Cocconeis dirupta var. dirupta Gregory

Joh, Gyeongje, 2021, Distribution of the genus Cocconeis (Bacillariophyceae) along the Seogwipo coast of Jeju Island, South Korea, Phytotaxa 528 (3), pp. 149-179 : 157

10. Cocconeis dirupta var. dirupta Gregory ( Figs 27–32 , 95 )

Basionym: Gregory 1857, p. 491(19), pl. IX(1), fig. 25.

References: Hustedt 1933, p. 354, figs 809a–c; Kobayasi & Nagumo 1985, p. 99, pl. 2. figs 16–27; Witkowski et al. 2000, p. 105, pl. 39, figs 1–5; Riaux-Gobin et al. 2011, p. 26, pl. 3, figs 9–11, pl. 41, figs 1–6; Lobban et al. 2012, p. 286, pl. 40, figs 1–3; Riaux-Gobin et al. 2016, p. 11, figs 4, 35–38

Description: Valves broadly elliptical, 9–23 μm long, 5–14 μm wide. SV: sternum narrow to broadly lanceolate, fusiform, sometimes slightly retracted in the middle. Transapical striae radiate, crossed by hyaline and longitudinal rows, 15–22 in 10 μm, one median stria often shorter on one or both side of the central area. RV: raphe slightly sigmoid. Sternum narrow and straight. Central area not developed or extended as transversely narrrow fascia, not reaching to the margins. Transapical striae radiate, 24–28 in 10 μm.

Remarks: The specimens described herein have valve outlines that are more elliptical rather than round-elliptical to discoid, as described for the lectotype designated by Riaux-Gobin et al. (2016). C. dirupta is distinguished from C. dirupta var. flexella by the straight axial area in the SV. In the literature, Cocconeis dirupta corresponds to two forms: one having large valves with a wide sternum ( Kobayasi & Nagumo 1985, Riaux-Gobin et al. 2016) and the other having valves with a strong retraction in the middle of the sternum ( Witkowski et al. 2000, Lobban et al. 2012). This species has a wide range in length and width dimensions, and both forms were found in the present study. The identifications were conducted according to the large forms designated by Hustedt (1933), and the small retracted forms by Lobban et al. (2012). The retracted forms collected in local areas have smaller dimensions, 9–17 μm long and 5–9 μm wide. In the smaller forms, transapical striae are abruptly denser at apices.

This taxon was reportedly common along oceanic coasts ( Witkowski et al. 2000), and in tropical coasts ( Riaux-Gobin et al. 2011, Lobban et al. 2012). Here, frequent on seaweeds of the Seogwipo coast of South Korea.

Figures

FIGURES 25–44. LM micrographs of Cocconeis species (2). Figs 25, 26. Cocconeis cf. diruptoides, RV (Fig. 25) and SV (Fig. 26). Figs 27–32. C. dirupta var. dirupta, SVs (Figs 27–31) and RV (Fig. 32). Figs 33–37. C. dirupta var. flexella, SVs (Figs 34–36) and RVs (Figs 33, 37). Figs 38–41. C. delicata, RVs (Figs 38) and SVs (Figs 39, 40, 41). Figs 42–44. C. convexa, SVs (Figs 42, 43) and RV (Fig. 44). Scale bar = 10 μm (Figs 25–44).

FIGURES 94–99. SEM micrographs of Cocconeis species (9). Fig. 94. Cocconeis cf. diruptoides, the internal view of SV. Fig. 95. C. dirupta var. dirupta, the internal view of SV. Figs 96–98. C. shikinensis, the external view of SV (Figs 97, 98), the internal view of RV (Fig. 96). Fig. 99. C. suzukii, the external view of SV. Scale bars = 1 μm (Figs 94, 98, 99), 2 μm (Figs 95–97).