Staurosirella eruciformis Van de Vijver, 2023

Vijver, Bart Van De, 2023, Staurosirella eruciformis, a new Staurosirella (Bacillariophyta) species from a historic sample from Scotland, UK, Phytotaxa 595 (3), pp. 296-300 : 296-297

Staurosirella eruciformis Van de Vijver sp. nov. ( Figs 1–14 LM, 15–18 SEM)

Frustules solitary. Valves linear, with parallel, irregularly undulating margins and broadly rounded, non-protracted apices. Occasionally, valves with constricted central part present ( Fig. 12 ). Continuous series of small aggregates of irregularly shaped, small, solid spines present along the valve face margin, located on the virgae ( Figs 15–17 ). Spines absent on the valve apices. Valve dimensions (n=25): valve length 10–30 µm, valve width 5.0–6.5 µm. Sternum fairly narrow, weakly lanceolate, narrowing towards the apices. Externally, valve face uneven with virgae having a narrow, raised ridge and adjacent striae slightly sunken in ‘punch hole-like’ depressions ( Figs 15–17 ). Vimines narrow, not raised. Striae alternating at both sides of the sternum, parallel to weakly radiate near the valve middle, becoming distinctly more radiate towards the apices, 9–11 in 10 µm, composed of long, slit-like, linear areolae, running parallel to the apical axis. Occasionally areolae doubled by short struts between the vimines ( Figs 16, 17 ). Striae extending uninterruptedly from valve face onto mantle, areolae in size gradually narrowing at both ends ( Fig. 17 ) giving the striae a lanceolate appearance. Small apical pore fields present at both apices, almost similar in size and shape, located at the valve face/mantle junction, isolated from neighboring striae ( Fig. 15 ). Pore fields well delimited, composed of small pores, arranged in several irregular rows ( Fig. 16 ). Internally, striae distinctly sunken between the flattened virgae and sternum ( Fig. 18 ). Doubling of the areolae well visible. Areolae occlusions not observed, probably due to corrosion associated with the cleaning of the valves.. Girdle structure not observed.

Type:— UNITED KINGDOM. Hawthorden , Scotland, Walker Arnott sample WA245, leg. Gregory (holotype slide BR-4785 = Fig. 4 , isotype slide 423 in Collection University of Antwerp, Belgium) .

Registration:— http://phycobank.org/103762

Etymology:— The specific epithet “ eruciformis ” refers to the shape of the valves resembling a large caterpillar. The Latin word “ eruca ” means caterpillar.

Ecology & associated diatom flora:— The only information available on sample WA245 is that it was collected in Hawthorden by Gregory. Only two samples in the entire Walker Arnott collection came from this locality in Scotland, samples WA244 and WA245, both gathered by Gregory. The sample is dominated by Denticula tenuis Kützing (1844: 43) , Odontidium hyemale ( Roth 1800: 506) Kützing (1844: 44) , Fragilariforma nitzschioides (Grunow in Van Heurck 1881: pl. 44: fig. 10) Lange-Bertalot (in Hofmann et al. 2011: 268) and various (at present unidentified) taxa in the genera Achnanthidium , Denticula , Planothidium and Nitzschia . This diatom flora mostly points to more or less oligotrophic, alkaline conditions ( Hofmann et al. 2011).

Comments:— Staurosirella eruciformis can hardly be confused with other Staurosirella species, given its rather robust valve outlook, the absence of protracted apices, the linear outline with occasionally constricted margins and the typical spine gatherings on the valve margin. Most Staurosirella species have lower valve dimensions and often possess a more lanceolate valve outline. Staurosirella neopinnata E.A. Morales et al. (2019: 82) does not exceed 4.7 µm in valve width (contrary to 5.0–6.5 µm in S. eruciformis ) and has a linear to linear-lanceolate valve outline with weakly convex, not concave, margins ( Morales et al. 2019). Van de Vijver (2022) described recently S. coutelasiana Van de Vijver (2022: 165) , a somewhat longer species (length up to 35 µm) but with a typical lanceolate valve outline, contrary to the broad, linear outline in S. eruciformis .

A few other diatom taxa show some resemblance but at present they can only be compared using the original drawing as the type material of these has never been revised. For instance, Odontidium ventriculosum Schumann (1862: 184) has a similar elongated valve outline but with a clear central inflation ( Schumann 1862, fig. 10). It is also possible that Fragilaria pinnata fo. parallela Mayer (1919: 196) is conspecific. Mayer (1919, 1937) show drawings of several valves with an elongated, broadly rounded valve outline and a valve width of 4–5 µm, slightly lower than S. eruciformis . It is unclear where the Mayer material is currently conserved making a comparison rather impossible. As Mayer (1919) described his taxon as a variety, the name will not have priority in species rank and therefore a possible conspecificity will have no taxonomic consequences later.

Staurosirella eruciformis has previously been reported, although never identified up to species level. Kulikovskij et al. (2011, plate 2 figs 13 & 14) illustrated 2 valves that might represent the new species and identified them as Staurosira sp. Similarly, Peeters & Ector (2017, p. 268) also show some valves that have a comparable valve outline. In both publications, it seems that the valve width is always lower (<4.5 µm) than in S. eruciformis and longer valves, typically found in the type population of the new species seem to be absent in the French and Belarussian population. A comparative analysis of all populations will be necessary to establish conspecificity. Finally, in the Republic of Northern Macedonia, a population of S. eruciformis was observed showing that the species most likely has a larger distribution range than being simply restricted to Scotland (Levkov, pers. comm.).

The observation in the historic Walker Arnott sample shows once more the importance of historic collection samples, often gathered in pristine environments, lost nowadays due to pollution and anthropogenic impact. In the past year, several new species were published based on the analysis of the diatom flora in historic slides ( Van de Vijver 2022, 2023, Van de Vijver & Wetzel 2022). A better analysis of this historic material can provide valuable insights in for instance reference environmental conditions and previous biogeographical distributions of red-list species, nowadays almost extinct due to habitat loss. More efforts should be directed to the conservation, digitisation and study of these historic diatom collections in order to preserve and disclose this valuable knowledge.

Figures

Figures 1–18.Staurosirella eruciformisVan deVijver sp.nov.LMand SEM images takenfrom the holotypesample (BR-4785, Hawthorden, Scotland, UK, Walker Arnott sample WA245). 1–14. LM views of the population arranged in decreasing length. 15–17. Three SEM external views of an entire valve. Note the presence of the irregular groups of marginal spines and the rather well-developed apical pore field at each apex. The virgae and sternum are clearly raised with the striae as punch-holes. 18. SEM internal view of an entire valve. Scale bars = 10 µm.