Nupela tenuicephala Lange-Bertalot

Nupela tenuicephala (Hustedt) Lange-Bertalot (Figs 10 BK-BY; 15 A-F)

Bibliotheca Diatomologica 27: 157 (Lange-Bertalot 1993). — Navicula tenuicephala Hustedt, Archiv für Hydrobiologie 39 (1): 113 (Hustedt 1942). — Navicula tridentula var. tenuicephala Cleve-Euler, Kongliga Svenska Vetenskaps-Akademiens Handligar, ser. 4, 4 (5): 190 (Cleve-Euler 1953).

Nupela paludigena (Scherer) Lange-Bertalot, Bibliotheca Diatomologica 27: 158 (Lange-Bertalot 1993). — Anomoeoneis paludigena Scherer, Diatom Research 3 (1): 149 (Scherer 1988).

ECOLOGY AND DISTRIBUTION. — Nupela tenuicephala was found in 14 lakes from Tursujuq National Park. It was not abundant, with a mean relative abundance of 1.1%, although lakes 16-H and 18-P had populations reaching 38.6 and 9.6% relative abundance. While there were no evident distribution patterns linked to water chemistry, this species was more prominent in low conductivity and acidic waters with a pH optimum of 5.9 (Appendix 3), and associated with low DIC and Mg (Appendices 4; 5). Nupela tenuicephala is common across eastern North America in acidic waters according to Siver & Hamilton (2011) (as N. paludigena), Fallu et al. (2000) (as N. tenuicephala), and Camburn & Charles (2000) (as Anomoeoneis paludigena).

The frustules are rectangular and narrow in girdle view. The valves exhibit an asymmetric shape, being linear on the primary side and linear-elliptic on the secondary side, with constricted rostrate to capitate ends (Fig. 15A, B, C). The valve dimensions, based on a sample size of 25, range from a length of 9.5 to 14 µm and a width of 2.5 to 3.5 µm. The stria density is 54-55 in 10 µm. Externally, the valve face is flat, and the margin abruptly bends (not curves) towards the mantle. The axial area is linear to lanceolate and narrow. The central area is asymmetric, being elliptic on the primary side with four marginal striae, and it exhibits a slightly elevated fascia on the secondary side (Fig. 15A, C, D). The raphe is linear and curved, without any kink-like irregularity halfway between the mid-valve and apex. Externally, the central raphe fissures are linearly expanded with teardrop rounded ends (Fig. 15D). The terminal raphe fissures curve and bend, extending down to the lower mantle. Internally, the raphe is located on the sternum and appears straight. The proximal raphe fissures are small and hooked to the secondary side (Fig. 15E), while the terminal fissures end on small elevated helictoglossae, isolated from the apex mantle (Fig. 15F). The striae are parallel to weakly radiate close to the mid-valve, and they become parallel at the ends. On the valve face, the striae are straight to weakly arched and interrupted at the valve edge. Voigt faults on the secondary side of the valve are indistinct, located at approximately 2/3 of the distance between the mid-valve and apex. The striae are interrupted by the raphe at the apices. Areolae are round to rectangular, with five to six per stria on the secondary side and three to four on the primary side. They are covered with a poroid velum that exhibits fine circular rows of small pores.

This taxon was originally described by Hustedt as a Navicula species from Lapland, Abisko, Sweden. There were three slides with this taxon named by Simonsen (1987: 232) with one slide selected (P2/57, Lapland 189, Abisko, Tümpel) as the lectotype. The LM images of Simonsen and specimens from Bahls & Potapova (2015) match our specimens. Scherer (1988) also described a similar species (Anomoeoneis paludigena) from Georgia (United States), which was transferred to Nupela by Lange-Bertalot (1993) and later emended by Siver et al. (2007). At this time, N. tenuicephala is in synonymy with N. paludigena. See Siver & Hamilton (2011) for a more thorough discussion about N. paludigena. Nupela tenuicephala is also similar to N. giluwensis Vyverman & Compère, but is distinguished by the differences in outline, striae density and formation of the central area. Siver & Hamilton (2011) suggest there is little difference between N. tenuicephala (synonym: N. paludigena) and N. giluwensis and they propose a more thorough comparison between these taxa. Another similar species is N. subinvicta (Krasske) Lange-Bertalot observed from Chile, but this taxon is different from N. tenuicephala with respect to valve symmetry, the large capitate apices and a broad central area.