Cocconeis sp. 1

Riaux-Gobin, Catherine, Frankovich, Thomas, Witkowski, Andrzej, Agudelo, Pablo Saenz-, Esteve, Peter, Ector, Luc & Bemiasa, John, 2021, Cocconeis tsara sp. nov., C. santandrea sp. nov. and allied taxa pertaining to the new section Loculatae, Phytotaxa 484 (2), pp. 145-169 : 159

Cocconeis sp. 1 , SEM Figs 55–58 , 64

Description: Small taxon. Valve elliptic to round-elliptic, length 13–17 µm (mean 16 µm ± 1.4); valve width 7–11 µm (mean 9.5 µm ± 1.4); L/ W 1.7 ± 0.2; RV striae (19 ± 3 in 10 µm); SV striae (13 ± 0.7 in 10 µm) (n = 9, SEM). The RV is marginally shortly biseriate ( Fig. 58 , arrowhead), and all SV loculate areolae are composed of two perpendicular bars, with a rhombic central and hymenate sector, giving a ‘diamond-like’ appearance to the areolae ( Fig. 56 ). The loculate areola structure is symmetrical, and each sector is closed by a hymen with slits perpendicular to the frame of the sector ( Fig. 64 ). RVVC fimbriae only each three to four virgae and very short SVVC fimbriae ( Fig. 57 , arrow).

Remarks. Rare taxon. Cocconeis sp. 1 is observed along with C. tsara sp. nov. in the West coast of Anakao (sample 17–ANAK, Table 1). The structure of the SV loculate areolae in C. sp. 1 is diamond-like and more or less similar to that in large specimens of C. scutellum var. posidoniae as illustrated in De Stefano et al. (2008: fig. 11). Nevertheless, the latter specimens are larger in length than C. sp1, and their associated RV are not described as different from that of C. scutellum var. posidoniae , so, with a long marginal biseriate section, while only biseriate on a short section in C. sp.1 ( Fig. 58 , arrowhead). More information is needed about C. sp. 1, before giving it a definitive position. One possibility is that the diversity of the assemblage observed by De Stefano et al. (2008) is higher than reported, and that the diamond-like structure of loculate areolae may be a strong descriptor when present on the all SV and not only as occasionally present on the periphery of the valve. Note that De Stefano et al. (2008: fig. 124) illustrated a specimen as C. scutellum var. posidoniae , while it may as well illustrate C. scutellum var. posidoniae f. decussata De Stefano [renamed as C. scutellum f. decussata (De Stefano) Kociolek & Riaux-Gobin in Riaux-Gobin et al. 2019b] with two sectors delineated by a unique axial bar. The specimen illustrated in fig. 147 (loc. cit.) as C. scutellum var. sullivanensi s De Stefano, may also pertain to the same latter taxon. As long as no genetics are available for these taxa it will be difficult to prove there real specificities.

Figures

FIGURES 55–58. Cocconeis sp. 1. SEM (Anakao, West coast of Madagascar). SV external view with diamond-like loculate areolae (55– 57, arrow indicates the position of the valve apex). Entire broken frustule (57), RV biseriate on a short marginal section (58, arrowhead), RVVC with short fimbriae every three virga, oblong papillae without furrows (58, framed arrow), SVVC with short fimbriae (57, arrow). Scale bars = 3 µm (55), 2 µm (57), 700 nm (58), 500 nm (56).

FIGURES 61–69. SV loculate areolae from Cocconeis pertaining to the Loculatae section (61–68), compared to SV areola occlusions in C. scutellum Ehrenberg (69). All images at the same scale. Cocconeis scutellum cf. f. decussata (61), C. scutellum cf. var. posidoniae (62), C. tsara sp. nov. (63), C. sp. 1 (64), C. nosybetiana (65), C. sp. 2 (large specimens) (66), C. santandrea sp. nov. (67), C. scutellum cf. var. sullivanensis (68), C. scutellum (69). Arrows indicate the pole/apex of the valve. Scale bars = 300 nm.