Stephanodiscus minutulus ( Kuetz .) Cleve & J.D. Moeller , Collect. Diat.: 300. 1882.

Bilous, Olena P., Genkal, Sergey I., Zimmermann, Jonas, Kusber, Wolf-Henning & Jahn, Regine, 2021, Centric diatom diversity in the lower part of the Southern Bug river (Ukraine): the transitional zone at Mykolaiv city, PhytoKeys 178, pp. 31-69 : 31

Stephanodiscus minutulus ( Kuetz.) Cleve & J.D. Moeller, Collect. Diat.: 300. 1882.

Basionym.

Cyclotella minutula Kütz. Kieselschal. Bacill.: 50, pl. 2, fig. 3. 1844.

Synonyms.

S. astraea var. minutula ( Kütz.) Grunow, S. minutulus ( Kütz.) Round, S. parvus Stoermer & Håk., S. perforatus Genkal & Kuzmin, S. rotula var. minutulus ( Kütz.) R. Ross & P.A. Sims.

Morphological description.

The frustule is disciform, valves flat or with slightly convex or concave centre, 8.8-9.1 μm in diameter, striae double to triple, numbering 10 in 10 μm (Fig. 5G, H ). A central process is present. The spines are short, growing from each rib (Figs 5G, H , 6A ).

Ecology.

It is a planktonic taxon, described as eutraphentic ( Hofmann et al. 2018). Stephanodiscus minutulus reaches greatest abundance in productive nearshore regions, in the mouths of large rivers and coastal embayments. This taxon is an indicator of increased TP concentrations, alkaliphilic, mesosaprobic indicator ( Stoermer and Yang 1969; Bradbury et al. 2002; Reavie and Kireta 2015).

Distribution.

Stephanodiscus minutulus occurred at the Mykolaiv city and downstream the Southern Bug river sites (Table 1 ). This taxon is distributed over the entire territory of Ukraine, common for such rivers as the Dnipro with its reservoirs and tributaries, the rivers Dnister, Danube, Siverskyi Donets, estuaries of main rivers ( Tsarenko et al. 2009).

Concerning worldwide distribution, Stephanodiscus minutulus is a widespread taxon, recorded for Europe (i.e. Estonia, Finland, France, Germany, Great Britain, Hungary, Ireland, Moldova, Norway, Russia, Ukraine), Asia (Armenia, Georgia, Iran, Israel, Russia, Japan), North America (Canada, Greenland, USA) ( Tsarenko et al. 2009; Kiss et al. 2012; Kulikovskiy et al. 2013; Genkal et al. 2020).

Comments.

Houk et al. (2014) considered Stephanodiscus minutulus to be different from Stephanodiscus parvus (and Stoermer and Håkansson 1984) and noted the main difference between S. minutulus and S. parvus the convex-concave valve relief in contrast to flat valves, respectively. However, many authors have shown that in S. minutulus the valve relief varies from convex-concave to flat, and therefore S. parvus was treated as a synonym (see Genkal 2010), and we adhere to this point of view.

Figures

Figure 5. SEM images of species found in the transitional zone at Mykolaiv city (A, B, E-H valve views, C, D girdle views): A Minidiscus proschkinae with arrows showing rimoportula and marginal fultoportula B-D Skeletonema subsalsum with arrows showing marginal fultoportulae E Stephanodiscus hantzschii with arrow shown spines F Stephanodiscus makarovae with arrows showing marginal and central fultoportulae G, H Stephanodiscus minutulus with arrows showing spines and central fultoportula. Scale bars: 3 μm (A); 4 μm (B, F, H); 5 μm (C, D, G); 10 μm (E).

Figure 6. SEM (A-F, H) and LM images (G) of species found in the transitional zone at Mykolaiv city, valve views: A Stephanodiscus minutulus with arrows shown marginal and central fultoportulae B, C Cyclotella atomus var. atomus with arrows shown marginal and central fultoportulae D-F Cyclotella atomus var. gracilis with arrows shown marginal and central fultoportulae on D picture, only central fultoportula on E and central fultoportula and alveolae on (F) G Cyclotella choctawhatcheeana with arrow shown central fultoportulae H Cyclotella cryptica with arrows shown marginal and central fultoportulae. Scale bars: 3 μm (B, C); 4 μm (D, F); 5 μm (A, E, H); 10 μm (G).