Morphology of Reimeria ovata (Hust.) Levkov & Ector in comparison with similar Reimeria species

The precise morphological description of the R. ovata cell, containing light microscopy and SEM observations, was presented by Levkov and Ector in 2010. Cells taken from the studied samples and observed under SEM were very similar to those described in the work of Levkov and Ector (2010). The studied cells were narrower and slightly longer. The single cells are narrower, longer and have more striae per 10 µm than those described by Levkov and Ector.

Under a light microscope, valves of R. ovata were morphologically very similar to those of R. sinuata (Fig. 2a-u). However, most of the observed R. ovata cells were larger and wider in comparison with R. sinuata (Table 2). Small valves looked identical under a light microscope. Detailed SEM analysis has shown that all observed cells wider than 4.6 µm belong to R. ovata.Based on this, the authors distinguished both species under a light microscope (Fig. 2a-u). Although there was a difference in the width of R. ovata and R. sinuata cells, the width cannot be used as a distinguishing feature in the case of other populations. Therefore, precise distinction between these species was only possible using SEM. R. ovata has C-shaped areolae on the valve face and the striae are slightly depressed (Fig. 3d). In the case of R. sinuata, many researchers have stated that the areolae on the valve face and the valve mantle have small, round foramina which are alternately biseriate (Krammer, Lange-Bertalot 1986; Kociolek, Stoermer 1987; Sala et al. 1993; Levkov, Ector 2010). In our samples, valves of R. sinuata looked similar (Fig. 4a-c). The third examined species, R. uniseriata, is clearly distinguishable from the two described above (Fig. 2v-ad). Striae are uniseriate and extend onto the valve mantle. Areolae have C-shaped foramina. The interstriae are much wider than the striae (Fig. 4d-e) (Sala et al. 1993; Levkov, Ector 2010). According to Levkov and Ector (2010), R. ovata is very similar to R. fontinalis Levkov, but was not recorded in our material.

Figure 4

There is no detailed information on the occurrence of this species, because it was only recently distinguished from Reimeria sinuata (Cymbella sinuata). C. sinuata f. ovata was often considered a synonym of C. sinuata (Kociolek and Stoermer 1987). The species was initially described as Cymbella sinuata var. ovata by Hustedt (1922). The species was separated from Reimeria sinuata in 2010 by Levkov and Ector and described as R. ovata. In Poland, the species was often described as Cymbella sinuata f. ovata, and less commonly as Cymbella sinuata var. ovata (Siemińska 1964; Siemińska, Wołowski 2003).

According to Levkov and Ector (2010), R. ovata occurs often in oligotrophic cold waters, but never in large numbers. Our studies confirm this information. The streams, in which R. ovata was recorded in the course of our study, were characterized by alkaline or close to neutral pH, very high saturation of oxygen (always >10 mg l^-1) and a low nutrient content. As the study was conducted only in spring and autumn, the water temperature did not usually exceed 12°C. Electrolytic conductivity values at most sites were at moderate levels, except for autumn 2013 and 2015 when values exceeded 200 µS cm^-1 (Table 1). In the collected material we always observed only single cells. (Table 2). Achnanthidium pyrenaicum (Hust.) Kobayasi was the main dominant species at each site in all the studied streams and often exceeded 50% of the species composition of diatom assemblages (mainly in the Rzeczyca and Wołosaty streams). Achnanthidium minutissimum (Kütz.) Czarnecki var. minutissimum and Diatoma ehrenbergii Kütz were less numerous (up to 30% of the species composition). All taxa are common in Europe. Achnanthidium pyrenaicum and Diatoma ehrenbergii thrive best in the calcium-rich waters of mountain regions and uplands where they often occur in large numbers (Krammer, Lange-Bertalot 1986, 1991; Hofman et al. 2011).

Reimeria sinuata is a common species in Poland, occurring all over the country (Siemińska 1964; Rakowska 2001; Siemińska, Wołowski 2003; Wojtal, Sobczyk 2006; Wojtal 2009, 2013; Bąk et al. 2012; Kawecka 2012; Noga et al. 2014). It is a cosmopolitan species, widespread throughout Central Europe in different types of waters, particularly in alpine and subalpine areas (Krammer, Lange-Bertalot 1986; Hofmann et al. 2011). It is also common throughout the Czech Republic (Fránková et al. 2009). It also develops in aerial biotopes, mainly on wet rocks, among mosses (Ettl, Gärtner 1995) and very rarely on soils (Noga et al. 2014).

Reimeria uniseriata was noted in the Kobylanka stream (Kraków-Częstochowa Upland) in 2006. It was the first information on this species from Poland (Wojtal, Sobczyk 2006; Wojtal 2009). Currently, it is frequently found in south-eastern Poland where it develops individually in different types of flowing waters (Żelazna-Wieczorek 2012; Noga 2012; Pajączek et al. 2012; Noga et al. 2014, 2015). A precise morphological description was made by Sala et al. (1993) and Levkov & Ector (2010) based on observations under both a light microscope and an SEM, but still little is known about the ecology of this species. It is probably a cosmopolitan species, common in benthic habitats (Wojtal, Sobczyk 2006; Wojtal 2009; Potapowa 2010). According to Bąk et al. (2012), it grows in mesotrophic rivers and lakes. Considering the presence of R. uniseriata in the studied rivers and streams of SE Poland (Noga 2012; Pajączek 2012; Noga et al. 2013; 2014; 2015) as well as the Wołosaty stream and its tributaries, it can be concluded that the species has a wide range of occurrence in various types of flowing waters. It develops in alkaline or neutral waters (usually pH 7-8), oligo-, meso- and eutrophic waters, at the source or in the middle and lower sections, and always in the form of individual specimens (<0.5%). It was often found in mesotrophic waters, but also as single cells.

Reimeria ovata has not been previously reported as Cymbella sinuata f. ovata Hust., but the morphological description of this species presented by Siemińska (1964) (“small valves, short and wide, almost elliptical, 10-12 µm long and 6 µm wide”) does not correspond to the cell morphology recently described as Reimeria ovata. All specimens in this study were bigger than reported by Siemińska (1964).

The species was identified in each of the streams surveyed in this study, at all the selected sites: the Wołosatka, Wołosaty, Rzeczyca and Terebowiec streams. The species was always present as individual cells. The species was observed more frequently only in the Terebowiec stream, but with the maximum abundance of only 0.5% in the total species composition of diatom assemblages. At each study site, it was associated with Reimeria sinuata, which was also very rare (<1%). The third species, Reimeria uniseriata, occurred least often in the studied material and only single specimens were found on slides.

The studies carried out in the Wołosaty stream and its tributaries suggest that R. ovata could also develop in other oligotrophic rivers and streams, either in SE Europe or in other regions of the country. However, it could easily be confused with R. simuata, due to their similarity under a light microscope. Therefore, to properly determine whether the species is present or not in a given material, it is advisable to perform SEM observations in order to dispel doubts in the identification of R. sinuata and R. ovata.