Physarum pusillum

Physarum pusillum was first described by Miles Berkeley and M.A. Curtis in 1873 as Didymium pusillum.{{cite book |last1=Cooke |first1=M.C. |title=Grevillea (Vol II) |date=1873 |publisher=Williams and Norgate |location=Berkeley |page=53}} The holotype (K-1492) was collected from South Carolina and kept in the Kew herbarium,{{cite book |last1=Lister |first1=A. |title=A monograph of Mycetozoa: a descriptive catalogue of the species in the Herbarium of the British Museum |date=1911 |publisher=The British Museum |location=London |page=302 |edition=2nd}} however, a comprehensive search of the herbarium in 2018 was unable to locate it.{{cite journal |last1=Lado |first1=C. |last2=Wrigley de Basanta |first2=D. |title=Typification of the myxomycete taxa described by the Listers and preserved at the Natural History Museum, London (BM) |journal=Phytotaxa |date=2018 |volume=341 |issue=1 |page=43 |doi=10.11646/phytotaxa.341.1.1|bibcode=2018Phytx.341....1L |hdl=10261/159611 |hdl-access=free }} In 1911, the species was reassigned to Physarum by Gulielma Lister, with accompanying description and illustrations recognising two morphotypes: one with a subglobose sporotheca and another (rarer one) with a flatter, lenticular sporotheca.{{cite book |last1=Lister |first1=A. |title=A monograph of the Mycetozoa a descriptive catalogue of the species in the herbarium of the British Museum |date=1911 |publisher=Longmans & Co. |location=London |page=64 |edition= revised by Guilielma Lister, 2nd}} The latter "oblate" form was recognised as a distinct species in 2020 following morphological and molecular anyalyses, leading to the reinstatement of the name P. gravidum.{{cite journal |last1=Cainelli |first1=R. |last2=de Haan |first2=M. |last3=Bonkowski |first3=M. |last4=Fiore-Donno |first4=A.M. |title=Phylogeny of Physarida (Amoebozoa, Myxogastria) based on the small-subunit ribosomal RNA gene, redefinition of Physarum pusillum s. str. and reinstatement of P. gravidum Morgan |journal=Journal of Eukaryotic Microbiology |date=2020 |volume=63 |issue=3 |pages=327–336 |doi=10.1111/jeu.12783|pmid=31904883 |doi-access=free }} Further morphological differences that distinguish P. gravidum from P. pusillum include: the presence of an umbilicus, thin lime nodes in the capillitium, and spores largely ornamented with more-or-less evenly distributed warts and occasional large, poorly defined clusters of warts.

Synonyms of Physarum pusillum include: Badhamia nodulosa, Craterium nodulosum, Physarum nodulosum, Physarum calidris, Lignydium calidris, and Physarum mucoroides.{{cite web |last1=Lado |first1=C. |title=An online nomenclatural information system of Eumycetozoa: search Physarum pusillum. |url=https://eumycetozoa.com/data/report.php?busca=physarum&por=gensi&numr=833&tipo=Rtax |publisher=Real Jardín Botánico |access-date=7 May 2025}}

The fruiting bodies of Physarum pusillum are a stalked sporangia, often gregarious, between 1–2{{nbsp}}mm tall. Each sporocarp has subglobose sporotheca, approximately 0.4–0.5{{nbsp}}mm in diameter, and a stalk that is membranous, limeless, longitudinally pleated, widened at the base, and standing either straight or with a slight curve at up to 1.3{{nbsp}}mm tall. The upper half of the stalk is translucent and orange or reddish-brown, while the lower half is opaque and dark brown. Its peridium is a single layer and encrusted with granular white to grey-white lime, although it is often orange-brown or reddish-brown and limeless at the base. Its capillitium consists of enlarged, irregular lime nodes, filled with lime granules and connected by hyaline, branching threads. It does not have a columella or pseudocolumella. Spores are globose, black in mass and brown by transmitted light, 8.5–12.5{{nbsp}}μm in diameter, and minutely warted. These warts are evenly distributed, with distinct groups of denser, larger warts. They are watery white as plasmodia.{{cite book |last1=Stevenson |first1=S. |title=Fungi of New Zealand Volume 3: Myxomycetes of New Zealand |date=2003 |publisher=Fungal Diversity Press |location=Hong Kong |isbn=962-86765-4-7 |pages=212–213}}

Physarum pusillum is found on both herbaceous and woody plant debris, such as dead leaves and twigs, worldwide, including New Zealand, South Africa, Croatia, and South Africa. In general, P. pusillum favours warm temperate to tropical regions.

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Category:Physaraceae

Category:Protists described in 1873