An Unusual Ecological Association Between Higher Fungi and Myxomycetes
Steven L. Stephenson*
*Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701.
Abstract
In the lower canopy of lowland tropical rainforests, the system of aerial rhizomorphs produced by certain marasmioid agarics intercepts and holds a considerable amount of litter, mostly in the form of dead leaves. The biodiversity of the assemblage of myxomycetes associated with this aerial litter microhabitat actually appears to be higher than the assemblage associated with ground litter in the same forest. As such, rhizomorph systems clearly influence the distribution and ecology of myxomycetes in rainforests. This ecological association has not been recognized previously by those biologists who study these organisms.
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No. 4 Neotropical Naturalist 2020
An Unusual Ecological
Association Between
Higher Fungi and
Myxomycetes
Steven L. Stephenson
NEOTROPICAL NATURALIST
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Fruiting bodies of Physarum melleum, a colorful myxomycete commonly associated with leaf litter.
Photograph © Kim Fleming
Neotropical Naturalist
S. L. Stephenson
2020 No. 4
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2020 NEOTROPICAL NATURALIST 4:1–5
An Unusual Ecological Association Between
Higher Fungi and Myxomycetes
Steven L. Stephenson
Abstract - In the lower canopy of lowland tropical rainforests, the system of aerial rhizomorphs
produced by certain marasmioid agarics intercepts and holds a considerable amount of litter, mostly
in the form of dead leaves. The biodiversity of the assemblage of myxomycetes associated with this
aerial litter microhabitat actually appears to be higher than the assemblage associated with ground
litter in the same forest. As such, rhizomorph systems clearly influence the distribution and ecology of
myxomycetes in rainforests. This ecological association has not been recognized previously by those
biologists who study these organisms.
In the lower canopy of lowland tropical rainforests, a number of species of marasmioid
agarics often form an intricate system of aerial rhizomorphs (César et al. 2018). Typically,
these are members of the genus Marasmius (Marasmiaceae) but can also include some
species in other genera such as Crinipellis (Marasmiaceae) and Gymnopus (Omphalotaceae).
Such aerial rhizomorphs (Fig. 1) are tough and persistent, typically brown or black (but
sometimes white) in color and usually between 0.1 and 1.5 mm in diameter (Snaddon et al.
2012). The system of rhizomorphs intercepts a substantial amount of litter, mostly in the
form of dead leaves. Snaddon et al. (2012) calculated that approximately 257 kg of leaf
litter per hectare was intercepted and held in a tropical rainforest in Malaysia. This material,
which has never been in contact with the ground, is referred to as aerial litter (Schnittler and
Stephenson 2000). Aerial rhizomorphs are collected and used as nesting material by certain
species of birds (Freymann 2008, Koch et al. 2018, Elliott et al. 2019). In fact, some nests
may consist almost entirely of rhizomorphs. Snaddon et al. (2012) reported that the system
of aerial rhizomorphs also serves as a microhabitat for certain insects. Field observations in
the Neotropics (S.L. Stephenson, unpubl. Data) indicate that this is also the case for various
other invertebrates—including arachnids, such as mites (order Acari) and spiders (order
Araneae).
Myxomycetes (plasmodial slime molds or myxogastrids) are a group of funguslike
organisms associated with dead plant material in virtually every type of terrestrial
ecosystem investigated to date, with approximately 1000 species known worldwide
(Lado 2005–2020). The myxomycete life cycle encompasses two very different trophic
(feeding) stages: one consisting of uninucleate amoebae, with or without flagella (the term
“amoeboflagellate” is used to refer to both types), and the other consisting of a distinctive
multinucleate structure, the plasmodium (Martin et al. 1983). Under favorable conditions,
the plasmodium gives rise to one or more fruiting bodies containing spores. The fruiting
bodies produced by myxomycetes are somewhat suggestive of those produced by higher
fungi, although they are considerably smaller—usually no more than 1–2 mm tall.
In tropical rainforest ecosystems, myxomycetes are associated with a number of
different microhabitats, including decaying coarse woody debris, woody twigs, lianas,
the bark surface of living trees, and both aerial litter and ground litter on the forest floor
(Fig. 2). Although the fruiting bodies of myxomycetes can be collected in the field from
Department of Biological Sciences, University of Arkansas, Fayetteville, Arkansas 72701
Manuscript Editor: Danny Haelewaters
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S. L. Stephenson
2020 No. 4
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all of these microhabitats, the use of moist chamber cultures, as they apply to the study
of myxomycetes, is a much more productive method of studying both types of litter
microhabitats. These cultures are prepared and then checked for myxomycetes in the
manner described by Stephenson and Stempen (1994).
It has long been recognized that various species of myxomycetes are commonly
associated with ground litter (Martin and Alexopoulos 1969, Stephenson 1989), but the
fact that these organisms are associated with aerial litter is a more recent discovery (e.g.,
Schnittler and Stephenson 2000, Black et al. 2004, de Haan 2011). Although only a limited
number of comparative studies have been carried out, the aerial litter microhabitat appears
to support a higher biodiversity of myxomycetes compared to the ground litter microhabitat
in moist tropical forests. For example, Schnittler and Stephenson (2000) reported data for
two types of tropical forests—primary and secondary—in Costa Rica. In the primary forest,
137 cultures prepared with samples of ground litter yielded an average of 0.7 species of
myxomycetes, whereas 100 cultures prepared with samples of aerial litter produced an
average of 1.3 species. For the secondary forest, 45 cultures prepared with ground litter
yielded an average of 3.0 species, whereas samples of aerial litter produced an average
of 4.5 species. In northern Queensland, Black et al. (2004) found that 92% of 61 cultures
prepared with aerial litter yielded myxomycetes, whereas only 55% of 11 cultures prepared
with samples of ground litter were positive for these organisms.
Figure 1. Dead leaves trapped by
aerial rhizomorphs in a tropical
rainforest in the Republic of
Cameroon in Central Africa (image
courtesy of Todd Elliott).
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2020 No. 4
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Figure 2. Fruiting bodies of Physarum melleum (top, image courtesy of Kim Fleming) and Physarum
compressum (bottom, image courtesy of Laurie Leonard), two species of myxomycetes commonly associated
with the leaf litter microhabitat in tropical rainforests.
Neotropical Naturalist
S. L. Stephenson
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In the types of comparative studies mentioned above, the assemblages of myxomycetes
recorded from ground litter and aerial litter usually are comprised of mostly the same
species, even though they are typically more abundant in the latter microhabitat. Although
it is not uncommon for a particular species of myxomycete to be very abundant in one of
the two microhabitats and largely absent from the other, there is no evidence to suggest
that some species of myxomycetes are absolutely restricted to either microhabitat (S.L.
Stephenson, unpubl. Data). However, this does not discount the possibility that some of the
rarely recorded species do display a certain degree of microhab itat specificity.
Schnittler and Stephenson (2000) suggested that the apparent displacement of
myxomycetes from the forest floor to aerial microhabitats is related to the differences that
exist for environmental moisture levels. Myxomycetes appear to be better adapted to survive
under fluctuating moisture conditions (more likely to be found in aerial microhabitats) than
under conditions of continuously high moisture levels (more likely to exist on the forest
floor). Even exposed to daily rainfall, aerial microhabitats tend to dry out rather quickly,
whereas microhabitats on the forest floor are likely to retain at least a film of moisture.
Under the latter conditions, the fruiting bodies of myxomycetes are often colonized by
filamentous fungi, thus severely restricting successful production and dispersal of spores
(Rogerson and Stephenson 1993). Schnittler and Stephenson (2000) indicated that the
influence of moisture is suggested by the results of the comparative data they had for Costa
Rica. The secondary forests they studied had numerous openings, whereas the sampled
primary forests had a closed canopy. The overall drier conditions in the former for both
aerial litter and ground litter would be expected to be more favorable for myxomycetes, and
the data outlined above appear to reflect this suggestion.
It thus seems apparent that the presence of an aerial network of rhizomorphs clearly
influences the distribution and ecology of myxomycetes in the forests in which the former
occurs. This association has not been recognized previously by those biologists who
study these organisms but actually represents a fascinating subsystem within the tropical
rainforest ecosystem.
Acknowledgements
Field work that generated the comparative data on myxomycetes associated with aerial
litter and ground litter was supported in part by grant DEB-9705464 from the National
Science Foundation. Appreciation is extended to Todd Elliott for supplying the image of leaf
litter trapped by aerial rhizomorphs. Kim Fleming and Laurie Leonard supplied the images
of myxomycetes associated with litter.
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