Halamphora attenuata Stepanek & Kociolek, 2015

Kociolek, Patrick, 2015, Three new species of the diatom genus Halamphora (Bacillariophyta) from the prairie pothole lakes region of North Dakota, USA, Phytotaxa 197 (1), pp. 27-36 : 32-34

Halamphora attenuata Stepanek & Kociolek sp. nov. ( Figs 29–44 )

Valves are semi-lanceolate with a smoothly arched dorsal margin and a straight to slightly convex ventral margin. Valve length 20.0–33.0 μm, valve breadth 3.5–4.5 μm. The valve ends are protracted, narrowly rounded and slightly bent ventrally. The raphe is arched with nearly straight raphe branches. Proximal raphe ends terminate closely and are slightly deflected dorsally. The distal raphe ends are striae (arrows), and fused virgae at the dorsal valve center (thin arrow). Figs 43, 44 . Internal whole valve view. Scale bars = 1 μm.

difficult to see in the LM. The axial area is narrow throughout, expanding slightly near the ventral valve center. Dorsal striae are fine, 23–25 in 10 μm, and are finely areolate and weakly radiate throughout. In many specimens a broad raphe ledge can be seen in the LM, appearing as a ‘longitudinal line’ or ‘shadow’ running the length of the dorsal striae. Ventral striae number 26–29 in 10 μm.

A light siliceous thickening or ‘semi-stauros’ can be observed at the dorsal central area in some of the specimens. Depending on the angle at which the valve is lying, the ventral portion of the valve can appear broad or narrow.

In the SEM, externally, the raphe is slightly biarcuate with the proximal ends slightly deflected dorsally and the distal ends hooked past the raphe ledge. The dorsal raphe ledge is very broad ( Fig. 40 ), although many specimens exhibited significant dissolution of this feature.

The dorsal striae are crossed by many irregularly spaced vimines creating round to ovoid areolae of variable size and spacing. The initial axial row of areolae is elongate and biseriate in contrast to the single areolate structure of the rest of the striae ( Fig. 41 , arrow).

It appears that this biseriate row of areolae is typically covered by the dorsal raphe ledge, and can only be observed here due to the partial dissolution of the raphe ledge ( Figs 40, 41 ). The ventral striae are composed of a single row of elongate areolae. A marginal ridge is not apparent in this taxon and the areolate striae continue to the valve margin. Internally, the axial row of biseriate dorsal striae make up nearly half of the dorsal striae before being crossed by vimines ( Figs 42 arrows, 43, 44). The dorsal virgae near the valve center are slightly thickened and fused near the central nodule ( Figs 42 thin arrow, 43, 44).

Type: — USA. North Dakota, Kidder County: Salt Alkaline Lake , N46.95448°, W99.52382°, 3 November 2011, Holotype: Slide ANSP GC 65214 , Fig. 31 ; Isotypes: cleaned material ANSP GCM 5692 , slide and cleaned material JPK 7981, Kociolek Collection at COLO .

Observations: — Halamphora attenuata resembles several taxa with fine areolate striae in general valve dimensions and striae counts, including Halamphora borealis (Kützing) Levkov (2009: 175 ; basionym: Amphora borealis Kützing 1844: 108 ) and Halamphora salinicola Levkov & Diaz in Levkov (2009: 220). Of these taxa only H. borealis is widespread, being found in brackish waters throughout Europe. Halamphora attenuata can be distinguished from these taxa by its broad, often convex, ventral valve, broad raphe ledge, and thickened dorsal virgae near the valve center.

Figures

FIGURES 29–39: LM. Halamphora attenuata sp. nov. Single valves showing size range. Fig. 31. Holotype specimen. Scale bar = 10 μm.

FIGURES 40–44: SEM. Halamphora attenuata sp. nov. Fig. 40. External whole valve view. Fig. 41. Detail of external valve center showing axial row of biseriate striae (arrow). Fig. 42. Detail of the internal valve center showing the transition from biseriate to uniseriate