Prestauroneis tumida Levkov, 2011

Levkov, Zlatko & Williams, David M., 2011, Fifteen new diatom (Bacillariophyta) species from Lake Ohrid, Macedonia, Phytotaxa 30, pp. 1-41 : 11-14

Prestauroneis tumida Levkov , sp. nov. ( Figs 69–89 )

Synonym: Parlibellus protractus (Grunow) Witkowski, Lange-Bertalot & Metzeltin sensu Levkov et al. (2007 : figs 64: 10–12)

? Navicula crucicula Grunow sensu Gallik (1935 , fig. 6)

Valvae lineari–lanceolatae quoad specimina maiora sed elliptico–lanceolatae quoad specimina minora, apicibus protractis, vel subprotractis et late rotundatis. Longitudo valvae 22–44 µm, latitudo valvae 9–11 µm. Fissurae raphis filiformis, rectis extremis proximalibus fere rectae poris centralibus inconspicuis. Area axialis fere angusta, linearis. Area centralis vix formata vel parva plus minusve lanceolata ad ellipticam. Striae transapicales paulo radiantes in media parte, tum paulatim transeunt denique modice convergentes 14–17 in 10 µm. Puncta striarum non discernanda microscopio photonico.Chloroplasti duo disciformes contra quoque latera cingulis positi.

Type:— MACEDONIA. Lake Ohrid, Biological Station Transect, mud 12 m depth, collection date :

03.04.2007 (accession No. MKNDC 001231 ). Slide BM 101471 (holotype), Slide MKNDC 001231 (isotype) .

Valves linear-lanceolate in larger specimens, to elliptic–lanceolate in smaller specimens, with protracted to subprotracted and broadly rounded ends, 22–44 µm long and 9–11 µm wide. Raphe filiform, straight, with proximal ends externally straight, central pores inconspicuous. Axial area narrow, linear. Central area slightly variable to barely developed, mostly small lanceolate or elliptic. Striae slightly radiate in central parts becoming gradually subparallel, finally moderately convergent 14–17 in 10 µm. (LM, areolae invisible in LM). Two plate-like chloroplasts, one against each side of the girdle.

External valve face flat, transition of valve face to mantle gradual ( Figs 83–85 ). Raphe branches straight with slightly widened and unilaterally deflected central pores. Distal raphe fissures prolonged, passing on valve mantle and unilaterally curved in opposite way to proximal ends ( Fig. 87 ). Striae uniseriate, regularly spaced. Usually one central stria; if two, one is shortened. Areolae variable in shape and orientation, usually elongated slits in oblique position ( Fig. 86 ). Internally ( Figs 88, 89 ), with distinct pseudosepta at apices. Raphe straight, proximally terminates with teardrop-shaped central pores, and distally with small helictoglossae. Interstriae wider than striae. Internal openings of areolae round, occluded by hymenes ( SEM).

Observations:— Navicula protracta Grunow sensu Gallik (1935 : fig. 5) has the most similar valves to Prestauroneis tumida , characterized by linear–lanceolate valves with shortly protracted and broadly rounded ends. However, its valves are slightly wider (B= 12.7 µm) and the margins are parallel at the middle. The valves of N. protracta illustrated in Krammer & Lange-Bertalot (1986: figs 7, 8) and Witkowski et al. (2000: fig 109: 9, 10, fig 134: 12, 13) are characterized by a linear outline, abruptly protracted to subcapitate ends and more distantly spaced central striae. Such features, in combination with narrower valves (B= 7–8 µm), enables easy differentiation from P. tumida . The valve identified as N. protracta in Krammer & Lange- Bertalot (1986: fig. 54: 5) is most likely a specimen of N. protracta f. elliptica Hustedt (see below).

In his treatment of N. crucicula (W. Smith) Donkin, Gallik (1935) described the new taxon N. protracta var. elliptica Gallik (1935: 65 , fig. 7), which is characterized by narrowly elliptical valves without protracted and broadly rounded ends, providing features to differentiate it from P.tumida . Hustedt (1957: 283) described a different taxon under the same name ( N. protracta f. elliptica ) which clearly differs from P. tumida in several respects: its valve length (L= 52 µm), the absence of protracted valve ends, and presence of finely punctate striae ( Simonsen 1987: figs 659: 1, 2).

Prestauroneis tumida can be easily differentiated from N. protractoides Hustedt (1957 : figs 32, 33) by the latter's valve size (L= 17–19 µm, B= 4–5 µ m), shape (linear with abruptly protracted ends) and striae morphology (distinctly punctate). Valves of N. crucicula (W.Smith) Donkin (1871 : fig. 6: 14) are lanceolate to elliptic–lanceolate with cuneate to obtusely rounded ends. The valve width is much greater than P. tumida (B= 16–18 µm) and the striae are more finely punctate. Navicula cruciculoides Brockman (1950: figs 4: 7–10) has lanceolate valves with cuneate to slightly protracted and narrowly rounded ends, and the striae are distinctly punctuate in LM, features not present in P. tumida .

Navicula crucicula var. lanceolata Frenguelli (1945 : fig. 8: 32) has a larger valve size (L= 54–57 µm, B= 12–13 µm) and strictly lanceolate valves. A similar valve outline is found in N. pseudocrucicula Kobayasi & Ando (1975 : figs 59–62). Both N. crucicula var. lanceolata and N. pseudocrucicula can be easily differentiated from P.tumida by their valve outline and the distinctly punctated striae.

Navicula crucicula var. alaskana Patrick & Freese (1961 : fig. 2: 1) has a comparable valve size to P. tumida but there are significant differences in the shape of valve ends (distinctly rostrate and capitate).

Navicula crucicula var. obtusata Cleve (in Cleve & Grunow 1880: fig. 2: 37) has a similar valve size and shape to P. tumida but occurs in brackish water habitats and is characterized by its distinct, lanceolate central area, unilaterally deflected proximal raphe endings and obtusely rounded and not protracted valve ends.

Finally, Prestauroneis was erected to accommodate species from section Microstigmaticae sensu Hustedt (1961 –1966) of the genus Navicula , e.g., species related to Navicula integra (Grunow ex Cleve) Fricke. Some species from this group have been earlier transferred to other genera, such as Parlibellus ( Cox 1988) and Proschkinia (Karayeva 1978) . In their study on marine diatoms, Witkowski et al. (2000) transferred several species of Navicula sensu lato species into Parlibellus , including N. crucicula , N. protracta and N. protractoides . However, these taxa share morphological and ultrastructural features with Prestauroneis and should be transferred there instead. In Prestauroneis , the striae radiate and are more distantly spaced in the mid-valve, separated with thickened costae giving the appearance of a stauros-like structure. However, a typical stauros (as illustrated in Cox 2001) is not present in Prestauroneis ( Figs 75–81 ). On the other hand, Parlibellus has a wide girdle region due to the numerous girdle bands, two deeply-lobed chloroplasts, external raphe fissures that terminate before the valve apices, internal raphe fissures with thickened axial ribs ( Cox 1988). Proschkinia has a wide girdle region composed of numerous punctuate bands, and single isolated stigma.

Figures

FIGURES 69–82: LM micrographs of Prestauroneis tumida. Figs 73, 74. LM of live cell with two chloroplasts. Scale bar = 10 µm.

FIGURES 83–89: SEM micrographs of Prestauroneis tumida. Figs 83–85. External view of whole valves. Fig. 86. Detail of mid-valve showing external foramina and proximal raphe ends. Fig. 87. Detail of valve end showing distal raphe ending. Fig. 88. SEM internal valve view. Fig. 89. Detail of mid-valve showing internal openings of areolae and proximal raphe endings. Scale bars in Fig. 83 = 5 µm; Figs 84, 85, 88 = 10 µm; Figs 86, 77, 89 = 2 µm.