Navicula volcanica Bahls & Potapova, 2015

Bahls, Loren & Potapova, Marina, 2015, Two new species of Navicula (Bacillariophyta, Naviculales) from the Cascade Mountains of the American Northwest, Phytotaxa 218 (3), pp. 253-267 : 255-258

Navicula volcanica Bahls & Potapova , sp. nov. ( Figs 2–17 )

Valves broadly rhombic with rounded apices. Length 100–176 μm, width 30–37 μm. Axial and central areas moderately wide, one-third to one-half the valve width, together forming a narrow lanceolate shape with a tumid center. Central area rhombic, surrounded by alternately long and short striae. Raphe lateral and somewhat bowed, concave to the primary side of the valve. Proximal raphe ends inflated and tipped slightly to the secondary side. Terminal raphe fissures shaped like question marks. Striae curved, strongly radiate, structured internally as deep grooves, 6–8 in 10 μm. Areolae about 28 in 10 μm. Internally, areolae apically elongated, occluded by hymens. External areolae openings small and fused into a narrow slit on external valve surface. Cells solitary with two plate-like chloroplasts lying along the girdle on each side of the valve.

Type:— USA. Oregon, Douglas County: North Umpqua River, 43.311º N, 122.155º W, 1262 m a.s.l., A. Johnson , 5 August 2011 (holotype: circled specimen on ANSP! GC 65221, illustrated in Fig. 5 . Isotypes: circled specimens on MONTU! 41-100, MDC! 126-94. Paratypes: MONTU! 11-51, MDC! T-3-14-18).

Etymology: —The specific epithet “ volcanica ” refers to the general environment of this species: among volcanoes.

Distribution: —Clearwater and North Umpqua Rivers, Oregon.

Observations:—When observed under LM, Navicula volcanica is most similar to N. ludloviana ( Figs 18–33 ), another large Navicula with a relatively wide axial area and strongly radiate striae, but with a much wider distribution ( Table 2). Patrick & Reimer (1966) reported that valves of N. ludloviana are 23–33 μm wide and 90–132 μm long. Valves of N. volcanica are larger than those of N. ludloviana , even considering the larger N. ludloviana valves that we observed ( Table 3). Our images also show that valves of N. ludloviana near the upper size limit have very narrow, attenuated apices ( Fig. 22 ). In smaller specimens, valve apices in N. ludloviana are subrostrate, but they are not protracted at all in N. volcanica , even in the largest specimens ( Fig. 2 ). Valves of N. volcanica are rhombic, while those of N. ludloviana are rhombic-lanceolate.

The similarity of these two species is superficial, however, as the ultrastructure of N. volcanica sets it apart from other known Navicula taxa. Our SEM images of N. ludloviana ( Figs 30–33 ) show that its frustular morphology is typical of Navicula sensu stricto in that external and internal areolae openings are approximately the same size and shape and striae internally are rather shallow troughs. Internally, striae of N. volcanica are extremely deep troughs and the virgae between them are inflated, so that the internal openings of areolae are partially obscured ( Figs 15–17 ). The internal openings of areolae are apically elongated and constricted in the middle. The median constriction of each areola corresponds to a median thickening of adjacent intercostal ribs ( Fig. 17 ). The hymens that occlude the areolae internally bulge out slightly above the internal surface of the striae ( Figs 15, 17 ). The external openings of areolae merge into a narrow slit ( Figs 11, 12 ) running the length of each stria. The slit is often interrupted, especially near the central and axial areas, creating what appear to be individual transapically elongated to isodiametric external areolae openings ( Fig. 11 ).

Internally, the raphe slit runs uninterrupted through an elevated central nodule ( Fig. 16 ). The elevated rib subtending the raphe is positioned inside a longitudinal groove in the middle of the axial area and appears to be fused with the inflated part of the axial area on the primary side of the valve ( Fig. 13 ), except in the middle part of the valve where it is positioned centrally. The axial area is slightly wider on the primary side.

Figures

FIGURES 2–10. Navicula volcanica, LM.3. Preserved specimen with intact chloroplasts and four large oil droplets.5. Holotype specimen. 2, 4, 6–10. Clearwater River, Oregon. 3, 5. North Umpqua River, Oregon. Scale bars = 10 μm.

FIGURES 11–17. Navicula volcanica, SEM, North Umpqua River, Oregon. 11, 12. External valve views. 13–17. Internal valve views. Scale bars: Figs 11, 13 = 10 μm; Figs 12, 14, 16 = 5 μm; Figs 15, 17 = 1 μm.

FIGURES 18–29. Navicula ludloviana, LM. 18. Iconotype, A. Schmidt 1876. 19–22. Specimens from fossil lacustrine population near Port Ludlow, Washington (ANSP! Boyer 1060). 23, 24. Specimens from fossil lacustrine populations near Pit River, California.23. ANSP! Boyer E-2-21. 24. ANSP! Febiger 5661. 25–29. Specimens from extant riverine populations in Oregon. 25–27, 29: Snow Creek, Oregon. 28: Lake Creek, Oregon. Scale bars = 10 μm for all images.

FIGURES 30–33. Navicula ludloviana, SEM. Specimens from fossil lacustrine population near Pit River, California (ANSP! Boyer 338). 30, 31. External valve views. 32, 33. Internal valve views. Scale bars: Figs 30, 32, 33 = 10 μm; Fig. 31 = 2 μm.