When slime gets spiky: slime-moulds of the family Didymiaceae

Many people are familiar with slime-moulds to some extent, with the most popular view being of a slowly streaming patch of goo, sometimes brightly coloured, much like a large amoeba - possibly unsurprising as they are Protozoa even though commonly treated as honorary Fungi. Another comonly encountered form is the 'dog'd vomit' type which appears as a pale yellowish lumpy patch among grass, including garden lawns. One common and widespread example of this second type is Mucilago crustacea which does indeed look like dog's vomit when freshly emerged. However, within about a day, the cortex hardens and the spore mass darkens, by which time it may well have moved up the stems of grass or other plants, and become visually quite different.

The aethelium (combined spore-bearing structure) of Mucilago crustacea on the stem of a reed at Winnall Moors, southern England.

In the above picture, the dark grey-black spore mass can be seen on the left side and there is the remnant of cytoplasmic streaming beneath where the specimen has moved up the reed stem. The cortex (the surface covering the aethelium) is powdery and spongy and any movement was sufficient to send flakes and presumably spores into the air.

The cortex in close-up showing the spongy, almost brain-like, texture and powdery surface.

The reason for this texture is the presence of crystalline calcium carbonate which impregnates the structure of the slime-mould, with the cortex forming calcareous powder and flakes.

A mass of spores and calcareous crystals under x40 magnification. The dark circles are air bubbles, but note the scattered spores which appear as tiny brownish circles in transmitted light.

One of the masses under x100 magnification. A cluster of brownish spores can be seen bottom right, each being a little over 10µm in diameter. The calcareous crystals are a similar size and can be seen as small spikes to the left of the picture.

Given the presence of abundant calcium carbonate in the structure of this species, it is found most commonly in limestone-rich areas such as calcareous grasslands (Winnall Moors is a wetland on chalk) and is found only rarely in acidic conditions. The outer covering is coated with calcium carbonate in various forms (powdery, scaly, compacted and so on) in all of the Didymiaceae, the family which includes M. crustacea. The large genus Didymium has a powdery covering of star-shaped crystals which in some species may aggregate to form a crust; the shape and size of these crystals can also help to identify individual species. However, the taxonomy of this family is unclear and it is likely that genetic studies will redefine the boundaries between genera.

Reference

Ing, B. (1999). The Myxomycetes of Britain and Ireland: An Identification Handbook. Richmond, Slough. The current standard work on species in the British Isles.