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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 The Neotropical Naturalist (ISSN # 2327-5472) is published by the Eagle Hill Institute, PO Box 9, 59 Eagle Hill Road, Steuben, ME 04680-0009. Phone 207-546-2821 Ext. 4, FAX 207-546-3042. E-mail: office@eaglehill.us. Webpage: http://www.eaglehill. us/neon. Copyright © 2020, all rights reserved. Published on an article by article basis. Special issue proposals are welcome. The Neotropical Naturalist is an open access journal. Authors: Submission guidelines are available at http://www.eaglehill.us/ neon. Co-published journals: The Northeastern Naturalist, Southeastern Naturalist, Caribbean Naturalist, Urban Naturalist, and Eastern Paleontologist, each with a separate Board of Editors. The Eagle Hill Institute is a tax exempt 501(c)(3) nonprofit corporation of the State of Maine (Federal ID # 010379899). Board of Editors David Barrington, Department of Plant Biology, University of Vermont, Burlington, VT, USA William G. R. Crampton, University of Central Florida, Orlando, FL, USA Paulo Estefano Dineli Bobrowiec, Instituto Nacional de Pesquisas da Amazônia, Brazil Valentina Ferretti, Universidad de Buenos Aires, Argentina Danny Haelewaters, Ghent University, Belgium Matthew Halley, Drexel University, Philadelphia, PA, USA Christopher M. Heckscher, Department of Agriculture and Natural Resources, Delaware State University, Dover, DE, USA Ian MacGregor-Fors, Instituto de Ecología Mexico, Veracruz, Mexico Klaus Mehltreter, Institute of Ecology, A.C., Xalapa, Veracruz, Mexico Jorge Ari Noriega A., Universidad de los Andes, Colombia Jason M. Townsend, Biology Department, Hamilton College, Clinton, NY, USA Judit Ungvari, Florida Museum of Natural History, Gainesville, FL, USA Fredric V. Vencl, Stony Brook University, Stony Brook, NY. 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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 1 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 Neotropical Naturalist S. L. Stephenson 2020 No. 4 2 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). Neotropical Naturalist S. L. Stephenson 2020 No. 4 3 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 2020 No. 4 4 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. Literature Cited Black, D.R., S.L. Stephenson, and C.A. Pearce. 2004. Myxomycetes associated with the aerial litter microhabitat in tropical forests of northern Queensland, Australia. Systematics and Geography of Plants 74:129–132. César, E., V.M. Bandala, L. Montoya, and A. Ramos. 2018. A new Gymnopus species with rhizomorphs and its record as nesting material by birds (Tyrannideae) in the subtropical cloud forest from eastern Mexico. MycoKeys 2018:21–34. de Haan, M. 2011. First records of protostelids and myxomycetes on aerial litter from the National Botanic Garden of Belgium. Sterbeeckia 30:38–50. Elliott, T.F., M.A. Jusino, J.M. Trappe, H. Lepp, G.A. Ballard, J.J. Bruhl, and K. Vernes. Neotropical Naturalist S. L. Stephenson 2020 No. 4 5 2019. A global review of the ecological significance of symbiotic associations between birds and fungi. Fungal Diversity 98:161–194. Freymann, B. P. 2008. Physical properties of fungal rhizomorphs of marasmioid basidiomycetes used as nesting material by birds. Ibis 150:395–399. Koch, R.A., D.J. Lodge, S. Sourell, K. Nakasone, A.G. McCoy, and M.C. Aime. 2018. Tying up loose threads: Revised taxonomy and phylogeny of an avian-dispersed Neotropical rhizomorph-forming fungus. Mycological Progress DOI.org/10.1007/s11557-018-1411-8. Lado, C. 2005–2020. An online nomenclatural information system of Eumycetozoa Available online at https://eumycetozoa.com/data/index.php. Accessed 14 December 2019. Martin, G. W. and C. J. Alexopoulos. 1969. The Myxomycetes. University of Iowa Press, Iowa City, IA. 561 pp. Martin, G. W., C. J. Alexopoulos, and M. L. Farr. 1983. The Genera of Myxomycetes. University of Iowa Press, Iowa City, IA. 102 pp. Rogerson, C.T., and S.L. Stephenson. 1993. Myxomyceticolous fungi. Mycologia 85:456– 469. Schnittler, M., and S.L. Stephenson. 2000. Myxomycete biodiversity in four different forest types in Costa Rica. Mycologia 92:626–637. Snaddon, J.L., E.C. Turner, T.M. Fayle, C.V. Khen, P. Eggleton, and W.A. Foster. 2012. Biodiversity hanging by a thread: the importance of fungal litter-trapping systems in tropical rainforests. Biology Letters 8:397–400. Stephenson, S.L. 1989. Distribution and ecology of myxomycetes in temperate forests. II. Patterns of occurrence on bark surface of living trees, leaf litter, and dung. Mycologia 81:608–621. Stephenson, S.L., and H. Stempen. 1994. Myxomycetes: A Handbook of Slime Molds. Timber Press, Portland, OR. 183 pp.