Several populations belonging to the genus Staurosirella have been observed in European rivers that were previously identified as Staurosirella pinnata. In light of the recent taxonomic revisions of the genus Staurosirella, the morphology of the unknown Staurosirella populations has been critically investigated using light and scanning electron microscopy. Following the comparison with previously described Staurosirella species, five taxa could not be identified using the currently available literature on the genus. These five taxa are described as new based on differences in valve outline; shape, size and structure of the apical pore fields; structure of the striae; and the presence, position and structure of the marginal spines. Two new species were described using historic collection material: Staurosirella binodiformis sp. nov. and Svanheurckiana sp. nov. Two new species were observed in samples from rivers in Flanders: S. marginostriata sp. nov. and S. stoksiana sp. nov. whereas a fifth species was observed in rivers from Iceland: S. jonssoniana sp. nov. All new species are compared with similar Staurosirella species worldwide. Notes are added on their ecological preferences derived from both physicochemical data and the associated diatom flora.

Europe, morphology, new species, Staurosirella, taxonomy

The genus Staurosirella D.M.Williams & Round was split in 1988 from the genus Fragilaria Lyngbye sensu lato (Williams and Round 1988) and is characterised by both isopolar or heteropolar valves with broad uniseriate striae composed of slit-like, linear, internally occluded areolae separated by narrow vimines. The original 1988 description was refined in 2006 (Morales and Manoylov 2006a). Since its erection, more than 50 Staurosirella species were either described as new species or transferred to it from other genera such as Fragilaria, Odontidium Kützing, Staurosira Ehrenberg or Opephora P.Petit (e.g. Morales and Manoylov 2006b; Van de Vijver et al. 2014; Almeida et al. 2015; Seeligmann et al. 2018; Guerrero et al. 2019; Morales et al. 2019, Osório et al. 2021; Van de Vijver 2022, 2023, Van de Vijver et al. 2022).

In the past 10 years, the number of Staurosirella species strongly increased, most likely due to a better understanding of several catch-all species such as Staurosirella pinnata (Ehrenberg) D.M.Williams & Round. An internet search for the name ‘Staurosirella pinnata’ resulted in more than 12.000 hits with an extra 38.700 for the name ‘Fragilaria pinnata’. For comparison, the name ‘Staurosirella neopinnata E.Morales et al.’ only resulted in 907 hits. Moreover, the name ‘Staurosirella (Fragilaria) pinnata’ was reported worldwide from the tropics to the poles with more than 80 records from Asia, Africa, Antarctica, Australia, North, Central and South America, and Europe [see AlgaeBase] (Guiry and Guiry 2023). It is clear that this worldwide distribution is the result of taxonomic drift on one side (severely broadening the original description of the species) and force-fitting (Tyler 1996) populations from all over the world into S. pinnata. Most of the confusion was linked to a lack of knowledge of the type material of S. pinnata as can be seen in Krammer and Lange-Bertalot (1991, plate 133) that shows the complex of species related to Fragilaria pinnata and F. leptostauron. The former proved to belong to the genus Denticula (Morales et al. 2013) resulting in the description of a new species, S. neopinnata to accommodate some of the populations, formerly identified as S. pinnata. Since our knowledge of the morphology and taxonomic identity of this seemingly widespread species improved significantly, a lot of species, previously included within S. pinnata, have been described, several of them from Europe, such as Staurosirella coutelasiana Van de Vijver, S. minutissima Van de Vijver, S. eruciformis Van de Vijver and S. lucectoriana Beauger, C.E.Wetzel & Van de Vijver (Van de Vijver 2022, 2023; Beauger et al. 2023).

During a survey of Staurosirella populations in European rivers in the framework of a routine water quality biomonitoring exercise, a large number of unknown taxa has been recorded that formerly were identified as S. pinnata s.l. Detailed morphological analysis of the different populations indicated several morphological differences with the type population of S. neopinnata (as the species should be called since 2019). Comparison with all available (recent) literature, exposed that several of these populations showed sufficient morphological differences to justify their description as new species despite a growing number of described Staurosirella species from all continents (see Material and Methods for a complete overview of all used literature). The present contribution describes five new species based on detailed light (LM) and scanning electron (SEM) microscopy and comparisons with all known taxa worldwide: Staurosirella binodiformis Van de Vijver, sp. nov., S. marginostriata Van de Vijver & V.Peeters, sp. nov., S. stoksiana Van de Vijver, sp. nov., S. jonssoniana Van de Vijver & Iris Hansen, sp. nov., and S. vanheurckiana Van de Vijver, Ballings & M.de Haan, sp. nov. Information on their ecological preferences is derived from the associated diatom flora and, when available, measured physico-chemical parameters.

In the present paper, a mixture of historic (herbarium) materials and recently collected samples have been investigated to detail the morphological features of the new Staurosirella species. The following samples have been used in this investigation:

• Sample APM21-91, Bosbeek (Maaseik, Province of Limburg, Belgium), leg. Vlaamse Milieu maatschappij (VMM), 51°5.6348'N, 5°45.894'E, coll. date 25 Jun. 2021.
• Sample Foged 29/1954, outflow from a small lake near Þingvellir, Iceland, coll. date 15 Jul. 1954, leg. Niels Foged (original material kept in C!)
• Sample 81a, Voorste Nete (Dessel, Province of Antwerp, Belgium), 51°13.9482'N, 5°7.4497'E, coll. date 06 Jul. 1994, leg. B. Van de Vijver.
• Sampling site at Græntorfa, Grenlækur, southern, Iceland, 63°43.955'N, 17° 58.067'W, coll. date 03 Jul. 2017, leg. Iris Hansen.
• Types du Synopsis sample 315, Leuven, Belgium, Van Heurck exsiccata set, leg. (probably) Père Gautier

A sub-sample of each of the selected materials was prepared for LM and SEM observations following the method described in van der Werff (1955). Small parts of the sub-samples were cleaned by adding 37% H₂O₂ and heating to 80 °C for about 1 h, after which the reaction was completed by addition of saturated KMnO₄. Following digestion and centrifugation (three times 10 minutes at 3700× rpm), the resulting cleaned diatom material was diluted with distilled water to avoid excessive concentrations of diatom valves on the slide and mounted on permanent slide using Naphrax (refraction index 1.73). The resulting slides were analysed using an Olympus BX53 microscope at 1000× magnification (UPlan FL N 100× oil objective, N.A. 1.30), equipped with Differential Interference Contrast (Nomarski) optics and the Olympus UC30 Imaging System, connected to the Cell Sense Standard program. For each taxon, the number of specimens, measured at random on the type slide, is indicated (n=X). For each species, at least 15, but often many more, valves are illustrated using LM to determine its morphological variability. An ecological characterisation of the new species is added based on the accompanying diatom flora, assessed by counting at least 100 diatom valves along random transects. Relative abundances, when given, are expressed as percentage of counted valves.

For SEM, part of the suspension was filtered through 5-μm Isopore™ polycarbonate membrane filters (Merck Millipore), pieces of which were fixed on aluminum stubs after air–drying and coated with a platinum layer of 20 nm, and studied using a JEOL-JSM-7100F field emission scanning electron microscope at 2 kV. Slides, samples and stubs are stored at the BR-collection (Meise Botanic Garden, Belgium). Plates were prepared using Photoshop CS5.

Terminology used in the description of the various structures of the siliceous cell wall is based on Ross et al. (1979, areola structure), Cox and Ross (1981, stria structure), Morales (2005, girdle structure), Williams and Round (1988, Staurosirella genus features) and Morales and Manoylov (2006a,b, Staurosirella genus features). For taxonomic comparisons, the following papers were consulted: Lange-Bertalot (1993), Morales (2005), Morales and Manoylov (2006a, 2006b), Morales et al. (2010a, 2010b, 2015, 2019), Almeida et al. (2015), Guerrero et al. (2019), Osório et al. (2021), Seeligmann et al. (2018), Van de Vijver et al. (2014), Van de Vijver (2022, 2023), and Beauger et al. (2023).

For typification of the species, we chose to use the entire slide as the type, following article 8.2 of the International Code of Nomenclature for algae, fungi, and plants (Turland et al. 2018). Diatoms show a broad variability along their cell cycle making the choice for the entire population on the slide more obvious, but because of admixtures, one valve was indicated to best illustrate the taxon (see Figures). All novelties are registered proactively according to Art. 42.3 (Turland et al. 2018).

The combination of morphological features places all new species in the genus Staurosirella. The striae are composed of linear, apically aligned areolae, separated by thin vimines. Broad, often well-raised virgae separate the striae. Rimoportulae are absent. Apical pore field present, usually similar in shape and size on both apices. These features fit well with the description of the genus Staurosirella in Williams and Round (1988) and Morales and Manoylov (2006a).

Each of the new species can be distinguished from similar small-celled araphid taxa worldwide. Tables 1–4 highlight the different features of similar species.

Staurosirella marginostriata shows most resemblance to both S. canariensis (Lange-Bertalot) E.Morales et al. and S. krammeri E.Morales et al. (Table 1). There are hardly any Staurosirella species with short and only marginal striae (Morales et al. 2010b). Staurosirella krammeri, described from Oregon, USA, has an almost similar valve outline and comparable valve dimensions, but lacks apical pore fields often replaced by a marginal stria and has typically only one large, acute to spatulate spine per virga (Morales et al. 2010b, figs 33–35). Contrarily, S. marginostriata shows well-developed apical pore fields and irregular series of small marginal spines located on the virgae. Staurosirella canariensis is much smaller (length up to maximum 7 µm), has a more elliptical, broadly rounded valve outline, and possesses small apical pore fields composed of only a few pores (Lange-Bertalot 1993). In LM, probably most confusion can be caused by Pseudostaurosira brevistriata, despite the latter belonging to a completely different genus, which becomes clear when comparing the ultrastructure of both species. Pseudostaurosira brevistriata forms long, chain-like colonies using well-developed linking spines, and possesses striae composed of one, transapically elongated areola. In LM, however, the marginal striae and the similar valve outline may result in an incorrect identification. Careful observation of the striae shows that they are more vaguely elongated in S. marginostriata, continuing into well-developed ghost striae whereas in P. brevistriata, the striae appear to be composed of one large, rounded, well-delimited areola (Morales et al. 2015).

Staurosirella binodiformis shows some resemblance to Staurosira binodis Ehrenberg, Staurosirella confusa E.Morales, and Staurosirella oldenburgiana (Hustedt) E.Morales (Table 2). The type material of S. binodis from Santa Fiore, Italy has recently been investigated (Van de Vijver, unpublished results). Staurosira binodis presents the typical structure of the genus Staurosira: frustules forming long, ribbon-like colonies, each frustule connected to the next using large, spathulate linking spines; large apical pore fields composed of numerous rows of small, rounded pores, long striae composed of rounded, small areolae and numerous girdle bands. In LM, the valve outline, linear with a clear central constriction, may cause some confusion with S. binodiformis, but the lack of colonies and the more narrow valves (max. 3 µm vs 4–6 µm in S. binodis) of the latter, limit the confusion. The most similar species appears to be S. oldenburgiana, a species described by Hustedt in 1959 from the Oldenburg Canal (Lower Saxony, Germany), although the valves are a little bit broader (up to 4 µm), with more elongated, narrower apices. The most distinct difference is the number, shape and structure of the marginal spines (Hustedt 1959; Morales 2005). Whereas S. binodiformis has a continuous series of very small, blunt spines, usually located on each side of the striae, S. oldenburgiana possesses only one, long, acute marginal spine located in the middle of the virgae (Morales 2005, Van de Vijver unpubl. res.). Hustedt (1959, p. 29) most likely had observed this taxon already as he mentioned in the discussion of his new taxon Fragilaria oldenburgiana, “Ähnliche Erscheinungsformen in Island [Similar forms in Iceland]”. Foged (1974, plate 3, fig. 3) illustrated one valve as Fragilaria construens var. binodis (Ehrenberg) Grunow, but the depicted valve clearly shows S. binodis and not the new Staurosirella binodiformis. Staurosirella confusa, finally, has a more lanceolate valve outline with usually convex margins and elongated, narrow rostrate apices, lacking in S. binodiformis. In Turkiye, a similar taxon was observed showing narrower valves (valve width max. 2.5 µm) giving that taxon a more slender, smaller outlook (C. Solak, pers. comm.). It is unclear whether those Turkish populations also belong to S. binodiformis. They were observed in a typical karstic environment, different from the Icelandic habitat.

Staurosirella stoksiana bears some similarity to a large number of European and North-American Staurosirella species (Table 3). The most similar species include S. ovata E.Morales and S. martyi (Héribaud) E.Morales & Manoylov, not in the least because of their heteropolar, often ovoid valve outline. Both, however, entirely lack spines, and have a more heteropolar valve outline with a clear difference between the very broadly rounded headpole and the more acutely rounded footpole (Morales and Manoylov 2006b, Van de Vijver et al. in press). Staurosirella martyi (syn. Staurosirella dubia (Grunow) E.Morales & Manoylov) usually has larger valves with a valve width up to 10 µm, whereas in S. stoksiana, the valve width rarely exceeds 5.5 µm. Larger valves of S. martyi also show a clear constriction just below the headpole, a feature lacking in S. stoksiana. Staurosirella neopinnata E.Morales et al. has typically parallel margins, isopolar valves, smaller apical pore fields and more regularly placed marginal spines (Morales et al. 2019) and S. coutelasiana Van de Vijver has longer, more slender valves, a less heteropolar valve outline, and a higher stria density (9–11 vs 8–9 in 10 µm (Van de Vijver 2022). The newly described Staurosirella vanheurckiana differs in a lower valve width (3.0–3.5 µm), a slightly wider sternum, a more clear heteropolar, ovoid valve outline with a broadly rounded head pole throughout its entire cell diminution series, the complete absence of marginal spines despite the analysis of a large population, and a different apical pore field structure with parallel rows of areolae along the apical axis (contrary to the transapical rows in S. stoksiana).

There are a few Staurosirella species that show some similarity to S. jonssoniana, based on the lanceolate valve outline with shortly protracted, rostrate apices (Table 4). The most similar is S. acidophila Almeida et al., described from southeastern Brazil. The species has a more elliptical-lanceolate central part with clearly convex margins and well protracted, elongated rostrate apices. The valves are always larger (width 4.5–7 µm vs 3.0–3.5 µm), have distinct apical pore fields (contrary to S. jonssoniana lacking apical pore fields), and with a different spine density (at least 2 per virga) and structure (lack of the pit-like depressions) (Almeida et al. 2015). Staurosirella confusa is also slightly larger (3.5–4.5 µm) with larger apical pore fields and at least 2 spines per virgae, each not located in a depression (Morales 2005). There are hardly any Staurosirella species lacking an apical pore field and most of these form long colonies such as S. mutabilis (W.Smith) E.Morales & Van de Vijver and S. lapponica (Grunow) D.M.Williams & Round (Van de Vijver et al. 2022). In LM, confusion may also arise with members of the genus Punctastriata D.M.Williams & Round, described in 1988 (Williams and Round 1988). Especially P. linearis D.M.Williams & Round has a more or less similar valve outline, although this species is always typically heteropolar with a more broadly rounded head pole and a more acute foot pole. The ultrastructure is entirely different with the typical Punctastriata feature of striae composed of several rows of small, rounded areolae. Punctastriata linearis also possesses one apical pore field on the foot pole and a clear depressed head pole, although these features are difficult to see in LM (Wetzel and Ector 2021). The larger Staurosirella subcapitata E.Morales differs in the presence of apical pore fields and a lower stria density (7–9 in 10 µm vs 12–13 in 10 µm) (Morales and Manoylov 2006b).

Finally, Staurosirella vanheurckii shows some resemblance to S. stoksiana but as highlighted here above, both can be separated based on valve outline, valve width, absence/presence of marginal spines and the structure of the apical pore fields (see Table 3). Staurosirella ovata, also lacking spines, is much wider (3.5–7.0 µm, i.e. double the valve width of S. vanheurckiana) and has a different apical pore field structure (Morales and Manoylov 2006b). The same applies for S. martyi that has a different valve outline, especially in the longer valves and a higher valve width (5–10 µm). Both S. ovata (6–9 in 10 µm) and S. martyi (6–7 in 10 µm) have a lower stria density compared to S. vanheurckiana (8–10 in 10 µm).

The description of these five new European Staurosirella species is the result of the continuous revision of the genus Staurosirella following the careful analysis of the historic type material. Catch-all species such as S. pinnata (now S. neopinnata), S. mutabilis and S. martyi revealed a much higher diversity in the genus Staurosirella than previously understood. It is clear that more species will be discovered in the near future, when more populations, in the past identified as S. (neo) pinnata will be investigated. Morphological criteria such as valve outline and valve dimensions need to be completed with a better analysis of other structures such as the shape, size and structure of the apical pore fields, the structure and density of the marginal spines, the presence (or absence) of a head pole depression and the structure of the striae in combination with the virgae.

Dr Christian Lange and Prof. Dr Nina Lundholm are thanked for their help in the Foged collection for retrieving the original material for Foged sample 29 from Iceland. The Vlaamse Milieu Maatschappij is thanked for granting the permission to use one of their samples (APM21-91, Bosbeek, Belgium).