Cavinula scutelloides (W. Smith) Lange-Bertalot

Cvetkoska, Aleksandra, Levkov, Zlatko, Hamilton, Paul B. & Potapova, Marina, 2014, The biogeographic distribution of Cavinula (Bacillariophyceae) in North America with the descriptions of two new species, Phytotaxa 184 (4), pp. 181-207 : 196

Cavinula scutelloides (W. Smith) Lange-Bertalot ( Figs 101–106 , 165–171 )

Basionym: Navicula scutelloides W. Smith 1856 (in Synopsis of British Diatomaceae, Vol. 2, 107, figs: 32–60, 61–62)

Valves elliptical, to round in smaller specimens with widely rounded apices. Valve length 12–26 µm, width 9.5–20 µm. Striae radiate, 8–10 in 10 µm, uniseriate with coarse elliptic to rounded areolae. Areolae internally occluded with hymen. Central area can be transversely expanded on one side. A few short striae present around valve center; all striae continue on mantle. Axial area narrow, linear-lanceolate. Raphe system linear; externally, proximal raphe features tear-drop like on valve face. Distal raphe fissures bent in the same direction, question mark-like, terminating on the valve mantle. Internally, raphe positioned on elevated sternum. Proximal raphe features curved in same direction, hook-like, and distal raphe features terminate with helictoglossae.

Type:––Omersby, Norfolk, United Kingdom. Here presented in Figs 103, 104 (type material) .

Observations:— This taxon is identified ( Figs 103, 104 from type material) based on shape, size and the enlarged areolae on the valve face. The closest taxon to compare with would be C. pseudoscutiformis , however they are separated by shape of proximal raphe fissures, length of alternating striae around the central area, and areolae size, and orientation. In addition, C. scutelloides and C. pseudoscutiformis are found under different water acidity conditions.

Distribution:— Currently this taxon is recognized as a cosmopolitan species across the temperate zone of the northern hemisphere, and relatively absent from the northern tundra with the possible exception of a small deep lake from northern Alaska ( Foged 1971). C. scutelloides was identified in 26 samples in the CANA database. According to those identifications, this taxon is present in the central part of North America e.g. the province of British Columbia, Ontario, Quebéc ( Canada) and New York and Massachusetts ( USA). The absence of C. scutiformis in the slides and samples in which they were reported, led us to the conclusion that specimens of C. scutelloides were often identified as C. scutiformis . Accordingly, C. scutelloides was verified from the following localities: Low Water Lake (Sudbury District, Ontario), Singleton Lake (southern Ontario), Flower Round Lake (southern Ontario), Hawkins Lake (Interior Douglas Fir, British Columbia) and Sneezie Lake (Cariboo Aspen-Lodgepole pine, British Columbia), Canada. In the northern United States verified specimens were observed from Walden Pond Massachusetts and Queer Lake (Adirondack Park, New York). Searches in the data from ANSP, indicated that C. scutelloides was identified from central North America in the states of Wisconsin, Maine, Georgia, Indiana, Iowa, Connecticut, Michigan, Illinois, Idaho and Minnesota.

C. scutelloides is absent in low pH water bodies, found from Florida to the high Arctic. However, in circumneutral to alkaline environment it was often observed. In western Canada, C. scutelloides was found to occur in lakes with a TP range of 7–39 µg /L ( Cumming et al. 1995).

Distribution Records:— Krammer & Lange-Bertalot (1986, fig. 59: 16–19, Europe), Levkov et al. (2007, fig. 68: 1–4, fig. 68: 6, fig. 69: 5, Europe), Watanabe (2005, fig: II 2B 3 -18: 16, 17, Japan), Foged (1971, fig. 15: 21, 22, Alaska).

Figures

FIGURES 101–131. LM micrographs. Figs 101–106. Cavinula scutelloides (W. Smith) Lange-Bertalot. Figs 107–111. Cavinula scutiformis (Grunow ex A. Schmidt) Mann & Stickle. Figs 112–122. Cavinula hilliardii Manguin ex Kociolek & Reviers. Figs 123,124. Cavinula variostriata (Krasske) Mann in Round, Crawford & Mann. Figs 125, 126. Cavinula lapidosa (Krasske) Mann. Figs 127–131. Cavinula pusio (Cleve) H. Lange-Bertalot in Werum & Lange-Bertalot. Figs 103, 104. W. Smith, Omersby, 1853 (type slide). Figs 126, 128. BRM N14/87a (Krasske material near Hessen). Scale bar: 10 µm.

FIGURES 165–171. SEM micrographs of Cavinula scutelloides (W. Smith) Lange-Bertalot. Figs 165–168. External valve view. Figs 169–171. Internal valve view. Figs 165–171. Hawkins L., Interior Douglas Fir, British Columbia, Canada. Scale bars: 5 µm in Figs 165–167, 169, 170; 2 µm in Figs 168, 171.