Cavinula pseudoscutiformis Mann & Stickle

Cavinula pseudoscutiformis (Hustedt) Mann & Stickle (Figs 44–73, 152–164) Basionym: Navicula pseudoscutiformis Hustedt 1930, p. 291, fig. 495 Valves rounded to slightly elliptical in larger specimens, with broadly rounded ends, not differentiated from valve body. Valve face flat, curving abruptly into a shallow mantle. Valve length 4.5–13 (17) µm, width 4–11 (14) µm. Axial area narrow, almost linear, transforming into a small elliptical, transapically elongated and unilaterally slightly expanded central area. Striae around central area alternate, short and long. Internally, axial area with slightly thickened lanceolate ridge. Raphe filiform; proximal and distal raphe endings slightly deflected in the same direction on the valve face. Central raphe endings larger than distal ends with tear-drop like central pores encircled with silica ridge. Deflected raphe endings, especially in smaller specimens, not clearly discernable in LM. Striae uniseriate, strongly radiate throughout, 21–24 in 10 µm. Each stria with fine small, rounded to somewhat transversely elongated areolae towards valve margins. Internally, areolae rows positioned between costae and each areola occluded with a hymen. In older material, hymenes often dissolved leaving unoccluded areolae. Type: –– Madebrökensee, Plön, Germany. Simonsen, 1987, fig. 194: 11, 12 (holotype). Observations:— According to Hustedt’s (1930) original description, this taxon has broadly rounded to elliptical valves with a valve length range between 9–15 µm, width range, 7–13 µm and stria density of 24 in 10 µm. In an investigation of Hustedt’s type material (Mal/97; Madebrökensee, Plön, Germany) Antoniades et al. (2009) found one valve in SEM matching the Holotype specimen designated by Hustedt. Cavinula vincentii Antoniades & Hamilton is distinguished from C. pseudoscutiformis by its more elliptic valve and the presence of an elongated pore-like fissure at both valve ends (not “s-shaped as in C. pseudoscutiformis).Valves reported by Snoeijs & Potapova (1995) from Zelenogorsk, Gulf of Finland, Russia, e.g. Length = 10–18 µm with an intermediate length of 13.7 µm and width = 8–14 µm, with an intermediate width of 10.7 appear to belong to C. vincentii. Cavinula scutiformis Grunow is distinguished by its larger valve size, more linearelliptical shape, denser striae, denser areolae and the differentiation of linear areolae along the axial area (see Krammer & Lange-Bertalot 1986, fig. 59: 10, 11). Cavinula pseudoscutiformis sensu stricto weakly resembles Navicula rotunda Hustedt (= Eolimna rotunda (Hustedt) Lange-Bertalot et al., see Levkov et al. 2007, fig. 66:1–13, fig. 67: 1–2) and Cavinula mollicula (Hustedt) Lange-Bertalot (sensu Lange-Bertalot & Metzeltin 1996, fig. 114: 5). C. pseudoscutiformis can be differentiated by the more spherical valve outline, the morphology of the raphe, mid-range stria density (not 20/10 µm or 32/10 µm), lower areola density and termination of the oppositely curved distal raphe ends on the valve face. Krammer & Lange-Bertalot (1986) reported a higher range of valve dimensions for C. pseudoscutiformis (L = 3.25–25 µm, W = 3–17 µm) and stria density between 20 and 26 in 10 µm. According to their description, C. pseudoscutiformis is a cosmopolitan taxon, present in North America, North and Eastern Europe, more often found in oligo to β-mesosaprobic waters. In the Canadian Archipelago, Antoniades et al. (2008) document valves with lengths between 8–18 µm, width 7–11 µm, stria density between 21–26 µm in 10 µm and 25–32 areolae in 10 µm. Morphological variability:–– During our observations we found three morphotypes of C. pseudoscutiformis. The first observed valve-group resembles the specimens with an elliptical valve outline and the following features: valve length 7.5–17 µm, width 5.5–14 µm, number of striae 20–22 in 10 µm (Figs 44–53). The second valve-group has a rounded valve outline with lanceolate axial area, punctate striae and the following features: valve length 4.5–12 (15) µm, width 4–10.5 (13) µm and number of striae 18–22 in 10 µm (Figs 54–58). During this study the same group characteristics observed from different provinces in Canada, were also found further south at Kernville, California, USA (Figs 59–63). The third valve-group contains specimens found in Nunavut, The Northwest Territories and Ontario, Canada (Figs 64–73). Valves have a rounded outline with broadly rounded ends, narrow linear axial area and a higher areola density. Valve length 5–9 µm and width 5–9 µm. Striae strongly radiate throughout the valve length, 20–22/10 µm. Axial area narrow, linear, not transforming into a defined central area. Raphe linear, filiform, the proximal and the distal raphe endings slightly deflected on the same side. The silica ridge that encircles the raphe in the previous valvegroup seems to be absent in the specimens belonging to this group. Based on these findings and the additional SEM observation of the different valve-groups, it is remarkable that although the first two groups exhibit a slight difference only in the valve outline, this character is not sufficient to separate the populations into different taxa. However, since the third valve-group differs in the valve outline, the morphology of the axial area, central area and the morphology of the raphe, we here suggest that these specimens represent a different morphotype of C. pseudoscutiformis. Distribution:— In the CANA collection, 189 localities for C. pseudoscutiformis have been identified across the Canada (excluding the provinces of Saskatchewan, Manitoba). C. pseudoscutiformis in Canada was low in abundance, typically lower than 5%, except at three localities: Baffin Island (9 %), Ellesmere Island (35%) and Ellef Ringnes Island (18%), all in the Arctic Archipelago. In the United States, C. pseudoscutiformis was verified across the north from Montana to Cape Cod and in the south from Florida to California. This taxon never exceeded 2.5% abundance from USA sites. In the CANA collection, its presence was confirmed in the states of New York, Florida and Massachusetts, but with abundances lower than 1.5 %. In the ANSP diatom collection, C. pseudoscutiformis was verified in the states of California and Georgia. However, a more detailed investigation is needed to verify the exact distribution of this taxon across the United States. C. pseudoscutiformis has a wide tolerance range for pH (6.3–8.0) and specific conductance (13–212 μS/cm), although preferring a more limited range of 6.5–7.1 in pH (Antoniades et al. 2008; Camburn & Charles 2000, Siver et al. 2005). This is an oligotrophic to weakly mesotrophic taxon occurring across central and northern North America (TP optima 8–12 μg/L) and prefers a moderate range in DOC (1.2–3.0 mg/L). For example, Caballero-Miranda (1996), reported C. pseudoscutiformis to be the most abundant taxon in the diatom assemblages from Lago del Sol, an acidic lake located in Nevado de Toluca (4 620 m asl.), Central Mexico and reported the following physical-chemical characteristics of the lake: pH=5.9, electric conductivity = 24.6 µS/cm, and total dissolved solids of 21.5 mg /L. Distribution Records:— Simonsen (1987, pg. 120, fig. 194: 11–12 = holotype, Germany), Antoniades et al. (2009, fig. 50: 5, 6; fig. 107: 1–3, Arctic Archipelago), Siver et al. (2005, fig. 38: 6–10, Massachusetts), Loseva (1982, fig. 66: 4a, 7a, 7b, Russia), Krammer & Lange-Bertalot (1986, fig. 457; fig. 8: 8, Europe), Metzeltin & Lange-Bertalot (1998, fig. 96: 6–9, Venezuela), Metzeltin & Witkowski (1996, fig. 2: 45, 46, Bear Island, Svalbard), Foged (1977, 26: 12, Ireland), Foged (1979, fig 30: 29, New Zealand), Foged (1981, fig. 30: 15, 16, Alaska), Watanabe (2005, fig. II B3 -18: 10–15, Japan), Potapova (2014, fig. 213, Bering Island, Kamchatka Peninsula, Russia).

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