Acaulon muticum
Common Name: Rounded Pygmy Moss, Conservation Status - Near Threatened (Baudraz, et. al., 2019)
Historically, Acaulon muticum was considered a singular species with two subspecies, var. muticum and var. mediterraneum, but in 2006 var. mediterraneum was upgraded to full species status and given the name A. mediterraneum. The species were separated based on differences in spore shape (BIBS, n.d.). Both species can be found in the UK, although A. mediterraneum is significantly rarer, and only found in southern England (Blockeel, 2014a; Callaghan, 2022).
Appearance and Identification
A. muticum is a tiny acrocarpous moss, which stands at just 1-2mm tall, with highly concave, ovate leaves of a similar length. The upper (perichaetial) leaves are generally larger than the lower leaves, with a slightly excurrent nerve, and blunt teeth along the upper edge (may require a magnification to be visible). The perichaetial leaves are so curved that they form a bud-like structure which surrounds the capsules and obscures them from view (BIBS, n.d.; Lawley, 2010). The capsules themselves are cleistocarpous, meaning they have no operculum, and contain fine, granulose spores which are about 30-50µm long (BIBS, n.d.; Hill et al., 2017). In A. muticum capsules are very common and can be seen in all seasons except summer, with the number of mature capsules peaking during the winter months (Blockeel, 2014b; Lawley, 2010). A. muticum does not form organised colonies and usually grows as scattered vertical shoots, although in densely populated areas these aggregations can resemble turfs (Hill et. al., 2017)
Similar species:
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Acaulon mediterraneum - Distinguishable solely by the shape of their spores, A. mediterraneum has prickly spinulose spores, while A. muticum has rough granulose spores (BIBS, n.d., Lawley, 2010).
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Acaulon triquetrum - Requires habits with more sun, less moisture, and significant levels of calcium carbonate in the soil, as a result it is incredibly rare in the UK and can only be found on the chalk and limestone cliffs of southern Britain. Other distinguishing features are its curved seta, recurved leaf margins, and overall triangular look to the plant when viewed from above (Hill et. al., 2017, Lawley, 2010).
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Dioecious Bryum sp. - Young male Bryum plants have a similar bud-like appearance, and their brown perigonia can resemble A. muticum capsules. Perigonia and capsules can be distinguished under a microscope by seeing whether they contain spores or antheridia (BIBS, n.d., Lawley, 2010).
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Microbryum curvicolle - Unlike A. muticum the capsules are not hidden behind the leaves, and are held outside of the bud-like structure by curved seta (Lawley, 2010).
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Tortula acaulon (Phascum cuspidatum) - The perichaetial leaves do not have teeth, are larger, and also more pointed than those of A. muticum (Lawley, 2010). T. acaulon also prefers basic soils, while A. muticum is found in acidic soils (Hill et. al., 2017).
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Weissia longifolia - has much longer and narrower leaves than A. muticum (Lawley, 2010).
(Greiff, 2020)
(Greiff, 2020)
Distribution
A. muticum is widely distributed and can be found throughout most of Europe, as well as certain parts of North America, North Africa, and Eastern Asia (Baudraz et al., 2019: Urmi et al., 2007). Unfortunately, A. muticum populations are scattered and rare across much of its range. It is only present in certain patches of Norway, Sweden, and the UK, and is absent or has a very limited distribution in most of the Balkan countries, with the exception of Greece and Bulgaria (Baudraz et al., 2019). As a result, this species is regionally threatened in several European countries, and may recently have become locally extinct in Finland and Ireland (Hodgetts, 2015; Hodgetts & Lockhart, 2020; Hodgetts & Lockhart, 2025). Where A. muticum is found it will generally occupy temperate grasslands, or artificial landscapes such as arable land, plantations, and pastureland (Baudraz et al., 2019).
Historically A. muticum has been present throughout the entirety of Britain, Ireland, and the Channel Islands, however, since 1950 it has only been recorded in two hectads for both Ireland, and the Channel Islands. It is most abundant on England's southern coast, and sightings become rarer as you travel inland and further north (Blockeel, 2014b; Hill et al., 2010). The best habitats to find A. muticum in the UK are sea cliffs, although not in actively saline soils due to their low salt tolerance, or patches of bare earth such as mole hills, earthy walls, and the woodland rides which comprised the majority of Jans A. muticum sightings (Blockeel, 2014b; Hill et al., 2017). Despite arable land being listed as a key habitat for this species by the IUCN , within the UK it is actually quite rare to find A. muticum on arable sites (Baudraz et al., 2019; Preston et al., 2013).
UK Altitudinal Range: 0-596m (Blockeel, 2014).
Regional Threat Level:
Britain: Regionally classed as Least Concern and considered neither nationally scarce or rare. However, while the population is not at present declining, with the species recently recorded in just 119 hectads it is close to being considered nationally scarce (Callaghan, 2022; Pescott, 2016).
Ireland: Regionally extinct (Hodgetts & Lockhart, 2025).
Habitat and Ecology
A. muticum can be found throughout the year, however, because it is an ephemeral moss, populations are highly transitory and rarely found in the same place twice (Blockeel, 2014). If you want to see this species with its mature capsules then these begin to appear in November, and from there can be sighted up until April (Hill et al., 2017).
A. muticum is generally found in area's where there are few other bryophyte competitors, favouring area's of disturbed or bare earth with low nutrient, acidic or neutral, loose textured soils (Blockeel, 2014; Meinunger & Schröder, 2007; Nebel & Philippi, 2001). While this species can tolerate partial shade, it prefers to grow in well lit area's with moderately moist soils. In the UK it has been recorded on the following substrates; soil, soil on rock, gravel and sand, and in the following habitats; rocky cliffs ledges and shores, dry grasslands, inland areas with sparse vegetation, arable land, active mines and quarries, and finally rides in broadleaf deciduous forest (Hill et. al., 2017). With the exception of the UK, this species flourishes in arable fields which are post-harvest but pre-tilling, and this tends to be where the largest populations are found. Unfortunately, this makes A. muticum vulnerable to habitat loss from both the intensification of agriculture, and the retirement of agricultural land which often results in forest encroachment (Baudraz et. al., 2019; Meinunger & Schröder, 2007; Urmi et al., 2007).
Associated Species: Riccia sorocarpa, Bryum argenteum, Bryum dichotomum, Bryum subapiculatum, Ceratodon purpureus, and Tortula truncata (along coast lines). Phascum cuspidatum (in arable land) (Blockeel, 2014)
References
Baudraz, M.E.A., Schnyder, N., Bisang, I., Caspari, S., Hedenäs, L., Hodgetts, N., Kiebacher, T., Kučera, J., Ştefănuţ, S. & Váňa, J. 2019. Acaulon muticum (Europe assessment). The IUCN Red List of Threatened Species 2019: e.T84534637A87738446. Accessed on 02 September 2026.
Blockeel, T. J. (2014a). Acaulon mediterraneum. In T. J. Blockeel, S. D. S. Bosanquet, M. O. Hill, & C. D. Preston (Eds), Atlas of British and Irish Bryophytes (Vol. 2). Pisces Publications. https://www.britishbryologicalsociety.org.uk/wp-content/uploads/2020/12/Atlas-of-British-and-Irish-Bryophytes-V2-235.pdf
Blockeel, T. J. (2014b). Acaulon muticum. In T. J. Blockeel, S. D. S. Bosanquet, M. O. Hill, & C. D. Preston (Eds), Atlas of British and Irish Bryophytes (Vol. 2, p. 234). Pisces Publications. https://www.britishbryologicalsociety.org.uk/wp-content/uploads/2020/12/Atlas-of-British-and-Irish-Bryophytes-V2-234.pdf
British and Irish Bryological Society. (n.d). Acaulon muticum. British and Irish Bryological Society. https://www.britishbryologicalsociety.org.uk/learning/species-finder/acaulon-muticum/
Callaghan, D. A. (2022). A new IUCN Red List of the bryophytes of Britain, 2023. Journal of Bryology, 44(4), 271–389. https://doi.org/10.1080/03736687.2023.2185393
Greiff, G. (2020). Dwarf Earth Moss (Acaulon muticum) in the United Kingdom [Graphic]. https://www.inaturalist.org/photos/107936444. https://commons.wikimedia.org/wiki/File:Acaulon_muticum_107936444.jpg
Greiff, G. (2020). Dwarf Earth Moss (Acaulon muticum) in the United Kingdom 2 [Graphic]. https://www.inaturalist.org/photos/107936589. https://commons.wikimedia.org/wiki/File:Acaulon_muticum_107936589.jpg
Hill, M. O., Preston, C. D., Bosanquet, S. D. S., & Roy, D. B. (2017). BRYOATT - Attributes of British and Irish Mosses, Liverworts and Hornworts. Centre for Ecology and Hydrology.
Hodgetts, N. G. (2015). Checklist and country status of European bryophytes – towards a new Red List for Europe. National Parks and Wildlife Service.
Hodgetts, N., & Lockhart, N. (2020). Checklist and country status of European bryophytes – update 2020. National Parks and Wildlife Service.
Hodgetts, N., & Lockhart, N. (2025). Bryophytes (Mosses, Liverworts & Hornworts). Ireland Red List No. 14. National Parks and Wildlife Service, Department of Housing, Local Government and Heritage.
Lawley, M. (2010a). Acaulon muticum. In I. Atherton, S. Bosanquet, & M. Lawley (Eds), Mosses and liverworts of Britain and Ireland: A field guide (First edition, p. 492). British Bryological Society.
Meinunger, L., & Schröder, W. (2007). Verbreitungsatlas der Moose Deutschlands—Band 2. Oliver Dürhammer für die Regensburgische Botanische Gesellschaft.
Nebel, M., & Philippi, G. (2001). Die Moose Baden-Württembergs. Ulmer.
Pescott, O. (2016). Revised lists of nationally rare and scarce bryophytes for Britain. 115, 22–30.
Preston, C., Hill, M., Porley, R., & Bosanquet, S. (2013). Survey of the bryophytes of arable land in Britain and Ireland 1: A
classification of arable field assemblages. Journal of Bryology, 32, 61–79. https://doi.org/10.1179/037366810X12578498135959
Urmi, E., Schubiger-Bossard, C., Schnyder, N., Müller, N., Küchler, M., Hofmann, H., & Bisang, I. (2007). Zwei Jahrhunderte Bestandesentwicklung von Moosen in der Schweiz. Haupt.
Written By: Lucy Hudson Last Updated: 2nd September 2026