Gomphonema firmum Skvortzow 1937

Vishnyakov, Vasily S., 2022, Expended range of Gomphonema firmum (Bacillariophyceae), once considered a Lake Baikal endemic, with notes on the identity of G. lanceolatum var. maximum, Phytotaxa 530 (2), pp. 213-220 : 214-219

Gomphonema firmum Skvortzow 1937, Philippine J. Sci. 62: 353, fig. 10: 12.

LM ( Figs. 1–13 ). The valves are heteropolar, clavate-lanceolate with broadly rounded poles almost equal in width; the broadest portion of the valve is near the central area, length 54–125 µm, width 16.5–21 µm. The axial area is relatively broad, linear, and expanded at the middle to form a mostly transapically elliptical to rhombo-elliptical central area. Both axial and central areas have unevenly scattered rounded impressions appearing as dark spots. Raphe branches are straight, distinctly lateral in the middle, with rounded external central pores somewhat deflected toward the isolated stigma. Striae are distinctly punctate, weakly radiate throughout, 11–12 in 10 µm; those striae bordering the central area are irregularly shortened. The headpole bears a single marginal spine, yet is difficult to be discerned in a valve view. The opposite one has a large bilobed pore field, which is clearly separated from the adjacent striae by a transverse hyaline strip.

SEM, external view ( Figs. 14–20 ). The valve surface has obvious relief due to depressed areolae, pitted areas, and the unevenly silicified valve margin ( Figs. 14–20 ). The striae are resolved into series of rounded loculi, where each areola opens into individual loculus. The striae bordering the central area comprise transapically elongated loculi, where two areolar openings may be visible ( Fig. 19 ). The areolae are commonly occluded with a single flap of silica (vola) arising from the areolar margin. The volae are round to reniform, smooth-edged, leaving С-shaped areolar slits. In some cases, volae are incised, or two or three volae occlude individual areola leaving irregular slits ( Fig. 20 ). The numerous shallow impressions without areolar openings present on both axial and central areas. The raphe fissures are accompanied by shallow grooves in proximal parts, and terminate with simple central pores ( Fig. 19 ). The headpole bears a considerably large oblate-shaped spine, which is straight or deflected on to the terminal fissure or to another side ( Figs. 14–17 ). In addition, several small plicate ribs may be visible along the junction of the valve face and mantle, particularly frequent close to the headpole ( Fig. 17 ).

SEM, internal view ( Figs. 20–26 ). The valve surface presents a wide sternum and strongly silicified virgae (costae). The rounded areolae are contained in deep grooves (alveolae) and separated with relatively wide vimines. The vimines bear stubs, which can be recognized as small paired papillae ( Fig. 24 ). The hemispherical central nodule bears recurved proximal raphe endings and a single slit-like transapical stigma opening ( Fig. 22 ). The raphe branches terminate distally in the elongated narrow helictoglossae ( Figs. 23–26 ). The pseudoseptae present on both valve poles; the footpole pseudoseptum is somewhat wider than headpole pseudoseptum ( Figs. 23, 24 ). The cingulum of each valve consists of two open valvocopulae having polar septae ( Figs. 25, 26 ).

Morphology of G. firmum from the newly observed locality clearly corresponds with amended descriptions in terms of valve shape, striae pattern, striae density, the presence of scattered impressions on the external valve sternum and the large spine at the headpole ( Skabitschewsky 1984, Kulikovskiy & Kociolek 2014). Meanwhile, size diminution series in Gyrgyntui include specimens shorter than previously described (54 vs. 70 µm). The presented SEM observation clarifies the fine structure of the valves, since previous SEM study dealt only with heavily eroded specimens (cf. Kulikovskiy & Kociolek 2014: fig. 45–49); however, it does not seem to contradict the SEM description of specimens from Lake Baikal. When Kulikovskiy and Kociolek (2014: 514) described areolae as having “small silicious projections into the areolar openings”, they obviously considered reduced volate occlusions. In G. firmum, most of the areolae are occluded externally by smooth-shaped or incised simple volae, as can be seen from our micrographs. The presence of plicate ribs along the valve margin, which seem to form in the same manner as the headpole spine, is a feature common in the Gyrgyntui specimens. In some strongly silicified specimens, the ribs can be arranged into two intermittent marginal ridges, which can be better seen from a girdle view (e.g., Figs. 15, 17 ). Although this feature was not mentioned in the previous study, the plicate structures nearly identical those in Gyrgyntui specimens may also be visible from micrographs of the Lake Baikal specimens (e.g., Kulikovskiy & Kociolek 2014: figs. 45–47).

Hence,both LM and SEM studies confirm the existence of G. firmum outside of Lake Baikal.From a biogeographical perspective, this means that the species can no longer be defined as narrow endemic, since a taxon, in particular a species, is considered to be endemic to a given area if it occurs only in that area ( Anderson 1994). In Gyrgyntui, the ecological conditions, i.e., shallow running water, are in sharp contrast with those in Lake Baikal, but a relatively high species abundance indicates these conditions as quite suitable too. We have no evidence to suggest Gyrgyntui as unique in some way, since there are countless streams similar to Gyrgyntui in the Taiga zone of Siberia; this, in turn, suggests that the species might be found at greater distances from the locus classicus.

In view of the above, another giant Siberian Gomphonema became the focus of our attention. Gomphonema lanceolatum var. maximum Poretzky in Proshkina-Lavrenko (1950: 296) is a neglected name, which was introduced for the diatom that occurred once in Lake Teletskoe , the Altay Mountains (51.719°N, 87.652°E). The name was first published with a Russian description and micrograph in 1950 ( Fig. 27 reproduced here). In 1953, the same description translated into Latin together with a line drawing ( Fig. 28 ) and precise data on the collection site was additionally published ( Poretzky & Sheshukova 1953: 152). Poretzky described the variety as having lanceolate valves of length 127–132 µm and width 20–24 µm with wide axial area, linear-lanceolate central area, and slight radial pattern of coarsely punctate striae, 8–10 in 10 µm. The light micrograph, which seems to be the only remaining element of the original material, shows the diatom without any differences from G. firmum in valve shape, dimensions, and striae pattern. The line drawing, although not as precise as the light micrograph, still shows the characteristic features of the same diatom.

Poretzky ascribed his variety to G. lanceolatum Ehrenberg (1843: 306), a species with intricate taxonomic history ( Krammer & Lange-Bertalot 1986). The identity of the species is still unknown. Reichardt (1995) did not find this diatom in the type material from Cayenne. Ehrenberg’s figure (1843: 306, fig. 2/1: 37) shows a Gomphonema with pronounced rhombo-lanceolate outline and very acute headpole, Gomphonema lanceolatum var. maximum has obviously no relations with G. lanceolatum, since the latter differs considerably in outline and size. We therefore propose G. lanceolatum var. maximum as a synonym of G. firmum. A new synonymy further widens the species distribution.

As was repeatedly outlined, the Lake Baikal diatom flora comprises several biogeographical elements, i.e., taxa restricted to the lake, or endemics, and those ones distributed more widely in Siberia or even in the Holarctic. There are both relicts and recently arisen taxa among endemics ( Flower 2005, Kulikovskiy et al. 2012). Gomphonema firmum provides a new example of putative endemic species originally described as from Lake Baikal, that is not actually restricted to the lake, due to prior attention to its diatom flora. In light of the presented data, the species appears to be widely distributed across southern Siberia. It can also be characterized as a rare species with a disjunct distribution pattern, being limited to a small number of localities, which are considerably isolated from each other. However, it is hard to explain this pattern due to lack of extensive sampling efforts throughout Siberia and a paucity of paleontological data.

Figures

FIGURES 1–13: Gomphonema firmum from Gyrgyntui, valve view, LM. Size diminution series including specimens shorter than previously described. Scale bar: 10 µm.

FIGURES 14–20: Gomphonema firmum from Gyrgyntui, external views, SEM. Fig. 14. Frustules in valve (a) and girdle (b) views. Figs. 15, 16. Frustules from external view. Fig. 17. Headpole showing the marginal spine and plicate ribs along the junction of valve face and mantle (arrows). Fig. 18. Footpole showing terminal raphe fissure and bisected pore field, which is separated from the adjacent striae by transverse hyaline strip. Fig. 19. Detail of central portion of the valve showing proximal raphe endings, round stigma opening and scattered impressions. Fig. 20.Areolae with volate occlusions; areolae with more than one vola are indicated by arrows.

FIGURES 21–26: Gomphonema firmum from Gyrgyntui, internal views, SEM. Fig. 21. Valve with cingulum consisting of two open valvocopulae. Fig. 22. Detail of central portion of the valve showing hemispherical central nodule, proximal raphe endings (black arrows) and slit-like stigma opening (white arrow), note the pattern of strongly silicified virgae and alveolae containing internal areolar openings. Fig. 23. Headpole showing pseudoseptum (black arrow) and distal raphe ending on helictoglossa (white arrow). Fig. 24. Footpole showing pseudoseptum (black arrow) that masks apical pore field, distal raphe ending on helictoglossa (white arrow), note very small, hardly recognizable paired stubs (small white arrows). Fig. 25. Headpole, valvocopula with septum, which masks helictoglossa (white arrow). Fig. 26. Footpole, valvocopula with septum, note porelli of the bisected pore field (arrows).

FIGURES 27, 28: Gomphonema lanceolatum var. maximum, reproduced iconographs. Fig. 27. Micrograph from Proschkina-Lavrenko (1950: Fig. 5). Fig. 28. Line drawing from Poretzky and Sheshukova (1953: Fig. 10). Scale bar: 10 µm.