Rhoicosphenia stoermeri 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 : 3-6

Rhoicosphenia stoermeri E.W. Thomas & Kociolek , sp. nov. ( Figs. 1–37 )

Frustules clavate and slightly flexed in girdle view. Valves heteropolar in valve view, narrowly lanceolate to lanceolate with elongated acute apices in larger specimens, smaller specimens oblanceolate with head pole more blunt and rounded than acute foot pole, 25–84 μm long, 6–9 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 with minor undulations, proximal raphe ends 3–10 μm apart, dilated externally, crook-shaped internally in same direction, distal raphe ends curved in same direction externally ending in helictoglossae internally. Axial area narrow at apices, becoming wider towards central area, central area elongated and panduriform, ovate in smallest specimens. Striae radiate in center of the valve and parallel at apices, 11–13 striae in 10 μm at center of valve, 14–16 striae in 10 μm at apices, composed of lineolate areolae, 30 in 10 μm. D-valve: raphe branches 4–6 μm long at head pole, not extending beyond pseudoseptum, and 5–7 μm long at foot pole, external proximal ends slightly inflated, internal proximal ends crook-shaped in same direction and distal ends not inflated externally, terminate in helictoglossae internally. Striae parallel in middle, slightly radiate at apices, 11–12 striae in 10 μm in middle of valve, 14–15 striae in 10 μm at apices, composed of lineolate areolae. Both valves with pseudosepta at each apex, 3–13 μm long. Both valves with apical pore field at foot pole, porelli 4 per 1 μm. Girdle bands open. In the SEM, external views of the R-valve ( Figs. 23, 25–27 ) show valve outline and large panduriform central area. Proximal raphe ends on Rvalve ( Figs. 23, 26 ) are dilated and drop-shaped. The axial area is narrow near the ends, becoming inflated around the proximal raphe ends, becoming narrow again in between them ( Figs. 23, 26 ). Distal raphe ends on the R-valve continue onto the mantle ( Figs. 25, 27 ). Apical pore fields are present only at the foot pole and porelli are linear and obliquely arranged ( Fig. 27 ). Internally, the valvocopula is modified to fit over and near entirely cover the pseudoseptum and has an aperture at the head pole ( Figs. 24, 28, 30 ). Internal valve views show panduriform central area and proximal raphe ends strongly hooked in the same direction ( Figs. 24, 29 ). Also, the areolae can be seen in troughs between the virgae ( Fig. 29 ). External views of the D-valve show the shortened raphe branches and lineolate areolae ( Figs. 31, 33, 34 ). Distal raphe end on D-valve terminates on valve face at head pole ( Fig. 33 ) and continues onto mantle at foot pole ( Fig. 34 ). Proximal raphe ends on D-valve ( Figs. 31, 33, 34 ) are dilated and drop-shaped. Internal SEM of the D-valve shows troughs between the virgae as well as lineolate internal openings to the external areolae ( Fig. 32 ). Pseudosepta are present at each pole and raphe branches extend beyond the pseudosepta at each pole ( Figs. 35, 36 ). The valvocopula is modified to cover pseudosepta on the valve interior ( Figs. 35, 36 ). Interior views of head and foot pole with crook-shaped internal proximal raphe ends ( Figs. 35, 36 ). In girdle view, valve flexure is illustrated and girdle elements are each ornamented with one row of simple poroids ( Fig. 37 ).

Type:— USA. California: Bear Creek, Los Angeles County, 34.24154º N, 117.88599º W, M. Brady, A.E. Fetscher, J.P. Kociolek, & E.W. Thomas, November 5, 2007 (holotype ANSP! Circled specimen on slide GC XXXXX made from ANSP GCM XXXXX, illustrated in Fig. 10 ; isotype JPK! 2627, slide and material, University of Colorado, Museum of Natural History , Kociolek Collection , Boulder , Colorado, USA) .

Etymology:— This species is named in honor of Dr. Eugene F. Stoermer, one of the true leaders in research on diatoms.

Taxonomic remarks:— Rhoicosphenia stoermeri is distinguished from other Rhoicosphenia taxa by its shape, size and large panduriform central area. Morphologically, R. stoermeri most closely resembles R. marina var. intermedia M. Schmidt (1899 : pl. 213, figs. 36–39). However, there are 11–13 striae in 10 μm at the center of the valve in R. stoermeri and 14–16 striae in 10 μm in R. marina var. intermedia . In addition, in specimens of similar size (lengths of approximately 50–65 μm) the shape of R. stoermeri valves is lanceolate while R. marina var. intermedia valves are oblanceolate. In terms of ecology, M. Schmidt reports R. marina var. intermedia as being found in marine environments of ‘coastal California’ and R. stoermeri is found in the San Gabriel Mountains of Southern California in very low conductivity streams.

Rhoicosphenia stoermeri is also morphologically distinct from R. curvata var. subacuta M. Schmidt. The specimens of R. curvata var. subacuta that are most similar to R. stoermeri are from marine environments of China (‘ Insel Hainan’, Schmidt 1899, pl. 213, Figs. 6–7 ). These two taxa are similar in shape, but R. stoermeri is distinguished by its large panduriform central area, distance between proximal raphe ends, and larger valve size. R. stoermeri also has more dense striae, 11–13 striae per 10 μm, as opposed to 9–11 striae per 10 μm in R. curvata var. subacuta .

Rhoicosphenia stoermeri is also similar to R. affinis Levkov in Levkov et al. (2010: 160), but the shape of R. affinis , ‘subclavate, with attenuate and subprotracted head pole’ ( Levkov et al. 2010: 160) distinguishes it from R. stoermeri , especially with regard to the head pole. R. stoermeri is most similar in its morphology to R. lacustris Levkov et al. (2010: 159) , but it has less dense striae 11–13 per 10 μm (as opposed to 13–15 per 10 μm in R. stoermeri ) and has a greater size range 25–84 μm (vs. 25–62 μm in R. stoermeri ). Rhoicosphenia stoermeri has a more distinctly panduriform central area with greater separation between proximal raphe ends as compared with R. lacustris .

Compared to the type material of R. abbreviata as documented by Levkov et al. (2010), R. stoermeri can be distinguished by several features. First, the size range of R. stoermeri is 25–84 μm long and 6–9 μm wide, both longer and wider than reported for R. abbreviata at 14–52 μm long and 5–7 μm wide. Second, the narrowly-lanceolate to lanceolate valve shape distinguishes R. stoermeri from linear to narrowly clavate valves of R. abbreviata . Striae density is also different between the two; R. stoermeri has distinctly punctate striae, 11–13 in 10 μm at the center while R. abbreviata has 9–12 in 10 μm at the center and are not distinctly punctate ( Levkov et al. 2010).

Distribution and ecological notes:— Found in lower elevation sites from Los Angeles to Redding, CA. Most sites are close to the Pacific Ocean with the exception of three sites and are generally characterized by low nutrients, slightly alkaline, and low conductivity.

Figures

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.

FIGURES 23–30. Type material of Rhoicosphenia stoermeri from Bear Creek, Los Angeles County, California, USA. SEM. 23, 25–27. External R-valve outline, panduriform central area, and dilated proximal raphe ends (23, 26). Distal raphe ends on the R-valve continue onto the mantle (25, 27). Apical pore fields are present only at the foot pole and porelli are linear and obliquely arranged (27). 24, 28–30. Internally, the valvocopula is modified to fit over and near entirely cover the pseudoseptum and has an aperture at the head pole (24, 28, 30). Internal valve views show panduriform central area and proximal raphe ends strongly hooked in the same direction (24, 29). The areolae can be seen in troughs between the virgae (29). Scale bars are 10 μm (23–24) and 1 μm (25–30).

FIGURES 31–37. Type material of Rhoicosphenia stoermeri from Bear Creek, Los Angeles County, California, USA. SEM. 31, 33, 34. External views of the D-valve show the shortened raphe branches and lineolate areolae (31, 33). Distal raphe end on D-valve terminates on valve face at head pole (33) and continues onto mantle at foot pole (34). Proximal raphe ends on D-valve (31, 33, 34) are dilated and drop-shaped. 32, 35, 36. Internal D-valve with troughs between the virgae as well as lineolate internal openings to the external areolae (32, 35). Pseudosepta are present at each pole and raphe branches extend beyond the pseudosepta at each pole (32). Valvocopula modified to cover pseudosepta on valve interior (35, 36). Interior views of head and foot pole with crook-shaped internal proximal raphe ends (35, 36). 37. In girdle view, valve flexure is illustrated and girdle elements are each ornamented with one row of simple poroids Scale bars are 10 μm (32, 37), 5 μm (31), and 1 μm (33–36).