Cocconeis molesta, KUTZING, 1844

COCCONEIS MOLESTA KÜTZING COMPLEX AND COCCONEIS CF. DIAPHANA W. SMITH

A rare and small epiphytic taxon, with a round-elliptic shape and a reduced RV fascia ( Figs 21– 22 View Fig twin arrowheads), is closely related to C. dirupta Gregory (see Kobayasi & Nagumo 1985, fig. 19; Riaux-Gobin et al. 2016, figs 35–38), while lacking the characteristic bi-lanceolate large SV sternum, more delicate SV areolae, and denser SV and RV stria ( Table 3). Note the strongly sigmoid RV in C. dirupta (refs cit.), while our taxon ( Figs 19–22 View Fig ) has helictoglossae only slightly bent in the opposite direction, such as observed in the C. molesta type ( Riaux-Gobin et al. 2016). The latter taxon from Mangareva ( Figs 19–22 View Fig ) is here provisionally assigned to C. molesta Kützing.

An oblong-elliptic and relatively large taxon ( Figs 23–30 View Fig View Fig , Table 3), was common in the subtidal samples Rik13, Rik15 and Rik16, and characterized by its SV short areolae arranged in a regular decussate pattern, with apical rows arranged along a 135° grid-pattern ( Fig. 27 View Fig , white lines). The open SVVC ( Fig. 29 View Fig , ellipse) has a smooth edge ( Fig. 30 View Fig , arrowhead). The SV sternum is narrow, irregular, concave, with a reduced to absent central area ( Fig. 27 View Fig arrowhead). SV striae are more or less parallel in median valve and abruptly radiate near the apices ( Fig. 28 View Fig arrowhead). At the apices, a marginal row of SV areolae are perpendicular to the margin. The RVVC was not observed. The RV fascia is narrow and straight, reaching the margins, and central raphe endings are close to each other ( Fig. 24 View Fig ). Helictoglossae slightly curved ( Fig. 26 View Fig ). RV areolae present at the apices ( Figs 23, 25–26 View Fig ). This taxon is provisionally assigned to C. cf. diaphana Smith 1853 (see Remarks).

Remarks: The latter taxon ( Figs 23–30 View Fig View Fig ) has affinities with Cocconeis diaphana Smith var. diaphana , except for a RV fascia reaching both margins, and striae on both valves much denser than in the C. diaphana var. diaphana type (BM 23161, in Riaux-Gobin et al. 2016). We can also compare our images with those illustrating the iso-lectotype of C. diaphana Smith var. diaphana (mica labelled as Cocconeis diaphana n. sp., Jersey, Aug. 14 1852, deposited in the Van Heurck collection in Meise, in Riaux-Gobin et al. 2016, figs 21–26). In the latter taxon, the RV and SV stria densities are also dissimilar, being less dense than in the Mangareva taxon ( Table 3). Furthermore, the SV and RV striae (in the iso-lectotype) are often marginally dichotomous while uniseriate until the margin in the Mangareva taxon. The latter also has some affinities with Cocconeis molesta , except for its larger dimensions, oblong shape, and RV fascia expanding up to both margins. Also, there are some affinities with C. molesta var. crucifera Grunow in Van Heurck (1880, pl. 30), as illustrated by Kobayasi & Nagumo (1985, figs 1–12), and by De Stefano et al. (2000, figs 33–36), but in the latter taxon, the RV fascia is much shorter, and the valve shape is less oblong than in the Mangareva taxon. Note that C. molesta var. crucifera was proposed as a synonym of C. molesta ( Riaux-Gobin et al. 2016) .