Mediolus, Patterson, 2013

Patterson, R. Timothy, 2013, Mediolus, a new genus of Arcellacea (Testate Lobose Amoebae), Palaeontologia Electronica (28 A) 17 (2), pp. 1-8 : 3-6

Genus Mediolus n. gen.

www.zoobank.org/ E43633B7-2017-48CB-B887- 849D86D3F230

Type species. Difflugia proteiformis (sic) (Ehrenberg) subspecies D. globularis (Dujardin) var. D. corona (Wallich) WALLICH 1864 p. 244 , pl. 16, figures 19, 20 (reillustrated here as Figure 2.1-2 ).

Diagnosis. A genus of Difflugidae characterized by a symmetrical globular test, a test most typically comprised of agglutinated particles from the environment, round aperture with multiple inward-oriented angular tooth-line crenulations, and a test typically characterized by a variable number of spines extending outward on the test.

Description. Test free, unilocular; ovoid to subspherical to spheroid; circular in section, about the vertical aperture through the fundus; test wall comprised of agglutinating particles variously composed of xenogenous mineral grains and/or organic material derived from the ambient environment; agglutinating particles attached together with an organic cement; circular aperture characterized by thin collar of secreted cement with variable number of inward-oriented angular crenulations also composed of cement; delicate spines may be present, spines long and narrow, hollow, and composed of very fine agglutinating particles.

Types and occurrence. Hypotype (Canadian Museum of Nature ( CANA 87186 ), recent from “Lake 10” near Yellowknife, Northwest Territories, Canada (62°30.924’N, 113°46.817’W) ; Hypotype ( CANA 91905 ), recent from Mew Lake, Algonquin Park, Ontario (45°34.575’N, 78°31.087’W) ; Hypotype ( CANA 87197 ), recent from Bell’s Lake , municipality of King (Greater Toronto Area), Ontario, Canada (43°N 56.597, 79°39.762W) ; Hypotypes ( CANA 87198 and CANA 87199 ), Holocene from Laguna de Quistococha , Iquitos, Peruvian Amazon (3°49.746’N, 73°19.157’W) .

Dimensions. 100-400µm; mean ~ 150 µm ( Medioli and Scott, 1983; Hu et al., 1997; Yang et al., 2004; Lahr and Lopes, 2006).

Remarks. The genus is readily distinguished from Protocurbitella Gauthier-Liévre and Thomas, 1960, as Mediolus lacks the distinctive diaphragm around the peristome. The peristomal diaphragm of Protocurbitella is similar to a similar structure characterizing Cuburbitella Penard, 1902. In addition, the pronounced lobed apertural collar of Cucurbitella is also very distinct from the apertural crenulations characterizing Mediolus . Cucurbitella is distinct from most other arcellaceans with regard to its distinctive relationship with some algal species. As the spring and summer growing season progresses and algae begins to grow into the water column Cucurbitella individuals go with it. In the absence of xenogenous grains to construct their test, late season specimens of this genus have the ability to produce idiosomic particles, which they use to construct their shell (Schönborn, 1984; Medioli et al., 1987). Mediolus differs from Loboforamina Jung, 1942 in that the latter is characterized by a lobate apertural region similar to that found in Cucurbitella .

As illustrated in this paper living representatives of Mediolus are found from the tropics to the poles. It is relatively common in mesotrophic to eutrophic lakes where it is often found attached to water plants ( Medioli and Scott, 1983; Roe et al., 2010; Patterson et al., 2012; Roe and Patterson, 2014). It is less common in ponds and ditches and has also been reported from sphagnum bogs. Individuals are heterotrophic using its lobose pseudopods to engulf organic material or smaller prey (e.g., diatoms and bacteria) ( Medioli and Scott, 1983).

The crenulated aperture typical of the type species D. corona is much more diagnostic of the genus that the spines. Using broods of D. corona, Jennings (1916) carried out what is now recognized as a classic experiment on hereditary in uniparental organisms. He found that although clones differed greatly in overall size and the number of spines produced through many generations, there was an almost perfect correlation between the number of apertural teeth produced within brood lineages. As the spines would sometimes disappear entirely in some broods it is the apertural teeth that are the single most distinguishing characteristic of Mediolus ( Figure 4 ). The seasonally planktic species Difflugia tuberspinifera Hu, Shen, Gu, and Gong, 1997 , a species endemic to east Asia, is morphologically similar to D. corona , differing in that the toothed aperture in D. tuberspinifera tends to be contained within a protruding apertural flange, and that the conical spines are restricted to the upper equatorial region ( Yang et al., 2004; Han et al., 2008). Yu et al. (2014) have recognized that, as also observed in D. corona , spineless strains also exist. Based on the general similarity of the apertural crenulations in both D. corona and D. tuberspinifera , the latter species is tentatively assigned to Mediolus .

The extreme range of habitat observed for the type species of Mediolus may provide indirect evidence that it may encompass two or more cryptospecies. A possible non-genetic analysis approach to eventually test this hypothesis has been demonstrated using planktic foraminifera ( Morard et al., 2013). Using transfer functions Morard et al. (2013) modeled the temperature dependence of several species of planktic species in the southern hemisphere and determined that several morphologically indistinguishable morphotypes are most likely distinct species.

Etymology. In honor of the late Professor Emeriti Franco Medioli (1935-2014), Department of Earth Sciences, Dalhousie University, in recognition of his extensive research on the ecology, systematics, and biostratigraphy of Arcellacea and related groups.

Stratigraphic Range. Quaternary, but particularly common in Holocene deposits.

Figures

FIGURE 2. Specimens of Mediolus corona from Wallich, 1864.1. illustrated as figure 19 in the original publication, Wallich described the specimen as “front view, showing crenulate margin of aperture; a six-horned variety”; 2. illustrated as figure 20 in the original publication, Wallich described the specimen as “a somewhat smaller four-horned variety; side view.”

FIGURE 4. Line drawings of Mediolus corona clones produced within six of Jennings (1916) broods, redrafted and reaarranged by Medioli and Scott (1983). The figure shows that although there is little variation within broods through only a few generations there is considerable variation in both size and process number and arrangement between broods. The research of Jennings (1916) indicated that although the crenulated aperture was a constant feature in all clones, some morphotypes can be processless.