Planothidium sheathii Stancheva, 2019

Stancheva, Rosalina, 2019, Planothidium sheathii a new monoraphid diatom species from rivers in California, USA, Phytotaxa 393 (2), pp. 131-140 : 132-133

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 CA 10030, 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.

Figures

FIGURES 1–18. Planothidium sheathii LM views. Figs 1, 2, 5–9, 11–18 are SV. Figs 3, 4, 10 are RV. The same valve is presented at different focal plane to show the cavum hood aperture on figures 5 and 6, 11 and 12, 13 and 14, 15 and 16, 17 and 18. Figs 8, 9 and 10 are the same frustule.Arrows show cavum hood aperture borders fusion with neighboring virga on SV, arrowheads show more distant central shortened striae on RV. Figs 1–16 from the type locality San Joaquin River. Fig. 7 represents the holotype. Figs 17, 18 from Merced River, California, USA. All scale bars: 5 μm.

FIGURES 19–24. Planothidium sheathii SEM views. Figs 19, 20. External SV view. Fig. 21. Internal SV view with cavum. Figs 22, 23. External RV view. Note the more distant central shortened striae on secondary side (arrowheads). Fig. 24. Internal RV view. All figures from the type locality San Joaquin River, California, USA.All scale bars: 5 μm.

FIGURES 25–29. Planothidium sheathii SEM views. Fig. 25. Internal view of RV areolae. Figs 26, 27. Detail of cavum aperture.Note the boarders of the aperture attached to neighbor virgae. Fig. 28. External view of SV periphery and mantle. Striae 5–6 seriate, interrupted at valve mantle junction. Note the separated groups of 8–13 rounded areolae, bordered by 2 elongated areolae on mantle in oblique position to each areola group (arrows). Fig. 29. External view of RV periphery and mantle. Striae 5–6 seriate, interrupted at valve mantle junction. Note the separated groups of 7 rounded areolae on mantle (arrows).All figures from the type locality San Joaquin River, California, USA. All scale bars: 1 μm.