Erugomicula, Nasser & Gregory & Singer & Patterson & Roe, 2021

Nasser, Nawaf A., Gregory, Braden R. B., Singer, David, Patterson, R. Timothy & Roe, Helen M., 2021, Erugomicula, a new genus of Arcellinida (testate lobose amoebae), Palaeontologia Electronica (a 16) 24 (1), pp. 1-13 : 4-8

Genus Erugomicula n. gen.

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Type species. Difflugia bidens Penard, 1902, p. 264 , figures 1-8 (figure 1 is re-illustrated here as Figure 2.1 )

Diagnosis. A genus tentatively assigned to the Hyalospheniidae characterized by laterally-compressed, ovoid test, typically comprised of agglutinated particles, but is relatively smooth, generally with short hollow spines ( Figure 2.7 ) oriented along the line of compression on the fundus ( Figure 3 ); aperture round, simple (never slit-like) ( Figure 2.2 ), occasionally with slight raised collar about the aperture ( Figure 2.6 ).

Description. Test free, unilocular, wide, and ovoid in face view with a smooth outline ( Figure 2.2 ); compressed in section ( Figure 2.3-4 ); test wall comprised of finely agglutinated xenogenous particles, attached with an organic cement; anterior positioned circular aperture occasionally surrounded by slight raised collar; delicate hollow spines ranging in number from zero to three (two most common), aligned along the line of compression, generally characterize the fundus ( Figure 3.1- 3 ).

Types and occurrence. In his description of Difflugia bidens , based on specimens collected from Lake Geneva, Switzerland, Pernard (1902) illustrated four distinct syntypes on page 265 (figures 1-4), but did not designate any as the holotype of the new species. Penard’s figure 1 (re-illustrated here as Figure 2.1 ) is the most typical morphotype of the species, being characterized by two basal processes from which the species name ‘bidens’ i s derived. The specimen in Penard’s figure 2 is characterized by three basal processes, and figures 3 and 4 have no basal processes. As per Article 74.1 of the International Code of Zoological Nomenclature (1999) we designate Penard’s figure 1 as the lectotype for the species. The additional hypotypes illustrated here are included to provide higher quality scanning electron microscope ( SEM) images of the morphologic variability within the species. They were obtained recently from Bell's Lake , Schomberg , Ontario, Canada (43°56.597’ N, 79°39.762’ W), Figure 2.2-4 ( CANA 129300 ), Figure 3.1 -6 ( CANA 129301 ). The SEM images of the specimens illustrated here were taking using a Tescan Vegall XMU SEM in the Nano Imaging Facility, Carleton University .

Dimensions. Hypotype specimens measured for this study (n = 24) were from Bell’s Lake, Ontario, Canada (length: 219 – 341 µm [mean = 274 µm]; width: 127 – 250 µm [mean = 207 µm]), with an average length:width ratio of 1.33 ( Table 1, Figure 4 ). Specimens from Bell’s Lake showed little morphological variability, with only minor deviation about the mean length:width ratio, despite large size variation between specimens ( Figure 4 ). Specimens of E. bidens measured by Penard (1902) from Lake Geneva, Switzerland, ranged from 250-270 µm in length, within the observed range of specimens from Bell’s Lake. Specimens retrieved from ditches in Naardermeer, Netherlands (Siemensma, 2017) were slightly larger than E. bidens found in Canada and Switzerland, ranging from 307-366 µm in length. The average apertural width of the Bell’s Lake hypotypes was 83 µm (50 – 103 µm), or approximately 40% of specimen width. Most specimens observed had two spines on the fundus, which was why the type species as named ‘bidens’, although specimens with zero, one, and three spines were also observed. It is likely that the presence or absence of spines is due to phenotypic plasticity and is not a suitable classification criterion (Jennings, 1916; Lahr et al., 2008; Gomaa et al., 2017).

Remarks. Erugomicula , n. genus, differs from Difflugia Leclerc, 1815 in the distinct compression of a wide ovoid test ( Figures 2-3 ). Erugomicula is distinguished by having a simple round aperture in contrast to Awerintzewia Schouteden, 1906 , which has a compressed to oval aperature and inhabits forest soils, and sphagnum and peat bogs. The new genus is also distinct from Heleopera Leidy, 1879 , which is characterized by a slit like aperture and roughly agglutinated aboral region. Erugomicula differs from Nebela (Leidy, 1874; sensu Kosakyan et al., 2016) by being composed of finely agglutinated xenogenous mineral particles, while Nebela is composed primarily of oval and circular siliceous plates or organic cement. Nebela species are also characterize by distinctive apertures which can be either linear, slightly or strongly curved.

The type species of the new genus, E. bidens , is usually present in small numbers in most lacustrine assemblages. Erugomicula bidens has been observed in lakes across Canada, including eastern Canada (Medioli and Scott, 1983; McCarthy et al., 2012; Patterson et al., 2012; Macumber et al., 2014), western Canada (Torigai et al., 2000; Neville et al., 2010), and the Northwest Territories (Nasser et al., 2016, 2020). It has also been found in Europe in Bulgaria (Serafimov et al., 1995; Golemansky et al., 2003), Estonia (Lokko et al., 2014), Finland (Kauppila et al., 2006; Kihlman and Kauppila, 2012), France ( Thomas, 1954), Lithuania (Šatkauskienė, 2014), Portugal (Camacho et al., 2015), and of course Switzerland (Penard, 1902; Golemansky et al., 2003), where the species was first described. Based on the similarly compressed test we also tentatively place Difflugia biconcava Ertl, 1965 and the possible junior synonym of that species Difflugia balcanica Ogden and Zivovich, 1983 in Erugomicula . Difflugia lucida Penard, 1890 is not placed in Erugomicula as that very coarsely agglutinated taxa has a very compressed aperture and due to the presence of a neck. Similarly, the compressed and coarsely agglutinated Difflugia nodosa Leidy, 1879 is also excluded from the new genus due to the presence of a pronounced neck.

Erugomicula is here tentatively placed in the family Hyalospheniidae based on recent molecular studies that suggest that test shape is an inherited fundamental trait of deep time ancestral phylogenetic importance (Lahr et al., 2019). Although genera of the family Hyalospheniidae are compressed, tests of current genera within that family are variously identified as chitinoid, clear, completely organic, or if agglutinated, comprised of the shell plates of small euglyphids. As E. bidens is comprised of mineral agglutinated particles further research will be required to determine whether Erugomicula is actually attributable to the Hyalospheniidae or Difflugiidae Stein 1859 where many agglutinated test genera are placed, or neither. If proven to be the case, then the test composition description of the Hyalospheniidae will require amendment. The work of Macumber et al. (2020) may provide additional support for placing the new genus in the Hyalospheniidae , based on recognition in that study of two distinct morphologicallybased clades (lanceolate and pyriform) within Difflugia . Interestingly the “pyriform” clade of Macumber et al. (2020) was determined to be a sister clade of Hyalosphenia papilio Leidy, 1874 (Family Hyalospheniidae ). Nevertheless, these recent studies provide only a preliminary indication of the relationship between the species of Difflugia , with much additional research required (Gomaa et al., 2012). Erugomicula be a key genus that requires particular attention in future molecular investigations of the group.

Erugomicula bidens is considered to be an indicator of increased terrigenous erosion of minerals and organic matter associated with land-use change (Patterson et al., 1985; Kihlman and Kauppila, 2012; Macumber et al., 2014). However, Patterson et al. (2002) did not observe an increase in D. bidens abundance in Swan Lake, Ontario in a stratigraphic section where an increase in sediment runoff into the lake was observed. The sediment runoff was predominantly a nutrient poor glacial clay; the lack of nutrients is most likely an additional important limiting factor on the distribution for D. bidens . An association of D. bidens with nutrient status was further supported by Patterson et al. (2012) as it was observed to have one of the higher optima and tolerances for Olsen's phosphorus (150-400 ppm). It has also been observed in mesotrophic and hypereutrophic lakes (Neville et al., 2010) and lakes impacted by industrial contaminants (Kauppila et al., 2006; Neville et al., 2011; Nasser et al., 2016).

Etymology. From the Latin Erugo, clear of wrinkles, smooth; and mico, shine, sparkle, f. dim., with reference to the relatively smooth surface of the type species Difflugia bidens Penard, 1902 .

Stratigraphic range. Although some arcellinidan species have been found in sediments dating as far back as the Permian (Singh et al., 2015), the type species of the new genus Erugomicula bidens has, to date, only been observed in Holocene lacustrine sediments (Medioli and Scott, 1983).

Figures

FIGURE 2. SEM images of Erugomicula bidens showing typical morphology; images 2.1-2.4 are all relative to a 100 µm scale bar. 2.1, Original drawing of E. bidens reproduced from Penard, 1902. This specimen has been designated herein as the lectotype for E. bidens (Penard 1902). 2.2, Apertural view of E. bidens showing circular aperture and lateral compression. 2.3, E. bidens showing ovoid shape and three spines in a slightly angled face view. 2.4, Oblique view of E. bidens specimen showing lateral compression from side view. 2.5, E. bidens specimen showing slightly raised collar around aperture. Scale bar equals 250 µm. 2.6, Enlarged view of E. bidens collar. Scale bar equals 100 µm. 2.7, Broken basal process from a E. bidens specimen showing that these processes are hollow. Scale bar equals 20 µm.

FIGURE 3. SEM images of Erugomicula bidens specimens from Bell’s Lake, ON, Canada, showing morphological variability. Scale bar equals 100 µm. 3.1-3.3, A series of E. bidens specimens displaying the varying number of basal processes typical of the species. The number of basal processes present, including specimens with no processes present is highly variable, although morphologies with two basal processes are most common. When present the basal processes are arranged along the fundus. 3.4-3.6, A series of E. bidens specimens in side view demonstrating variations in the length-to-width ratio.

FIGURE 4. Morphological parameters of Erugomicula bidens measured using specimens retrieved from Bell’s Lake (ON) Scott’s Pond (ON) and Yellowknife (NT) (n=24). Left, Density distribution plot of length and width of E. bidens; specimen width is represented by the light grey curve, and specimen length with the dark grey curve. Right, Scatterplot of length versus width of measured E. bidens specimens; the black line represents the median length:width ratio of 1.3.