Cocconeis cf. diruptoides Hustedt

Joh, Gyeongje, 2021, Distribution of the genus Cocconeis (Bacillariophyceae) along the Seogwipo coast of Jeju Island, South Korea, Phytotaxa 528 (3), pp. 149-179 : 159

12. Cocconeis cf. diruptoides Hustedt ( Figs 25, 26 , 91–94 )

Basionym: Hustedt 1933, p. 356, fig. 810.

References: Simonsen 1987, p. 133, pl. 219, figs 5–10, 13, 14; De Stefano & Marino 2001, p. 303, figs 15–27; Car et al. 2012, p. 445, figs S18–21.

Description: Valves oblong-elliptical to elliptical, 9–23 μm long and 5–13 μm wide. SV: sternum narrow and slightly sigmoid. Central area forming with lacking one striae, expanding into a short hyaline fascia on the one side of valve. Transapical striae slightly radiate towards apices, 25–37 in 10 μm, areolae on striae transapically elongated, 18–26 in 10 μm, forming wavy longitudinal lines, a few striae bifurcated in the margins of valve. RV: raphe slightly sigmoid, sternum very narrow. Central area as transversely narrow and hyaline fascia, not reaching up to the margins. Transapical striae slightly radiate, 28–35 in 10 μm, with 29–35 areolae in 10 μm of stria. Narrow hyaline area around the margins of valve.

Remarks: The local specimens match the description of De Stefano et al. (2000), based on Hustedt’s type specimen, characterized by the presence of hemifascia on SV and strial dichotomy in the margins. However, since the hemifascia is not found in Hustedt’s illustration and the exact type specimens are controversial on the other hand ( Riaux-Gobin et al. 2018), the local specimens are recorded as Cocconeis cf. diruptoides.

Specimens collected in the current study were often less than 18 μm long; some were as short as 9 μm. The species is similar to Cocconeis dirupta var. flexella in its valve morphology, but the latter is distinguished by a sigmoid raphe and sternum of the valve, a low density of transapical striae ( De Stefano & Marino 2001), and sparse strial areolae in the SV.

In the present survey, this is one of the most commonly observed Cocconeis . The valves are very variable in size and range from 9 μm to 23 μm long. In the assemblage, individuals commonly exhibited weakly sigmoidal raphe of valves, but widely differed in the density of striae for both valves.

This taxon is distributed in the Mediterranean ( Hustedt 1933), has been collected from the northern Adriatic Sea between Rovinj and Vis (De Stefano & Mario 2001), and is widely distributed at temperate and warm coasts ( Car et al. 2012). Along the Seogwipo coast, it was found frequently as epiphytes on seaweeds; it is herein newly described for 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 88–93. SEM micrographs of Cocconeis species (8). Figs 88, 89. C. pseudomarginata var. intermedia, the external view (Fig. 88) and internal view of SVs (Fig. 89). Fig. 90. C. dirupta var. flexella, the external valve of SV. Figs 91–93. C. cf. diruptoides, the external view of SVs (Figs 91, 92), and the internal view of RV (Fig. 93). Scale bars = 5 μm (Figs 88, 89), 2 μm (Figs 90, 92), 1 μm (Fig. 93).

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).