Cavinula cocconeiformis f. elliptica (Hustedt) 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 : 185-187

Cavinula cocconeiformis f. elliptica (Hustedt) Lange-Bertalot ( Figs 32–43 , 140–145 )

Basionym: Navicula cocconeiformis f. elliptica Hustedt (1954: 470)

Valves lanceolate, rhombic-lanceolate to elliptic in smaller specimens with rounded apices. Valve length 15–24 (34) µm, width 8–11 (11.5) µm. Striae radiate, with fine elliptical to rounded areolae, extending on valve mantle, 22–24 in 10 µm. Areolae not discernible in LM. Axial area linear-lanceolate, transforming into a small not clearly defined, transapically elongated, elliptic central area. Striae around central area alternate, long and short. Raphe distinct, central on valve. Externally, proximal raphe features tear-drop like pores. Distal raphe fissures terminate on valve face with small apertures bent in opposite directions at each apex. Internally, raphe on elevated sternum; proximal and distal raphe fissures terminate with helictoglossa. Copulae open bands, with one row of pores.

Type: –– Weinfelder Maar , Eifel, Germany; Simonsen, 1987, fig. 595: 38, 39 (holotype). Here presented in Fig. 34 from type slide BRM MA3 /39.

Observations:–– According to Simonsen (1987), Hustedt states that it is impossible to separate this form from C. cocconeiformis sensu stricto because this elliptical valve grades into the large rhombic-elliptical nominate form ( Fig. 34 ). Simonsen (1987) further suggested that Hustedt really considered this taxon to be a part of C. cocconeiformis . Lange- Bertalot (in Lange-Bertalot & Metzeltin 1996) transferred this taxon from Navicula cocconeiformis f. elliptica to C. cocconeiformis f. elliptica , however in the plates (op.cit. fig. 24: 10–12) it was proposed that Navicula cocconeiformis f. elliptica is a synonym of Cavinula mollicula , which was described in Hustedt (1945, fig. 41: 34–36) from Lake Prespa. The problem was in the selection of a lectotype for C. mollicula . Simonsen (1987, fig. 507: 36–37) selected an elliptical valve with fine striae and narrow linear central area as the lectotype while the isolectotypes specimens ( Simonsen 1987, fig. 507: 38–43) are more lanceolate with coarser striae and narrow central area formed by regular shortening of central striae. This problem is also illustrated in Levkov et al. 2007 (fig. 66: 45–47). Since Hustedt identified the holotype for C. cocconeiformis f. elliptica , while Simonsen selected the lectotype for C. mollicula , we agree with Lange-Bertalot that the epithet C. cocconeiformis f. elliptica should stand until a better circumscription of C. mollicula is completed.

Distribution:–– During the observation of a selected number of permanent slides from the Diatom Collection, deposited in the Canadian Museum of Nature, the presence of specimens we recognize as C. cocconeiformis f. elliptica was confirmed in a number of slides from different localities across Canada: Moot Lake (sediment core from the deepest part of the lake), Muskoka District and Wawashkesh Lake and Lake Abitibi in the province of Ontario, Lac aux Araignees and Madawaska in the province of Québec, Hilltout Lake, Cariboo Aspen-Lodgepole pine (British Columbia), the localities YUK021 and YUK045 (Yukon Territory) and in Yellowknife-Contwoyto Lake locality, Mackenzie District in the Northwest Territories and Baffin Island (Nunavut). C. cocconeiformis f. elliptica sensu lato was confirmed further south, in the United States, in samples from Delaware and South Carolina (deposited in the Academy of Natural Sciences in Philadelphia). Although, the taxon presented in those samples was similar to C. cocconeiformis f. elliptica , it could also be C. mollicula . A proper identification was difficult and uncertain and thus we cannot confirm definitely the presence of this taxon in the USA.

The estimated mean conductivity for the samples in which C. cocconeiformis f. elliptica was identified varies from 8 µS/cm (Northwest Territories) to 150.7 µS/cm (Québec), the mean pH range is between 5.6 and 8.2 (Ontario), the total mean nitrate concentration was 299 µg/L and the estimated mean total phosphorus was 18 µg/ L. The wide range in ecological data strongly suggests that the taxonomy of this valve form is still a concern .

Distribution Records:— Simonsen (1987, fig. 595: 38, 39, Europe), Lange-Bertalot & Metzeltin (1996, fig. 83: 17–20, fig. 104: 17–19, Europe).

Figures

FIGURES 2–43. LM micrographs. Figs 2–31. Cavinula cocconeiformis (Gregory ex Greville) Mann & Stickle. Figs 2–16. Cavinula cocconeiformis (Morphotype 2). Figs 17–31. Cavinula cocconeiformis (Morphotype 1). Figs 32–43. Cavinula cocconeiformis f. elliptica (Hustedt) Lange-Bertalot. Figs 28, 29. BM 689 (lectotype slide). Fig. 34. BRM MA3/39 (holotype slide). Scale bar: 10 µm.

FIGURES 140–145. SEM micrographs of Cavinula cocconeiformis f.elliptica (Hustedt) Lange-Bertalot.Figs 140, 142, 144, 145. External valve view. Figs. 141, 143. Internal valve view. Figs 140–145. Weinfelder Maar, Eifel, Rheinland-Pfalz, Germany (type material). Scale bars: 2 µm in Figs 143–145; 5 µm in Figs 140–142.