Rhoicosphenia lowei E.W. Thomas & Kociolek, 2015

Thomas, Evan W. & Kociolek, J. Patrick, 2015, Taxonomy of three new Rhoicosphenia (Bacillariophyta) species from California, USA, Phytotaxa 204 (1), pp. 1-21 : 6-11

Rhoicosphenia lowei E.W. Thomas & Kociolek , sp. nov. ( Figs. 38–74 )

Frustules clavate and slightly flexed in girdle view. Valves heteropolar in valve view, oblanceolate to linear-clavate with bluntly rounded head pole and rounded foot pole, 16–75 μm long, 5–8 μm wide. Frustules heterovalvate, one valve concave with long raphe branches (R-valve), one valve convex with shortened raphe branches (D-valve). R-valve: raphe filiform, proximal raphe ends inflated, crook-shaped internally in same direction, distal raphe ends curved in same direction externally ending in helictoglossae internally. Axial area narrow at poles widening central area, central area oval, two to three times broader than axial area, and sometimes constricted in center creating two distinct lobes. Striae radiate in center of the valve and slightly radiate throughout, 9–11 in 10 μm at center of valve and are composed of lineolate areolae, 30 in 10 μm. D-valve: raphe branches 2–3 μm long at head pole, not extending beyond pseudoseptum, and 5–7 μm long at foot pole, external proximal ends not expanded, internal proximal ends crook-shaped in same direction and distal ends not inflated externally, terminate in helictoglossae internally. Striae parallel in center, radiate at apices, 9–11 in 10 μm at center of valve, and are composed of lineolate areolae. Both valves with pseudosepta at each apex, 3–7 μm long at head pole and 2–6 μm long at foot pole. Both valves with apical pore field at foot pole, porelli 3 per 1 μm. Girdle bands open. In the SEM, external views of R-valve ( Figs. 59–62 ) show the bluntly rounded head pole, lineolate areolae, and dilated proximal raphe ends. The apical pore field is present only at the foot pole and the porelli are more densely arranged, smaller, and rounder than the stria areolae. The distal raphe ends on R-valve continue onto the mantle ( Figs. 60, 62 ). Valvocopula is modified to overlap with the pseudosepta at each end of valve ( Figs. 63, 65, 66 ). The internal proximal raphe ends are strongly crook-shaped in the same direction ( Fig. 64 ). The external of the D-valve is characterized by the shortened raphe branches and lineolate areolae ( Fig. 67 ). The distal raphe end on D-valve terminates on valve face and the head pole ( Fig. 68 ) and continues onto the mantle at the foot pole ( Fig. 69 ). The proximal raphe ends are dilated and drop-shaped ( Figs. 68–69 ). In girdle view, the foot pole has apical pore fields on each valve and the valvocopula has a single row of round poroids ( Fig. 70 ). Internal views of the Dvalve show troughs between the virgae ( Fig. 71 ) as well as lineolate internal openings to the external areolae ( Figs. 71–74 ). Prominent pseudosepta are present at each pole ( Figs. 71, 73, 74 ) and raphe branches extend beyond the pseudosepta at each pole ( Figs. 71, 73 ). The proximal raphe ends are crook-shaped in the same direction ( Figs. 71, 73 ).

Type:— USA. California: Ash Creek, Lassen County, 41.134220º N, 120.800250º W, SWAMP Field Crew , September 15, 2010 (holotype ANSP! Circled specimen on slide GC XXXXX made from ANSP GCM XXXXX, illustrated in Fig. 45 ; isotype JPK! 6204, slide and material, University of Colorado, Museum of Natural History , Kociolek Collection , Boulder , Colorado, USA) .

Etymology:— This species is named in honor of Dr. Rex L. Lowe, a good friend, teacher and mentor to the authors.

Taxonomic remarks:— Rhoicosphenia lowei is distinguished from R. stoermeri by its smaller cardinal points of its size range; R. lowei has its smallest valve length of 16 μm and largest of 75 μm, while the cardinal points of the other large species in this paper, R. stoermeri , are 25 μm for the smallest and 84 μm for the largest valves. In addition, the shape of R. lowei is more linear with blunt apices, versus the lanceolate valves with acute apices of R. stoermeri . Of these two species, R. lowei has the coarsest striae, 9–11 in 10 μm, while the other, R. stoermeri has a higher density of, 11–13 in 10 μm. Finally, with regard to these two species, R. lowei has a shorter distance between proximal raphe ends. Both of these species share the larger panduriform central area, however it is less pronounced in R. lowei .

Compared to images of R. abbreviata in multiple publications through time (as R. curvata, Patrick & Reimer 1966 , pl. 20, figs. 1–5; Krammer & Lange-Bertalot 1986, fig. 91, images 20–28; Levkov et al. 2010, figs. 1a–p), R. lowei can fit the broadest species concept of R. abbreviata in many aspects of its morphology. In terms of size, R. lowei is 16–75 μm long and 6–8 μm wide, and R. abbreviata has been reported to be 12–75 μm long and 4–8 μm wide (as R. curvata, Patrick & Reimer 1966 ), 10–75 μm long and 3–8 μm wide ( Krammer & Lange-Bertalot 1986) 14–52 μm long and 5–7 μm wide ( Levkov et al. 2010). However, when comparing Rhoicosphenia lowei to R. abbreviata , the large, oblanceolate to linear-clavate valve outline differentiates it from the linear to narrowly-clavate smaller valves of R. abbreviata . These two species have similar central areas and their striae densities overlap. In the SEM, R. lowei has C-shaped areolae along the axial area, which are not documented in R. abbreviata ( Levkov et al. 2010, Figs. 2c, 2e ).

In valve shape, R. lowei is also somewhat similar to R. marina var. intermedia M. Schmidt (1899 : pl. 213, figs. 37–39), but the valves of R. lowei are narrower. The striae of R. lowei are less dense at 9–11 in 10 μm, while R. marina var. intermedia has 14 striae in 10 μm. In addition, R. lowei is a freshwater species and R. marina var. intermedia is reported from marine habitats. Finally, the species R. lacustris Levkov in Levkov et al. (2010: 159, figs. 22a–x) is morphologically similar to R. lowei . Key differences can be found in shape, with the headpole of R. lowei being narrower than the bluntly rounded headpole of R. lacustris , size, the 16–75 μm long of R. lowei is greater than the 25–62 μm long of R. lacustris , however, no images of initial valves are provided in Levkov et al. (2010). In addition, the striae of R. lacustris are denser at 13–15 in 10 μm versus 9–11 in 10 μm of R. lowei and the areolar density is higher in R. lacustris at ~ 45 in 10 μm versus 30 in 10 μm in R. lowei . Rhoicosphenia lowei also has smaller septum like structures and pseudosepta, as well as a larger aperture in the pseudoseptum. Another key difference between the two species is habitat, with R. lowei being found in freshwater and R. lacustris being found in eutrophic freshwater to brackish environments.

Distribution and ecological notes:— Rhoicosphenia lowei is found throughout the state of California from the Oregon border in the north to Los Angeles in the south, but more commonly reported north of Santa Cruz. It is found in samples across a wide range of elevations, from 11 to 2000 meters above sea level. Most locations are characterized by low conductivity, as well as low nitrogen and phosphorus.

Figures

FIGURES 38–58. Type material of Rhoicosphenia lowei from Ash Creek, Lassen County, California, USA. LM. 45. Holotype specimen. 40, 41, 44–46, 48, 50, 51, 53, 55, 57, 58. R-valves. 42, 43, 47, 49, 52, 56. D-valves. 39, 54. Girdle views. 38. Post-auxospore. Scale bar is 10 μm.

FIGURES 59–66. Type material of Rhoicosphenia lowei from Ash Creek, Lassen County, California, USA. SEM. 59–62. External views of R-valve show the bluntly rounded head pole, lineolate areolae, and dilated proximal raphe ends. The apical pore field has porelli that are more densely arranged, smaller, and rounder than the stria areolae (62). The distal raphe ends on R-valve continue onto the mantle (60, 62). 63–66. Internal R-valve shows the valvocopula is modified to overlap with the pseudosepta at each end of valve (63, 65, 66). The internal proximal raphe ends are strongly crook-shaped in the same direction (64). Scale bars are 10 μm (59), 1 μm (60–66).

FIGURES 67–74. Type material of Rhoicosphenia lowei from Ash Creek, Lassen County, California, USA. SEM. 67–70. The external of the D-valve is characterized by the shortened raphe branches and lineolate areolae (67). The distal raphe end on D-valve terminates on valve face and the head pole (68) and continues onto the mantle at the foot pole (69). The proximal raphe ends are dilated and drop-shaped (68–69). In girdle view, the foot pole has apical pore fields on each valve and the valvocopula has a single row of round poroids (70). 71–74. Internal views of the D-valve show troughs between the virgae (72) as well as lineolate internal openings to the external areolae (71–74). Prominent pseudosepta are present at each pole (71, 73, 74) and raphe branches extend beyond the pseudosepta at each pole (71, 73). The proximal raphe ends are crook-shaped in the same direction (71, 73). Scale bars are 10 μm (67, 74), 1 μm (68–73).

FIGURES 1–22. Type material of Rhoicosphenia stoermeri from Bear Creek, Los Angeles County, California, USA. LM. 10. Holotype specimen. 1, 2, 4, 6, 7, 10, 12, 14, 15, 18, 20, 22. R-valves. 3, 5, 9, 11, 13, 16, 19, 21. D-valves. 8, 17. Girdle views. Scale bar is 10 μm.