Despite their vital role in soil ecology, the Myxomycetes go largely unnoticed by field naturalists due partly to their small size and unpredictable occurrence, but perhaps also to their absence from, or token appearance in, field guides.
A Cinderella group, sometimes also known as Mycetozoa or "fungus animals", the slime moulds have traditionally been studied by field mycologists because of the resemblance of their fruit bodies or sporocarps to those of some fungi, but they are now considered to belong to the kingdom Protozoa. They spend the greater part of their life in the soil or in decaying organic matter such as fallen logs in the form of single celled myxamoebae which feed by scavenging on organic matter such as bacteria, and even fungi. Unlike fungi, they are not true decomposers, and can not break down cellulose or lignin, but they play a vital role in the carbon cycle and also release nitrates and phosphates into the soil.
Fig 2. Life cycle of a slime mould.
Fig 1. Plasmodium of Badhamia sp .
Slime moulds only become visible to the patient observer at the fruiting stage. After fusion of compatible haploid myxamoebae the newly formed diploid cell undergoes multiple nuclear divisions without cell division to form a plasmodium (the 'slime' in slime mould – Fig. 1), which eventually migrates to the surface of the soil or log, or up the stems of plants to produce visible and often brightly coloured sporocarps, and to release their spores into the air. On germination, each spore releases a single myxoflagellate, this is a single cell with two flagella, one long and motile, the other usually short and rigid. These feed by ingesting bacteria, and divide repeatedly until under suitable conditions they absorb their flagella to become myxamoebae, and so the process continues (Fig.2).
Although most commonly sought in woodlands, Myxomycetes can be found almost anywhere in damp conditions after rain, and even around melting snowfields in the mountains. Most are no more than a few millimetres tall, some much smaller, but many species produce large clusters of sporocarps together, and many are brightly coloured, making them easier to find.
Fig 3. Structure of a typical sporangium.
The Myxomycete fruit body or Sporocarp typically consists of a single sporangium which may be supported by a stalk, or may be sessile (Fig. 3). The stalk may extend into the sporangium to form a columella, and the sporangium may also contain a capillitium, a network of more or less flexible threads or elaters, which support the spores and may assist in their dispersal. These threads may be in the form of separate elaters, as in Trichia (Fig. 4), or may form a reticulate network as in Stemonitis (Fig. 5), for example.
The sporangium is covered by a surface skin of one to 3 layers, the peridium, which disrupts to release the spores. The form and nature of the peridium and capillitium, together with the size and ornamentation of the spores, are important features for the identification of slime moulds. Individual sporangia may fuse to produce sausage shaped bodies called plasmodiocarps, or in some species many sporangia cluster together to form aethalia, in which individual sporangial walls are lost.
Fig 4 Capillitium and Spores of Trichia
Fig. 5 Capillitium of Stemonitis axifera
One of the largest fruit bodies belongs to Enteridium (or Reticularia ) lycoperdon (Fig 6), once thought to be a variety of puffball. Its aethalia form silvery cushions up to 10 cm across, usually on dead standing trees in Spring, especially alder, but also on large fallen branches and trunks, and even on worked timber such as window frames. As it matures, the silvery covering becomes whiter, and eventually ruptures to release a mass of cocoa brown spores.
Arcyria obvelata (Fig 7) produces stalked, cylindrical yellow sporangia up to 15 mm tall on fallen wood, often crowded together, which when mature look like tiny loofahs. The "spongy" part of the loofah is the capillitium - a network of elastic threads or elaters marked with prominent part-rings which support the spores and aid their dispersal. The capillitium is supported by a small yellow cup (the calyculus) at the top of the short stalk, which is the remains of the peridium, the membranous skin which covers the developing sporangium.
Fig. 8. Aethalium of Fuligo septica var. flava.
Fig. 9. Aethalia of Lycogala terrestre
Fig. 11 Mucilago crustacea
Fig. 6. Aethalium of Enteridium lycoperdon
Fig. 7. Sporangia of Arcyria obvelata
A slime mould known to almost all forayers is the “Flowers of Tan”, Fuligo septica (Fig 8), so called because of its former common occurrence on heaps of spent tan. It is now more commonly found on rotting pine stumps and even on old straw bales. It is sometimes possible to watch the slime emerging from its substrate, and over a few hours grow into a large colourful lump several centimetres across, somewhat reminiscent of a pile of scrambled egg! As the aethalium matures, the cortex ruptures, releasing a mass of dark brown spores.
Almost as conspicuous are the small puffball-like sporocarps of Lycogala terrestre (Fig 9).
These are peach coloured when young and exude orange milk when broken open. The mature sporocarps are duller and discharge their spores in much the same way as puffballs, sometimes even appearing to have a pre-formed pore.
One of the most striking and memorable species is Leocarpus fragilis (Fig 10). This has the appearance of shining miniature dates, and often occurs in large quantities investing living herbaceous stems and leaves, as well as leaf litter and other debris. The plasmodium is bright yellow and the developing sporangia retain this colour for some time.
Fig 10. Sporangia of Leocarpus fragilis
Mucilago crustacea (Fig 11) can be found encrusting plant stems and leaves with a white spongy mass of fused sporangia called an aethalium, which may be up to several cm long. The white colour is caused by lime in the sporangium walls, which dehisce to release a mass of black spores, but it is the immature creamy white or pale yellow plasmodial slime that gives this species its common name of "Dog's Vomit"!
The best time to look for slime moulds is a few days after a period of rain, when plasmodia can migrate into the open air and form fruit bodies without risk of drying out too quickly. As the fruit bodies, or sporocarps, mature, they dry out to release their powdery spores. In many cases, the capillitium, or elaters, expand on dehiscence to facilitate spore dispersal.
The appearance of slime moulds changes considerably as they mature, starting as no more than a film of slime, which may be watery, white like milk, yellow, or other colour depending on species, but it will usually change colour and darken as it matures. Partially developed fruit bodies are soft and moist, with a texture rather like fresh custard (with a skin on top!), and they will easily deform on touching to reveal still-fluid contents. In contrast, mature fruit bodies may be fragile ore robust, and will usually release spores on contact, but they will be dry and usually retain their appearance. The spores are usually evident with a hand lens, and are often strongly coloured yellow, orange, red, brown, violet, or black. The remains of old fruit bodies can sometimes be found looking like tiny cups, goblets, miniature feathers or limy encrustations.
For more critical identifications, as for fungi, a high power microscope is needed, with x40 and oil immersion objectives, but it is perfectly possible to begin enjoying the amazing beauty of “myxos” with nothing more than a sharp pair of eyes. A few tips for identification are given on the next page.
Fig 12. Developing pseudoaethalium of Tubifera ferruginosa, emerging from a white plasmodium.
The Myxomycetes, or Slime Moulds
A fair number of slime moulds can be identified, at least to genus, with nothing more than a x10 hand lens, but they are often discovered as immature white or coloured blobs, and at this stage it is quite impossible to identify them with any degree of certainty. Fortunately, it is possible to gather a sample, together with its substrate, and watch it developing in a small damp chamber. Not all collections will mature, and some species are particularly sensitive to disturbance, but it is always worth trying. Mature collections can be air dried and preserved in small containers such as matchboxes for future study. They will survive indefinitely if kept dry and free from infestation.
The most recent standard work on British Myxomycetes is “The Myxomycetes of Britain and Ireland” by Bruce Ing, 2nd edition 2020.
The recently published “Les Myxomycètes by Michel Poulain, Marianne Meyer and Jean Bozonnet (FMBDS 2011), although written in French, has the keys in both French and English, with exceptional colour photographs.
“Where the Slime Mould Creeps” by Sarah Lloyd (Tympanocryptis Press 2014) gives a good introduction to the natural history of slime moulds, as does “The Curious Observer’s Guide to Slime Mold” by Carrie Niblett (UCSC 2017) (published in Great Britain by Amazon).
Extensive information on Myxomycetes, amongst other things, can be found at discoverlife.org/ , and an introduction to the study of Myxomycetes at http://www.ambac-cumino.eu/introiogio.pdf . A brief search of the web will also reveal downloadable versions of such classics as Lister & Lister's “Mycetozoa”, and George Massee's “Monograph of the Myxogastres”, amongst others. These books are both vey old, but both contain excellent colour illustrations.
All illustrations © John Holden 2010 and 2019