, full of immature spores. This pink slime gives *Lycogala* its name, wolf's milk. Photo by Maria Morrow, CC BY-NC.

A pink slime mold with distinct columns of sporangia that have formed into a single (sort of spiky) cushion — Figure \(\PageIndex{8}\): Another option for a fruiting structure is the **pseudoaethalium,** where there are distinct sporangia but they still form together like a cushion. This is the type of fruiting structure formed by Tubifera ferruginosa, the red raspberry slime mold. Photo by Hiromi Karagiannis, CC BY-NC.

Four fruiting structures, each with a pale white stalk and dark sporangium that has an oil-sheen look to it (rainbowed) — Figure \(\PageIndex{9}\): Fruiting bodies of *Diachea leucopodia* have a distinct stalk and **sporangium**. The stalk in this species is white, while the elongate sporangium displays an oil-sheen rainbow of colors. Photo by Sypster, CC BY-NC.

Other Features

sporocarp with a distinct stalk and sporangium. From the sporangium, pink threads emerge. — Figure \(\PageIndex{10}\): This fruiting body of *Arcyria denudata* shows an interesting feature of slime mold anatomy. The sporangium is composed of a **peridium** (skin) that encloses the spores. The spores are often intermixed with a tangle of filaments called the **capillitium**, which helps disperse the spores. In this image, the pink capillitium (full of spores) can be seen bursting out of the sporangium. Photo by Alexander Shirokikh, CC BY-NC.

In addition to having interesting macroscopic morphologies, they also have interesting microscopic features! Ornamentation and size of the spores, as well as the appearance of the capillitial threads, can be necessary for identification.

Microscopic view of ornamented capillitial threads (like blackberry prickles) and spores — Figure \(\PageIndex{11}\): Ornamentation of the capillitial threads can be a helpful identification feature. On the left are the spiny ornamentations characteristic of *Arcyria obvelata*. On the right are the twisted threads of *Trichia botrytis*. Photo on the left by Maria Morrow CC BY-NC. Photo on the right by George Barron, for non commercial academic and research use only.

Bright yellow capillitial threads with a twisted pattern, viewed under the microscope — Figure \(\PageIndex{11}\): Ornamentation of the capillitial threads can be a helpful identification feature. On the left are the spiny ornamentations characteristic of *Arcyria obvelata*. On the right are the twisted threads of *Trichia botrytis*. Photo on the left by Maria Morrow CC BY-NC. Photo on the right by George Barron, for non commercial academic and research use only.