Protostelid slime molds are relatively unknown and easily overlooked. They were first recognized in the early 1960s and have been little studied since. There are 36 currently accepted species, and possibly twice that number of undescribed species. Most are microscopic. Only a few are visible to the naked eye.
Honeycomb coral slime mold (Ceratiomyxa fruticulosa) is the most commonly encountered protostelid slime mold and may be the most common slime mold of any kind in the world. It occurs on every continent except Greenland and Antarctica. It is found in forests on rotting fallen logs and branches. It can form extensive colonies one meter or more long. It is very short lived, appearing after a soaking rain and disintegrating in just a few days.
Honeycomb coral slime mold first appears as a thin, watery, translucent, mucus-like layer, creeping across the wood, engulfing bacteria, protozoa, and particles of nonliving organic matter. Eventually it fruits, forming clusters of erect, translucent columns. The columns have a frosted or powdery appearance due to a dense covering of tiny, white, spores on long, thread-like stalks.
Wasp nest slime mold (Metatrichia vesparium) is common and widespread. It is found in Europe, Asia, Africa, Australia, and North and South America. In the United States it is common east of the Great Plains, including Minnesota, less common in the west. It grows in open forests on dead and rotting wood, especially hardwood.
The fruiting body may be attached directly to the substrate or rise in a densely crowded group of up to twelve on a common stalk. The individual spore-producing structures are dark red or reddish-purple to nearly black, less than ⅛″ in height, and about 1 ⁄32″ in diameter. They are mostly cone-shaped and have a convex, shiny, iridescent, lid on top. When mature, the lid swings open like a jack-in-the-box, and the red or rust-red interior expands outward. When this dries out, the spores are disbursed by wind. Eventually, the expanded portion disintegrates. What is left looks like the nest of a paper wasp, giving this slime mold its common name.
Slime mold is a term of convenience grouping together several kinds of unrelated organisms. They were formerly placed in the Fungi kingdom because they produce structures containing spores (sporangia). The modern classification has them divided among several supergroups. The taxonomy of slime molds changes frequently—it is a work in progress.
Red raspberry slime mold (Tubifera ferruginosa) is one of the most commonly encountered slime molds in woodlands. It appears from June through November as a pink to bright red, pillow-shaped, tightly-packed mass on well-rotted logs, sometimes on moss. The surface is knobby, like a raspberry. It is not edible.