Cocconeis tsara Riaux-Gobin, Witkowski & Bemiasa, 2021

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 : 147-151

Cocconeis tsara Riaux-Gobin, Witkowski & Bemiasa sp. nov. LM Figs 1–8 ; SEM Figs 9–29

Description: Valve small, elliptic. Valve length 11–19 µm (mean 16 µm ± 1.5); valve width 7–12 µm (mean 10 µm ± 1.1); L/ W 1.7 ± 0.2; n = 64 (SEM). SV: convex, striae radiate and regularly spaced, uniseriate comprised of subround to square subdivided areolae with rounded corners ( Figs 9–12 ) becoming multiseriate on the valve margin (up to triseriate with alternate areolae, Figs 9, 12 ) comprised of smaller undivided areolae. SV striae 13–17 in 10 µm (mean 14 µm ± 1.0). SV areolae appear round-elliptic in phase-contrast ( Fig. 1 ), while squared in Nomarski differential interference-contrast ( Figs 4–6 ). Externally, hymenes of areolae nearly level with valve surface ( Figs 10–11 ). Areolae open internally by a sub-round to elliptic aperture giving access to a loculate areola ( Fig. 17 ). In internal view, one thin bar (up to two) axially arranged (= primary bar/s), merges with one bar transapically oriented on the bottom of the small chamber (= secondary bar), resulting in an irregular to asymetrical cross ( Figs 10–11 , 17 , 63 ). Exceptionally, presence of diamond-like structure (more or less irregular or incomplete) on rare marginal loculate areolae ( Figs 12–13 ). All sectors of loculate areolae externally closed by hymenes with short slits perpendicular to the frame of each sector ( Figs 10–11 ). SVVC strong and closed, ornamented by digitate fimbriae with irregular apex ( Figs 14, 16 ). SVVC fimbria present on each virga, or more rarely, on each three to four virgae ( Fig. 14 ). Narrow cingulum composed of few cingular bands/copulae. RV: Slightly concave, with a marginal hyaline rim, striae uniseriate up to the margin ( Figs 18–22 ). RV striae 18–25 in 10 µm (mean 20 µm ± 1.6), composed of regularly spaced round areolae. No RV areolae on apices. Central area reduced, external proximal raphe endings closely spaced, straight, slightly expanded ( Figs 23–25 ), terminal raphe endings simple, far from margin ( Figs 20–22 ). RV areolae externally closed by slightly concave hymenes with short slits perpendicular to the frame of the areolae. RVVC open, with hammer-like fimbriae taking place on an elevated-raised rim, each two to three virgae ( Figs 27–28 ). Between each large fimbria, an undulation, reminiscent a vestigial fimbria, faces each virga ( Fig. 29 , arrowheads). Each fimbria ornamented on its abvalvar side by an oblong and smooth papilla (no furrows, Figs 27–29 ). Second RV copula with a ligula.

Holotype, here assigned: Slide BM 101 951 ( NHM) from the sample 17– ANAK.

Isotypes: Slides mounted with the same material as the holotype sample 17– ANAK: slide SZCZ 26006 in A. Witkowski collection ( The Faculty of Geosciences , Szczecin, Poland) and slide 17– ANAK in C. Riaux-Gobin collection ( CRIOBE – CNRS, USR 3278 , Perpignan, France) .

Type locality: Coral reef on the Western coast of Anakao ( Madagascar, Indian Ocean), sample 17– ANAK ( Table 1).

Etymology: The specific epithet ‘ tsara ’ refers to the Malagasy word meaning beautiful.

Habitat: Epiphyte on Bleakeleya cf. notata (Grunow in Van Heurck) Round, living on macroalgae from a coral reef environment composed of diverse living corals.

Remarks: Cocconeis tsara sp. nov. is most similar to C. scutellum var. posidoniae De Stefano, D.Marino & Mazzella ( De Stefano et al. 2000, 2008). The two taxa can be distinguished by differences in frustule ultrastructure. Cocconeis scutellum var. posidoniae has SV areolae subdivided into two hymenate, and axially arranged, reniform structures (figs 75, 77– 78 in De Stefano et al. 2000), sometimes showing a central ‘pierced’ structure (fig. 79 in De Stefano et al. 2000). In contrast, the SV areolae of C. tsara are cruciately or near-cruciately divided by thin crossbars, irregular or distorted ( Figs 10–11 , 63 ) and the hymenate sectors are triangular, with rounded edges on the areola perimeter ( Figs 10–11 , 17 ). The RVVC of C. scutellum var. posidoniae is closed (fig. 83 in De Stefano et al. 2000) while that of C. tsara sp. nov. is open ( Fig. 20 ). The RVVC fimbriae of C. scutellum var. posidoniae are joined at their terminus forming large quadrangular fenestrae (figs 76, 83, 85 in De Stefano et al. 2000) while the fimbriae of C. tsara are free on the interior edge ( Figs 27–29 ), and ending by a spathulate or hammer-like structure ( Fig. 27 arrows). The RV striation is marginally biseriate in C. scutellum var. posidoniae (figs 82, 86 in De Stefano et al. 2000) while uniseriate in C. tsara ( Figs 18 , 26 ). The papillae on the RVVC fimbriae in C. scutellum var. posidoniae are ornamented by numerous furrows, while the papillae are smooth and rod-like in C. tsara ( Figs 27–29 ).

Valves slightly smaller and more elongate than C. tsara sp. nov. were found as epizoic on ‘Océane’ (juvenile Chelonia mydas Linnaeus, Haapiti, Moorea , South Pacific, Figs 30–33 ) [valve length 10–11 µm (mean 11 µm ± 0.6); valve width 4–6 µm (mean 5.4 µm ± 0.5); L/W 2 ± 0.2; n = 12. SV striae 18–21 in 10 µm (mean 19 µm ± 1.4)]. Cocconeis cf. tsara has a similar SV loculate areola arrangement, but the areolae are sometimes further divided by additional cross-bars forming a zig-zag or arborescent arrangement of the hymenate sectors ( Fig. 33 ). No RV was associated with certainty to the latter taxon (only two RV observed, with dense striation, that may as well pertain to Cocconeis coronatoides Riaux-Gobin & O.E.Romero , in Riaux-Gobin et al. (2011a: 88), which was also present in the sample in rare abundance). A vestigial raphe was also present on the SV ( Figs 30–31 , circles).

Figures

FIGURES 1–8. Cocconeis tsara sp. nov. LM (Anakao, West coast of Madagascar). SV with round-elliptical areolae, multiseriate on the margin, RV with dense striation and well-marked hyaline rim close to the margin (1). RVs with distinct areolae and discreet central area (2–3). SVs with round to square areolae in more or less regular axial rows (4–6). SVVC closed with digitate fimbriae not always regularly disposed (7). RVVC open with short and peged fimbriae irregularly arranged (8). Scale bars = 10 µm (phase-contrast: 1, 7–8; Nomarski interference contrast: 2–6).

FIGURES 9–12. Cocconeis tsara sp. nov. SEM (Anakao, West coast of Madagascar). SV in external view (9–12), regularly spaced and strongly radiate striae, round-square areolae in irregular axial rows (9, 12), loculate areolae with irregular cross, sometime incomplete (10, arrowhead). Each sector of the loculate areolae externally closed by hymenes with slits, perpendicular to the frame of the sector (10–11, arrows indicate the position of the valve apex). Scale bars = 4 µm (9, 12), 400 nm (10–11).

FIGURES 13–17. Cocconeis tsara sp. nov. SEM (Anakao, West coast of Madagascar). SV in internal view (13–15, 17), with roundelliptical to square foramina (13–15), narrow straight sternum. SVVC with digitate fimbriae locating on each virga (15). SVVC detached, with irregularly arranged fimbriae (14). Each fimbria with an irregular apex (16, arrowheads). Detail of each loculate areola, with an axial primary thin bar (17, arrowheads) and a perpendicular secondary bar on the bottom of the loculate areola (17, arrow indicates the position of the valve apex). Scale bars = 5 µm (14), 4 µm (13, 15), 1 µm (16), 700 nm (17).

FIGURES 18–25. Cocconeis tsara sp. nov. SEM (Anakao, West coast of Madagascar). RV in external view (18–25). Radiate striae, regularly spaced, uniseriate even after the marginal hyaline rim (18). Valve concave with a narrow cingulum (19). Round apices. RVVC open (20, arrowheads), second copula with a ligula (20, twin arrows). Apex void of areolae (21, arrow). Terminal raphe ending roundish. Marginal row of RV areolae enlarged (22, arrow) or axially split (20, arrow). Central area small (23, arrow), RV areolae externally concave (24, arrow). Proximal raphe endings roundish and close to each other (25). Scale bars = 4 µm (19), 3 µm (18), 2 µm (21), 1 µm (20, 22–23, 25), 700 nm (24).

FIGURES 26–29. Cocconeis tsara sp. nov. SEM (Anakao, West coast of Madagascar). RV in internal view (26–29). Raised marginal rim, proximal raphe ending bent in opposite sides, marginal areolae axially split in two parts (26, arrow). RVVC with fimbriae with hammerlike endings (27, arrows) and oblong and smooth papilla (27, framed arrowhead). Hammer-like fimbriae (28, arrowheads), fimbriae each two virgae. No areolae on apex (29, arrow), vestigial fimbriae (29, arrowheads) between each virga, helictoglossa discreet (29). Scale bars = 3 µm (26), 700 nm (28–29), 500 nm (27).

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.

FIGURES 30–33. Cocconeis cf. tsara. SEM (‘Océane’ Chelonia mydas, Moorea, Society Archipelago). SV in external view (30–33). Valve narrow-elliptic. Regularly spaced and slightly radiate striae, round-squared areolae (30, 32), loculate areolae with irregular cross, primary bar sometime double (33, arrowheads), secondary bar sometime arborescent or in zig-zag (33, arrows). Note the vestigial raphe on apices (30, 31, circles). Scale bars = 2 µm (30, 32), 1 µm (31), 500 nm (33).