Cocconeis shikinensis Suzuki

Joh, Gyeongje, 2021, Distribution of the genus Cocconeis (Bacillariophyceae) along the Seogwipo coast of Jeju Island, South Korea, Phytotaxa 528 (3), pp. 149-179 : 167

22. Cocconeis shikinensis Suzuki ( Figs 49–52 , 96–98 )

Basionym: Suzuki et al. 2001b, p. 138, figs 1–31.

Description: Valves elliptical, 19–50 μm long, 12–36 μm wide, the valve surface overall convex, but two-thirds of inner parts on valve face slightly concave. SV: sternum narrow and linear. Central area not developed, but very small circular in large specimens. Transapical striae transversely elongated, radiate towards the apices, 13–17 in 10 μm, crossed by several hyaline and zigzag longitudinal lines, elongated areolae in striae, alveolus, with 8–10 in 10 μm of stria. In some valves, often areolae arranged sparsely in the submarginal area. At the marginal area of the valve, one short striae arranged every 1–3 transapical striae, so the striae at the margins more dense than the middle.

RV: raphe straight. Sternum narrow, but the terminal ends of the sternum expanded with crescent-shaped hyaline area, resulting the raphe ends distant from the apices. Central area small and circular. Transapical striae radiate, 14–21 in 10 μm, areolae in striae circular-shaped, with 16–20 areolae in 10 μm of stria. Valve face between raphe and valve margins depressed concentrically.

Remarks: This species is similar to Cocconeis schmidtii Heiden in its valve morphology, but differs in having a crescent hyaline area on the terminal ends of the RV, in lacking a round central hyaline area, and in having a lower density of areolae on both valves ( Suzuki et al. 2001b). It was reported as important epiphytes on Caulerpa sp. (Chlorophyta) in Japan ( Suzuki 2001b). In the present study, it was mainly found in Rhodophyta, such as Plocamium sp. This species has morphological features that may cause confusion with C. placentula Ehrenberg and its allied taxa. The RV is not so, but the SV is similar to that of C. placentula ; thus, judging by the SV alone can be misleading. Prior to the naming of this species, the taxon attached to a carapace in Japan was recorded as C. placentula ( Takano 1962) . The two species, Cocconeis shikinensis and C. placentula , are sometimes found in the marine coasts. In the present study, the valve length of C. shikinensis ranges from 19 μm to 50 μm, while that of C. placentula does from 11 μm to 36 μm ( Lange-Bertalot et al. 2017).

Previously, these taxa had been reported strictly from Japanese coasts, including the type locality, Izu Islands of Tokyo ( Suzuki et al. 2001b, 2007). This taxon was encountered infrequently as epiphytes on seaweeds of the Seogwipo coast; it is herein newly described for South Korea.

Figures

FIGURES 45–56. LM micrographs of Cocconeis species (3). Figs 45–48. Cocconeis scutellum, RV (Fig. 45) and SVs (Figs 46–48). Figs 49–52. C. shikinensis, SVs (Figs 49, 52) and RVs (Figs 50, 51). Figs 53–56. C. schmidtii, RVs (Figs 54, 56) and SVs (Figs 53, 55). Scale bar = 10 μm (Figs 45–56).

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).