Synedra bacillaris (Grunow) Hustedt, 1932

Lobban, Christopher S., Ashworth, Matt P., Camacho, Terance, Lam, Daryl W. & Theriot, Edward C., 2022, Revision of Ardissoneaceae (Bacillariophyta, Mediophyceae) from Micronesian populations, with descriptions of two new genera, Ardissoneopsis and Grunowago, and new species in Ardissonea, Synedrosphenia and Climacosphenia, PhytoKeys 208, pp. 103-184 : 103

Synedra bacillaris (Grunow) Hustedt, 1932

Fig. 19

References.

Grunow 1877, p. 167, pl. 193, fig. 12a-c; Hustedt 1931-1959, p. 230, fig. 718; Sullivan and Wear 1995, p. 182, figs 9-12.

Description from literature.

As described from LM ( Hustedt 1931-1959), this taxon is easily distinguished from Ardissonea spp. by the prominent longitudinal costa on the midline and the absence of lateral longitudinal costae (i.e., no observable annulus). There is only one wall layer. Valves linear, usually slightly wider in the middle, 320-700 µm long, 16-20 µm wide, 8-9 striae in 10 µm. Robust transapical costae on each of the virgae. As seen in SEM ( Sullivan and Wear 1995), the valve has a single wall, areolae are circular, those at the poles slightly smaller and not organized into striae, elsewhere the striae are framed by the transapical costae. Sullivan and Wear stated that the costae extend from the margin to the median costa, but their fig. 12 shows some costae extending from the median costa and the costae are not consistently thick. Sullivan and Wear reported an absence of spines, polar pore fields and rimoportulae. Girdle bands closed, not more than five (three on the epicingulum): the valvocopula broad with a single row of small, circular pores near the advalvar edge of the pars exterior and a large flange that fits inside the valve mantle; there was said to be short fimbriae along its inner edge but they are not evident in the micrographs. There are additional rows of pores on the valvocopula just around the poles. The valvocopula has a flange that fits around the valve mantle but there is no apical notch. The copula is also broad but with several rows of pores (their fig. 11 shows these loosely organized in straight striae of at least three pores), and there is a narrow pleura ( Sullivan and Wear 1995); none of their images show the inner edge of the girdle bands. Although not specifically stated in these descriptions, the frustules are implicitly isopolar. Sullivan and Wear (1995) particularly noted the circular areolae, in contrast to the large oval areolae they saw in Ardissonea formosa . Grunow’s (1877) published drawings (as Synedra crystallina var. bacillaris ) do not include details of areolae but unpublished drawings archived in the Grunow drawing collection at W (W1901-5465) include several sketches where the areolae appear to be oval. Grunow’s drawings, like those of later authors, show this species as linear and isopolar, with bluntly rounded poles, but his unpublished girdle view shows a slightly tapered frustule.

Observations.

We were able to find a few fragments of Synedra crystallina var. bacillaris Grunow in an SEM preparation from Grunow’s Honduras gathering (Fig. 19 ). The external view (Fig. 19A-C ), of a fragment 172 µm long that appeared to reach the slightly inflated middle, was distinguished from Ardissonea formosa , also present on the stub, by the row of pores on the exterior of the valvocopula. The pole was blunt, with small spines (Fig. 19B , arrowheads), and the valve linear, although a little wider in the middle. Both valves had 11 striae in 10 µm, comprising oval areolae, and the small fragment, width 10 µm, showing the interior had the characteristic central longitudinal costa and prominent transapical costae (Fig. 19D ). A second frustule fragment (Fig. 19E-G ), 174 µm long, showed the single line of pores on the valvocopula were 19 in 10 µm, striae on the copula 18 in 10 µm. The broken tip of this valve showed the transverse costae (Fig. 19G ).

Taxonomic comments.

Specimens provisionally identified as S. bacillaris were found in samples from across Micronesia but there was a range of stria densities and dimensions. All had oval areolae and many had spines. However, there were few fitting the literature description, even overlooking the shape of the areolae, i.e., those with linear valves, blunt poles and 8-9 striae in 10 µm. Those in Guam were mostly among populations of lanceolate specimens and probably part of them (see next taxon). The small, circular areolae shown by Sullivan and Wear (1995) are certainly different from the oval areolae in our specimens but determining the identities of these specimens relative to one another and to the literature will require much further study.

Figures

Figure 19. Putative Synedra crystallina var. bacillaris from Grunow's Honduras material A-C specimen in external view with valvocopula, identified by the length, the straight sides and blunt pole, stria density (11 in 10 µm), and row of pores in the exterior of the valvocopula (arrows); small spines on apex (arrowheads on B) D fragment of valve, interior view, showing central longitudinal costa and transverse costae E-G fragment of frustule showing valvocopula and copula G showing transverse costae (arrow) and absence of inner wall, arrowhead shows additional pores on valvocopula pole. Scale bars: 25 µm (A, E); 10 µm (B-D, F); 5 µm (G).