Myxomycetes Species: A Guide to the World of Slime Molds

How Many Slime Mold Species Exist?

As of the most recent taxonomic surveys, approximately 1,000 species of Myxomycetes have been formally described. However, experts like Steven Stephenson and Martin Schnittler estimate that the true number could be significantly higher, potentially 1,500 to 2,000 species, because many tropical regions remain underexplored and molecular studies keep revealing cryptic species hidden within what were thought to be single species.

These organisms are found on every continent, including Antarctica, where species have been documented on sub-Antarctic islands. They are most diverse in temperate forests with abundant decaying wood, but they also occur in deserts, alpine meadows, and tropical rainforests.

Classification Overview

Myxomycetes (plasmodial slime molds) belong to the supergroup Amoebozoa. They were historically grouped with fungi due to their spore-producing fruiting bodies, but molecular evidence firmly places them among the amoebae. Here is the simplified classification hierarchy:

• Supergroup: Amoebozoa
• Phylum: Mycetozoa
• Class: Myxomycetes (also called Myxogastria)

Within Myxomycetes, taxonomists recognize several major orders:

Notable Species Worth Knowing

Physarum polycephalum (the laboratory star)

This is by far the most studied myxomycete. Its bright yellow plasmodium grows rapidly on oat flakes, making it ideal for research and education. It can solve mazes, form memories, and optimize networks. Its genome was sequenced and published in 2015. If you want to grow your own, see our complete growing guide.

Fuligo septica (dog vomit slime mold)

Perhaps the most commonly encountered slime mold in gardens and on mulch. Fuligo septica forms a large, foamy, bright yellow mass called an aethalium that can reach 20 centimeters or more in diameter. It often appears overnight on wood chip mulch after rain, alarming gardeners who mistake it for toxic fungus or, as the common name suggests, something a dog left behind.

Despite its unappetizing appearance, Fuligo septica is harmless. In parts of Mexico, it is actually eaten as a traditional food known as "caca de luna" (moon excrement), typically scrambled like eggs.

Stemonitis axifera (chocolate tube slime mold)

One of the most elegant myxomycetes. Stemonitis produces clusters of tall, slender, brown sporangia on hair-like stalks, often growing in dense groups on decaying logs. The fruiting bodies look like tiny chocolate-colored feathers, typically 5 to 20 millimeters tall. When mature, the outer wall (peridium) disappears, leaving a delicate network of capillitium threads visible to the naked eye.

Lycogala epidendrum (wolf's milk)

This species forms small, round, pinkish-orange aethalia (typically 3 to 15 millimeters in diameter) on rotting wood. When young, squeezing these globes releases a bright pink paste that resembles toothpaste. As they mature, the interior dries to a dusty olive-brown spore mass. Lycogala is common in temperate forests worldwide and is often the first myxomycete that nature photographers encounter.

Arcyria denudata (carnation slime mold)

Arcyria denudata produces striking clusters of bright red to pinkish sporangia on short stalks. As the sporangia mature, they expand and the capillitium unfurls like tiny pompoms. Found on decaying hardwood logs, this species is one of the most photogenic myxomycetes and appears frequently in macro photography collections.

Hemitrichia serpula (pretzel slime mold)

Rather than forming discrete upright sporangia, Hemitrichia serpula creates a plasmodiocarp: a network of interconnected tubes that follows the veins of the original plasmodium. The result looks like a tangled web of bright yellow or golden-brown pretzels spread across the surface of a log. It is one of the best examples of how a fruiting body can mirror the feeding network.

Badhamia utricularis

A close relative of Physarum that is increasingly used in research. Its plasmodium is white to gray, and it readily engulfs and consumes fungal fruiting bodies, making it one of the more aggressive feeders among myxomycetes. Badhamia utricularis is sometimes available from biological supply companies as an alternative to Physarum polycephalum.

Didymium squamulosum

A cosmopolitan species found from tropical forests to suburban leaf litter. It produces small white sporangia covered in crystalline lime flakes. Didymium squamulosum is notable because molecular studies have revealed extensive genetic diversity within what was previously considered a single species, suggesting it may actually be a complex of several cryptic species.

Cribraria cancellata (lattice slime mold)

This tiny species produces sporangia with an extraordinarily intricate peridium that resembles a lattice or net. Under a hand lens, the geometric pattern of the peridium is breathtaking. Cribraria species are favorites among myxomycete enthusiasts for their architectural beauty.

Ceratiomyxa fruticulosa (coral slime mold)

Technically placed in its own group (Ceratiomyxa is sometimes classified outside Myxomycetes proper), this organism produces tiny white, finger-like or branching coral-shaped structures on decaying wood. Unlike true myxomycetes, Ceratiomyxa bears its spores on the outside of the fruiting body rather than inside. It is abundant in moist forests worldwide.

Identifying Slime Molds in the Wild

Identifying myxomycetes requires careful observation. Here are the key features taxonomists use:

1. Plasmodium color: the color of the feeding stage (yellow, white, gray, orange, red) can narrow down possibilities, though it is not always reliable
2. Fruiting body type: sporangia (individual stalked bodies), aethalia (cushion-shaped masses), plasmodiocarps (vein-like networks), or pseudoaethalia
3. Spore color and ornamentation: examined under a microscope; spore surface features (spines, warts, reticulations) are key identification characters
4. Capillitium structure: the internal thread network within sporangia varies enormously between species
5. Lime deposits: presence or absence of calcium carbonate on the peridium, stalk, or capillitium
6. Substrate: what the organism is growing on (bark, leaf litter, dung, living plants)

For tips on finding slime molds outdoors, visit our guide on slime molds in the wild.

Habitats and Distribution

Myxomycetes occupy a wide range of habitats:

• Temperate deciduous forests: the richest habitat; decaying logs and leaf litter support hundreds of species
• Tropical rainforests: high diversity but understudied; many undescribed species likely exist in canopy litter
• Boreal forests: lower diversity, dominated by species adapted to cold and acidic substrates
• Deserts and arid regions: surprisingly, dozens of species thrive here, fruiting on the inner surfaces of dead cactus and succulent stems
• Alpine environments: specialized nivicolous (snow-line) species fruit near melting snow in spring
• Urban areas: mulched garden beds, compost bins, and greenhouses regularly host species like Fuligo septica and Physarum cinereum

How Myxomycetes Differ from Other "Slime Molds"

The term "slime mold" is applied to several unrelated groups of organisms. It helps to know which groups exist and how they differ:

Among these groups, only Myxomycetes and Dictyostelids are well-studied. The Myxomycetes are the most species-rich and visually spectacular group.

Ongoing Research and New Discoveries

Myxomycete taxonomy is far from settled. Key areas of active research include:

• Environmental DNA (eDNA) surveys: soil sampling and DNA barcoding are revealing that many species are more widespread than previously thought, and that cryptic diversity is rampant
• Tropical inventories: systematic surveys in tropical Africa, Southeast Asia, and South America continue to yield new species at a steady rate
• Nivicolous species: the specialized species that fruit near melting snow are increasingly threatened by climate change and are a focus of conservation-oriented research
• Molecular phylogenetics: DNA-based studies are reshuffling the classification of many genera, splitting some and merging others

A landmark resource for anyone interested in myxomycete diversity is the book Myxomycetes: Biology, Systematics, Biogeography and Ecology by Stephenson and Stempen, along with the regularly updated online database maintained by the University of Arkansas.

Conservation Concerns

While no myxomycete species is currently listed as endangered by the IUCN, there are growing concerns:

• Many species are known from only a handful of collections
• Habitat destruction (old-growth forest logging) eliminates the decaying wood substrates that many species depend on
• Climate change threatens nivicolous species whose life cycles are tied to snowmelt timing
• Some rare species may go extinct before they are even described

Citizen science projects like those on iNaturalist are helping to map distribution data and may prove valuable for identifying species of conservation concern.

Getting Started with Myxomycete Identification

If you want to explore beyond Physarum polycephalum, here are some steps to get started:

1. Get a hand lens (10x magnification minimum) for field observation
2. Photograph everything before collecting, since many species are fragile
3. Collect specimens by placing them (with substrate) in small boxes and allowing them to air-dry
4. Invest in a microscope (40x to 400x) for spore and capillitium examination
5. Use identification keys: "The Myxomycetes" by Martin and Alexopoulos remains a standard reference
6. Join online communities: Facebook groups and iNaturalist have active myxomycete identification communities

For those who prefer to start with a living culture, our guides on growing slime mold and where to buy slime mold cover everything you need to get your first Physarum culture up and running.

The world of Myxomycetes extends far beyond the yellow blob on oat flakes. With roughly 1,000 known species and likely hundreds more awaiting description, this is a group that rewards patient observation and offers a lifetime of discovery for naturalists, photographers, and scientists alike.