Planothidium sheathii

Planothidium sheathii Stancheva, sp. nov. (Figs 1–29)
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Description
Valve outline and dimensions: Valves broadly lanceolate to elliptic with obtusely rounded, never protracted apices, 6.3–8.8 μm wide, 14–29 μm long (50 valves were measured). Striae 11–13 in 10 μm on SV, 11–12 in 10 μm on RV measured at the central part of the valve face between valve sternum and margins, on SV measured at the valve part opposite to the unilateral expansion.

LM of SV (Figs 1, 2, 5–9, 11–18): Axial area narrow lanceolate, slightly expanded in the middle. Central area with a prominent cavum, which is oblong with parallel sides (Figs 5, 7, 11) to slightly V-shaped (Figs 2, 8, 13, 15, 17) when viewed at valve surface focal plane. Internally, the cavum is covered by a broad hood. At a deeper focal plane, the cavum hood aperture is visible as curved line (Figs 6, 7, 9, 12), which crosses the cavum and neighboring striae, and endings near the mantle at virgae (Figs 6, 7, 14, 16). Striae parallel at valve middle to weakly radiate towards apices.

LM of RV (Figs 3, 4, 10): Axial area very narrow linear. Central area transapically rectangular, bordered by two shortened striae on both sides (rarely three on one side). The short central striae more distant on one side. Striae parallel at valve middle to radiate towards apices. Raphe branches straight with expanded, drop-like proximal raphe endings.

SEM of SV (Figs 19–21, 26–28): Externally, striae multiseriate, composed of five to six rows of small rounded same sized areolae, interrupted at juncture of valve and valve mantle (Figs 19, 20, 28). Valve mantle shallow, with regular groups of 8–13 rounded areolae offset from the valve striae, each bordered by two elongated areolae in oblique position to each areola group (Fig. 28). Axial and central areas are irregular depressions on valve surface (Figs 19, 20). Internally, the large cavum is covered with broad hood (Figs 21, 26, 27). The hood aperture flares out at the valve margin with open borders fuse with the neighboring virgae (Figs 26, 27). Striae sunken between raised virgae (Fig. 21).

SEM of RV (Figs 22–25, 29): Externally, striae multiseriate, composed of four to six rows of small rounded same sized areolae on valve face (Figs 22, 23), with groups of seven to nine rounded areolae on mantle offset from the valve striae (Fig. 29). Two or three central striae shortened, parallel and more distant on secondary side of the valve (Figs 22, 23). Proximal raphe endings expanded, distal raphe endings curved terminating on valve face by a short fissure extending just beyond apical striae, unilaterally deflected (Figs 22, 23). Internally, proximal raphe ends not widened, end directly opposite each other on inner valve surface (Fig. 24). Helictoglossae small (Fig. 24).

Type locality USA, California, San Joaquin River, 36.7790352, -120.233343, collector Gary Ichikawa, July 09, 2018.
Holotype: Slide GC 65325 at The Academy of Natural Sciences of Drexel University Diatom Herbarium, Philadelphia, USA. Holotype specimen is illustrated on Fig. 7.
Isotype: Slide RS 056 and cleaned material RS 057, CSUSM, USA.
Etymology: The species is named in honor of Dr. Robert G. Sheath in recognition of his contributions to the stream algal bioassessment in California.
Distribution and ecological notes: Recorded in the type locality and in Merced River, California (site CA10030, 37.39213, -120.7909, July 16, 2018) with low relative abundance and within pH range of 7.5–7.7, specific conductivity 38–78 μS cm-1, and dissolved oxygen 88.2–97.9 mg L-1.

• Author
  Stancheva 2019
• Length Range
  14-29 µm
• Width
  6.3-8.8 µm
• Striae in 10µm
  11-13