2026 Volume 101 Issue 3 Pages 145-149
The genus Tabularia currently comprises 20 accepted diatom species, among which T. sinensis was first described from Poyang Lake, China. In this study, we re-examined Japanese specimens of T. sinensis using scanning electron microscopy, focusing on the structure of the valvocopula. In our previous report, we described the valvocopula of T. sinensis as open at one end. However, the present observations confirmed that it is closed at both ends. This feature has not been reported in other Tabularia species and is considered characteristic of the genus Ulnaria. The previously observed open-ended structures were likely artifacts caused by sample preparation damage. Recent molecular phylogenetic studies suggest that some species currently placed in Tabularia may require taxonomic reassessment. Morphologically, T. sinensis exhibits intermediate features between Tabularia and Ulnaria, suggesting a complex taxonomic position. (Continued from J. Jap. Bot. 95: 291–296, 2020)
Tabularia属珪藻は現在20種が認められており，そのうちT. sinensisは中国の鄱陽湖で初めて記載された．本研究では日本産T. sinensisを走査型電子顕微鏡で再検討し，接殻帯片(valvocopula)の構造に着目した．以前，本研究者らはT. sinensisの接殻帯片が片方開放している（開環型）と報告したが，今回の観察で両端が閉じている（閉環型）ことを確認した．これは従来のTabularia属には見られず，Ulnaria属に特有の特徴である．試料作製時の損傷で開放したように見えたが，詳細な観察により閉じているのが本来の形であると判明した．T. sinensisは形態的に分子系統解析では異なる系統群とされるTabularia属とUlnaria属の中間的特徴を示すことから，複雑な分類学的位置が示された．
Currently, 20 species of diatoms are recognized within the genus Tabularia (Kützing) D.M.Williams & Round (Guiry and Guiry 2025). Tabularia sinensis Yue Cao & al. is a freshwater diatom species that was first discovered and described as new to science by Cao et al. (2018), based on specimens collected from Poyang Lake, the largest freshwater lake in China. In Japan, T. sinensis was reported as a new record by Nishida et al. (2020), based on observations of specimens collected from rivers in Fukuoka Prefecture and on Iriomote Island, Okinawa Prefecture. Although considered to be native to Asia, T. sinensis has also been recorded outside its presumed native range, including in the Seine River, France, in 2018, and the Scheldt–Sea Canal in Brussels, Belgium, in 2021, as reported by Vijver et al. (2023).
We previously examined the fine morphology of Tabularia sinensis using scanning electron microscopy (SEM). In the present study, we prepared new material and conducted a detailed re-examination, focusing particularly on the structure of the valvocopula (VC). As a result, we obtained new insights into the morphology of the VC.
On 2 February 2016, diatom samples of Tabularia sinensis were collected from beneath the Omija Bridge in the Omija River, Iriomote Island, Okinawa Prefecture, Japan (24°23′39″N, 123°51′49″E). Samples were obtained by brushing the surfaces of stones on the riverbed to dislodge attached algae.
The collected samples were transported to the laboratory, where lipids were first extracted using ethanol. Organic matter was then removed following the method of Nagumo (1995) using a commercial drain cleaning detergent. Subsequently, residual organic material was eliminated by hot sulfuric acid treatment to obtain clean diatom frustules.
The cleaned frustules were air-dried on aluminum stubs and coated with gold using an ion sputter coater (Hitachi E-1010). Detailed morphological observations of Tabularia sinensis, including its valvocopula structure, were conducted using SEM (JEOL JSM-7100F).
SEM observations of Tabularia sinensis revealed that all girdle bands, except for the valvocopula, were open at one end, consistent with previous reports. However, detailed examination of the valvocopula from both ends showed that some valvocopula were closed at both ends rather than open-ended (Figs. 1–6).
Figs. 1–6. Scanning electron micrographs of Tabularia sinensis. The valvocopula is observed to be closed at both ends in all specimens shown in Figs. 1–6. 1. Whole frustule with the near-side valve absent. 2, 3. Enlarged views of both ends of the same frustule shown in Fig. 1. 4. Valve ends of a different specimen, comparing VCU and VCL, showing that VCU is thinner than VCL. 5, 6. Enlarged views of both ends of the frustule from another specimen. The lower valve is absent in this specimen. Both ends of VCL are shown, allowing comparison of its thickness within the same individual. VC: valvocopula; VCU: valvocopula attached to the upper valve in each figure; VCL: valvocopula attached to the lower valve in each figure; B2, B3: the second and third girdle bands, respectively; asterisk (*): indicates the open end of B2. Scale bars: 10 µm (1), 1 µm (2–6).
These observations indicate that the valvocopula of Tabularia sinensis is structurally closed in its native state. Instances where the valvocopula appeared open-ended likely resulted from damage to the extremely thin distal portions of the bands during the cleaning process or the drying procedure, particularly under thermal stress.
Closer inspection of the openings in the apparently open valvocopula revealed a lack of the bilateral symmetry characteristic of other girdle bands, suggesting these openings are not natural structures. Moreover, the thickness at the ends of the valvocopula was asymmetrical, with one end being markedly thinner than the other (Figs. 4–6).
To distinguish true structural openings from those resulting from processing-induced damage, we note that a natural opening—such as those indicated by asterisks in Figs. 4 and 6, depicting the natural opening of girdle band 2 (B2)—is a primary structure with regular and continuous margins. In contrast, openings caused by processing-induced damage, such as the left side of the asterisk on the valvocopula (VC) in fig. 11 of Nishida et al. (2020), display irregular margins and disrupted surface continuity. Thus, natural openings and processing-induced damage can be reliably differentiated based on the regularity of the margins and the continuity of the band surface.
Harper et al. (2009) reported that during drying treatments of Tabularia specimens, the ends of the valvocopula are prone to twisting and subsequent breakage, and presented SEM images clearly showing such damage (Harper et al. 2009: fig. 11). Our findings align with these reports, supporting the hypothesis that similar mechanical stresses caused processing-induced damage to the valvocopula in T. sinensis.
Taxonomic Position and Phylogenetic Implications of Tabularia sinensisUntil the 1970s, the genus Synedra Ehrenberg included both freshwater and marine species, representing multiple phylogenetic lineages. Lange-Bertalot (1980) proposed transferring freshwater Synedra species to the genus Fragilaria Lyngbye. Subsequently, marine species were reclassified into five genera, including the newly established genus Tabularia, by Williams and Round (1986). In the same year, Williams (1986) informally proposed a new genus Catacombas D.M.Williams & Round, with S. gaillonii (Bory) Ehrenberg as its type species, separate from Synedra, whose type species was considered to be S. ulna. However, because S. gaillonii is the lectotype of Synedra which was designated by Ehrenberg (1830), the creation of a separate genus with this species as its type contravened nomenclatural rules. As a result, Catacombas was illegitimate and superfluous. Consequently, several species, including S. gaillonii, remained within Synedra. Later, the genus Synedra, with S. ulna as the type species, was reclassified as the subgenus Alterasynedra Lange-Bertalot under Fragilaria Lyngbye (Lange-Bertalot in Krammer and Lange-Bertalot 1991). However, Compère (2001) pointed out that Alterasynedra is not a validly published subgeneric name and therefore cannot be used as a basionym. He also suggested that these positionless species clearly belong to a phylogenetic lineage distinct from Fragilaria and proposed the establishment of the independent genus, Ulnaria (Kützing) Compère.
Diatoms of the genus Ulnaria are characterized by having all girdle bands in a closed configuration (Williams 1986, Compère 2001). However, confirming this feature requires observation during the late stages of cell division, which is practically difficult. Liu (2023) examined Ulnaria species collected from various climatic regions in China and proposed that the closed configuration of the valvocopula serves as a diagnostic feature of the genus.
According to Liu’s proposal (Liu 2023), Tabularia sinensis, collected from freshwater habitats, could also be assigned to Ulnaria based on the valvocopula. However, this species exhibits morphological traits typical of Tabularia, including the overall frustule shape, areolae occluded by cribral plates, and well-developed apical pore fields at both valve ends. Although, Tabularia and Ulnaria also differ in six other morphological characters, as summarized in Table 1, i.e., frustule outline, valve shape, apical pore field, areola structure, copulae structure (including the valvocopula), and habitat tendency, Tabularia sinensis shares the first four traits of Tabularia, and the last two Ulnaria. Such character distribution indicates that although T. sinensis shares a closed valvocopula with Ulnaria, it exhibits an intermediate morphology between the two genera. Although the present study does not aim to resolve the precise taxonomic placement of T. sinensis, previous studies (Williams and Karthick 2021) have confirmed that Tabularia and Ulnaria belong to distinct phylogenetic lineages. Therefore, further molecular phylogenetic analysis including T. sinensis is expected to provide a more accurate basis for taxonomic assignment in future studies.
| Morphological character | Tabularia
Williams and Round (1986) Nishida et al. (2016) |
T. sinensis
Present study |
Ulnaria
Compère (2001) |
|---|---|---|---|
| Frustule outline | Linear to narrowly lanceolate | Linear | Linear-lanceolate to linear-elliptical |
| Valve shape | Straight or slightly curved | Straight or slightly curved | Often distinctly curved |
| Apical pore field | Well-developed at both ends | Well-developed at both ends | Weak or moderately developed at ends |
| Areola structure | Areolae occluded by cribral plates | Areolae occluded by cribral plates | Areolae not occluded; volae present |
| Copulae structure | All bands open (including valvocopula) | Valvocopula closed; other bands open | All bands closed (including valvocopula) |
| Habitat tendency | Marine / Brackish | Freshwater | Freshwater |
