Odontidium minutum Levkov & D.M.Williams, 2011

Levkov, Zlatko & Williams, David M., 2011, Fifteen new diatom (Bacillariophyta) species from Lake Ohrid, Macedonia, Phytotaxa 30, pp. 1-41 : 2-4

Odontidium minutum Levkov & D.M.Williams , sp. nov. ( Figs 1–9 ; 25–30 )

Frustula aspectu cinguli rectangulata. Valvae ellipticae (nec lanceolatae) apicubus rotundatae (nec cuneatae), 12–16 µm longae, 9–11 µm latae. Costae transapicales 2–5 in 10 µm, 4–6 striae inter costas. Striae simplices, paralellae in media parte, transientes radiantes vel paulo curvatae sub apicibus 24–30 in 10 µm facile discernandae microscopio photonico. Cingulum simplex et a 4–8 zonis apertis compositum.

Type:— MACEDONIA: Lake Ohrid, St. Naum springs, Brown filaments, collection date: 31 January 2003 (Accession No. MKNDC 000407). Slide BM 101466 (holotype). Slide MKNDC 000548 (isotype) .

Frustules rectangular in girdle view. Valves elliptical, with rounded poles, length 12–16 µm, width 9–11 µm. Transapical ribs nearly all appear primary, 2–5 in 10 µm, c. 4–8 rows of striae between pairs of ribs. Striae simple, parallel towards middle, becoming radiate and slightly curved at valve apices, 24–30 in 10 µm, indistinct. Cingulum simple, composed of 4–8 bands (LM).

External valve surface rippled, with numerous small spines (spinules) along valve face margin, each occurring on a virga, irregularly positioned, pointing out from valve in variable directions ( Figs 25, 27, 28 ). Outer valve surface covered with small protrusions (nodules); internally, with uniseriate striae; transapical ribs primary, with few secondary ribs ( Figs 29, 30 ). Sternum broad, ill-defined, extending from each pole into apical pore field ( Fig. 27 ). One rimoportula per valve, polar, oriented within or adjacent to a stria, replacing 3– 5 vimines, a simple slit externally ( Figs 27, 28 ), internally with protruding lips ( Figs 29, 30 ). Apical pore field simple, occurring at each pole, composed of round pores. APF pores are more closely packed than areolae density ( Figs 27, 28 ). Girdle composed of c. 6–10 open bands, all similar, with single series of poroids on pars interior ( Figs 25, 26 ), multiple sets of poroids on pars exterior ( Figs 25, 26 ). Bands become progressively narrower towards epivalve ( Figs 25, 26 ) and covered with small nodules ( SEM).

Observations:— Odontidium minutum can be characterized by its small valve size and shape, retained during its life cycle. It resembles some smaller specimens of Odontidium mesodon but can be differentiated from it by the valve shape (lanceolate to elliptic–lanceolate) and its smaller size. Additionally, the striae in O. mesodon are coarser and clearly visible with LM ( Williams 1985). Odontidium hiemalis has longer valves with a clearer linear outline, coarse striae and a distinct sternum ( Williams 1985), features not present in O. minutum . Diatoma vulgaris var. brevis Grunow was considered a synonym of D. vulgaris by Williams (1985) while Krammer & Lange-Bertalot (1991) considered it as an unnamed variant. Patrick & Reimer (1966) recognized Diatoma vulgaris var. brevis as a separate entity characterized by lanceolate shaped valves, larger size (L= 24–50 µm; B= 11–13 µm), coarser and distantly spaced striae (about 16 in 10 µm) and a distinct, linear sternum. Odontidium minutum can be easily differentiated from D. vulgaris var. brevis by its valve shape, size, number and density of ribs (6–8 in 10 µm in the latter), and presence of a distinct sternum.

Odontidium can be separated from Diatoma by the structure of the sternum—wide and diffuse in Odontidium —its complex girdle structure and the coarser structure of the striae and ribs of the valves ( Van Heurck 1896). The general features of Odontidium are present in these specimens, hence the generic placement of this taxon.

Genus Staurosirella Williams & Round (1987)

Figures

FIGURES 1–24. LM micrographs. Figs 1–9. Odontidium minutum. Figs 10–17. Staurosirella pulchella. Figs 18–24. Navicula subviridula. Scale bar = 10 µm.

FIGURES 25–30: SEM micrographs of Odontidium minutum (type material). Figs 25, 26. SEM external view of frustule and girdle bands. Fig. 27. SEM external valve view. Broad lanceolate sternum, apical pore fields (APF) on the valve apices and a single rimoportula present. Fig. 28. Detail of the valve end showing APF and external opening of rimoportula. Fig. 29. Internal valve view. Fig. 30. Detail of valve end showing internal opening of rimoportula, striae and APF. Scale bars in Figs 25, 27, 29 = 5 µm, Fig. 26 = 10 µm; Figs 28, 30 = 2 µm.