Arcellina Haeckel, 1894

Ichise, Satoshi, Sakamaki, Yositaka & Shimano, Satoshi D., 2021, Neotypification of Difflugia biwae (Amoebozoa: Tubulinea: Arcellinida) from the Lake Biwa, Japan, Species Diversity 26 (2), pp. 171-186 : 173-175

Suborder Arcellina Haeckel, 1894 Family Difflugiidae Wallich, 1864 Genus Difflugia Leclerc, 1815 Difflugia biwae Kawamura, 1918 [Japanese standard name: Biwako-tsubokamuri] ( Figs 3–5 )

Material examined. Neotype: TNS-AL –63110 (VSS-I- 10 in Appendix 1) in TNS ( Department of Botany , National Museum of Nature and Science ), on mounted slide with Euparal , 0–10 m in depth at St. III (35°18′04″N, 136°09′29″E) on 15 August 1961 by Mr. Kan-ichi Mita and Mr. Kenji Naka ( Ichise et al. 2004), deposited at Department of Botany, National Museum of Nature and Science, previously known as the National Science Museum.

Voucher specimen series, VSS I: The 19 specimens were individually mounted on slides. Fifteen specimens were deposited at TNS under the catalogue numbers TNS-AL-63101 to 63116 (VSS-I-1 to 16), and the remaining four were deposited at the Lake Biwa Museum in Shiga Prefecture, Japan, under the catalogue numbers LBM 2040000001–2040000004 (VSS-I-17 to 20) . The collection data for all the specimens is same as the neotype.

VSS II: TNS-AL- 63117 to 63140 in TNS (see Appendix 2 with vial data), a total of 51 specimens kept in 5% formalin solution in vials (see Appendix 2) . The collection data is same as the neotype.

The specimens of Morph#1, Morph#2, Measurement#1 and Measurement#2 were lost from a laboratory collection in Shiga Prefectural Institute of Public Health and Environmental Science. Sampling data were shown in Materials and methods part.

Designation of neotype and voucher specimens. The authors could not confirm the existence of the type series (holotype and paratypes) of D. biwae . The type series or any observed specimens are believed to have been lost in the past because no specimens were deposited by Kawamura (1918), which was confirmed by interviews of several of his colleagues (e.g., Kenichiro Negoro, personal communication); this was also the case for his simultaneous studies.

The original very short description of this species is incomplete, and thus its taxonomic status must be clarified. Furthermore, because the population of the type locality is considered to be extinct, a name-bearing type for D. biwae is deemed to be needed to verify their conservation status in the long term. Thus, the specimen ( TNS-AL –63110), which was collected from Lake Biwa as the type locality assigned by Kawamura (1918) ( Fig. 3A ), is herein designated as the neotype for D. biwae . The species was abundant with morphological variation in Lake Biwa population as the subdominant plankton species in August each year in that collection period in 1961. Accordingly, the present neotypication meets the conditions of Article 75 of the International Code of Zoological Nomenclature ( International Commission on Zoological Nomenclature 1999).

Diagnosis. Shell fusiform, transparent, and brown in fixed specimens, composed of mineral particles or diatom frustules as xenosomes, from their environment. The circu- lar oral aperture surrounded by a conspicuous low funnelshaped collar with a ragged margin. Shell circular in crosssection, narrowest at the base of the collar and gradually swelling to broadest at the posterior, then narrowing gently toward the aboral protuberance. The protuberance gently sinuous and pointed at the tip. One or two pseudopodia extended from the aperture to the back of the shell.

Redescription. Shell fusiform, transparent, and brown in fixed specimens, composed of mineral particles ( Figs 3B , 4E , 5A–G ) or diatom frustules [typically Praestephanos suzukii (A. Tuji and Kociolek, 2000) , same one species as previous “ Stephanodiscus suzukii ” and “ S. pseudosuzukii ” in Ichise et al. (2004)] as xenosomes, captured from their environment ( Fig. 5D ).

Body length [(TL) in Fig. 2 , all measurement in VSS-I, maximum–minimum, followed by the measurement of the neotype in parenthesis, the same hereinafter] 359–230 (312) µm, broadest body width ( BW) 79–54 (59) µm, narrowest body width ( NW) 52–40 (42) µm, broadest diameter of collar ( CD) 97–78 (83) µm, broadest diameter of aboral protuberance ( PD) 26–19 (24) µm, length of protuberance ( PL) 157–84 (155) µm ( Figs 4A–D , 5A ) . Shell circular in crosssection ( Fig. 5B ), narrowest below the collar and gradually swelling to broadest 1.98–1.29 (1.40), ( BW / NW in Fig. 2 , the same hereinafter) in the position of the posterior 0.19– 0.27 (0.19), ( BW /TL) of the body length, then narrowing abruptly toward the aboral protuberance ( Figs 4A–D , 5A ) . A conspicuous low-funnel-shaped collar surrounds the circular oral aperture ( Figs 4A–E , 5A–C ). Collar margin thin and slightly wavy in the outer periphery ( Figs 4E , 5A–C, E, F ) . Broadest diameter of body 0.90–1.59 (1.41), collar diameter ( CD / BW) . The aboral protuberance, gently sinuous and pointed in the tip ( Figs 4A–D, F , 5A, G ) . Broadest diameter of the protuberance is in the position of the posterior attachment part of body, at 0.07–0.05 (0.08) of the body lengths ( PD /TL) ( Figs 4A–D , 5A ) . All raw measurement data shown in Appendix 1 [one obviously irregular shell (VSS-I-19) excluded from all size discussion and morphometric analyses] . One or two pseudopodia extend from the aperture to the back of the shell ( Fig. 3A ; Kawamura 1918) . Food predominantly diatoms such as P . suzukii ( Fig. 5D; P . suzukii in shell of this species, as previously in cytoplasm).

Distribution. Lake Biwa, Shiga Prefecture, Japan (e.g., Kawamura 1918) and three Chinese lakes: Qiandao Lake, Zhejiang Province ( Li and Yu 2001); Poyang Lake, Jiangxi Province ( Wang et al. 2003); Mulan Lake, Hubei Province, China. However, Tsugeki et al. (2003) and Ichise et al. (2004) reported that D. biwae is extinct in Lake Biwa due to eutrophication caused by nutrient input from its surrounding river systems.

Remarks. Difflugia biwae resembles D. delicatula Gauthier-Lièvre and Thomas, 1958 , D. elegans Penard, 1890 and D. oblonga caudata Štěpánek, 1952 in its shell shape and aboral protuberance. However, D. biwae can be clearly distinguished from these species by the feature that its collar diameter is longer than its body width. Additionally, D. biwae differs from these species in that it has a long neck (vs. short neck in D. delicatula and D. elegans ) and the smooth appearance of its elongated fusiform shell (vs. rough appearance of elongated oviform shells in D. elegans and D. o. caudata). Yang and Shen (2005) have discussed the comparisons of these conjugates.

Yang and Shen (2005) referred to “its typical locomotive form on the substrate.” However, the authors did not observe pseudopodia of D. biwae in the present study. The description of pseudopodia in the redescription section is based on the original line drawings ( Fig. 3A ; Kawamura 1918) and the observations of Yang and Shen (2005).

Ichise et al. (2004) reported that when the body of D. biwae was crushed, shells of Centrales diatom, mainly P. suzukii , exuded from the cells, suggesting that this species mainly preyed predominantly on diatoms, such as P. suzukii ( Fig. 5D ).

In the present paper, we restore the Japanese standard name as “Biwako-tsubokamuri” written in the original description ( Kawamura 1918). Although Mizuno (1977) used “Biwa-tsubokamuri” as the Japanese name for the species, it was a miss-spelling at first. “Biwako” and “ko” represent “Lake Biwa” and “Lake” respectively in Japanese.

Figures

Fig. 3. Line drawings of Difflugia biwae. A, Lateral view with cytoplasm (original line drawing, originally no scale: after Kawamura 1918); B, lateral view of shell (present study, scale 100µm for B).

Fig. 4. Light micrographs of the shell of Difflugia biwae. A–D, Lateral views of whole shells of four different specimens with variations (A after “Photo. 1” in Ichise et al. 2004); E, apertural view showing the circular aperture is surrounded by a conspicuous great collar flare; F, apertural view showing the protuberance. Sampling data, Morph#1 in the Materials and Methods section. Scale 100µm in A for A–D; scale 100µm in E for E and F.

Fig. 5. Scanning electron micrographs of the shell of Difflugia biwae. A, Lateral view of whole shell; B, apertural view showing the circular aperture is surrounded by a conspicuous great collar flare; C, lateral-apertural view of collar flare; D, composed of mineral particles or diatom frustules (typically Praestephanos suzukii, endemic diatom of Lake Biwa) as xenosomes, captured from their environment; E, detail of surface of collar flare rim (inside) with small angular particles and flattish pieces of quartz; F, detail of surface of collar flare rim (outside); G, tip of the protuberance with flattish pieces of quartz. Sampling data of these specimens was given as Morph#1 in the Materials and Methods section. Scales are given in each photo.

Fig. 2. Shell outline and positions of measured axes used in present study: TL, total length; BW, body width; CD, collar diameter; NW, neck width; BL, body length; PD, protuberance diameter; PL, protuberance length.