Slime mould log
by Sarah Lloyd
May 16th 2016
The genus Cribraria has two features that make it easy to recognise in the field: the fruiting bodies have a distinctive net - the peridial net - that encases the spore mass; and small granules of calcium called dictydine granules (visible with a compound microscope) are found on the net, nodes and sometimes the stem and spores.
The difficulty in identifying Cribraria species is well documented and examining fresh material is preferable because their colour gradually changes. Their very small fruiting bodies (they rarely exceed 2 mm tall) mature over a relatively short time so even fruiting bodies arising from the same plasmodium can sometimes look different enough to be assigned to more then one species. Furthermore, although it is stated in some texts that the relatively size of stalk to sporotheca is constant in Cribraria species, my observations indicate that this is not always the case. (In many slime moulds, stalk length can be markedly different in fruiting bodies arising from the same plasmodium depending on whether they appear on the upper or underside of the substrate.) Further confusion arises because there seems to be little agreement among taxonomists and many belong to species complexes.
Cribraria species do not have certain features that can assist in the identification of species in other genera. They lack both a columella—the extension of stem inside the sporotheca, nor do they possess a capillitium—the threads inside the sporotheca. Therefore, the only features to observe are the relative size of the stalk to sporotheca, the peridial net and the spores, which in most species of Cribraria are of a similar size and relatively featureless.
Most of my texts describe European, American or New Zealand collections (there is still no text devoted to Australian myxos) and they almost invariably state that Cribraria spp. are associated with decaying pines. I occasionally find them on bark of dead trees, but they are more often collected from very large, very strongly decayed eucalypts (Eucalyptus spp.) and sometimes from dogwood (Pomaderris apetala).
It is only in the past several months that I have made a serious attempt to identify the 90 collections of Cribraria species I have amassed since 2010. Several are easy to identify even without resorting to microscopy, but most are difficult and in some cases impossible to identify with certainty.
Cribraria cancellata and C. mirabilis were until relatively recently included in a separate genus (Dictydium) because their peridial net is composed of 40-50 longitudinal ribs rather than a network of threads. C. cancellata, which often appears in extensive colonies, can be reddish-brown with the substrate stained the same colour. I also have a brown collection that some authors regarded as a variety - C. cancellata var. fusca. C. mirabilis also has longitudinal ribs, but they reach only about half of the sporotheca before grading into a net near the apex.
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