Cocconeis santandrea 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 : 152-155

Cocconeis santandrea Riaux-Gobin, Witkowski & Bemiasa sp. nov. SEM Figs 34–54

Description: Valves robust, oblong-elliptic. Valve length 16–24 µm (mean 21.5 µm ± 2.0); valve width 12–16 µm (mean 13 µm ± 1.5); L/ W 1.6 ± 0.2; n = 25 (SEM). SV: Convex. Striae radiate and regularly spaced, uniseriate on valve face comprised of larger areolae, becoming multiseriate (up to quadriseriate with alternate areolae, Figs 34, 36 ) and comprised of smaller areolae in the last 1/3 of the valve and mantle ( Figs 34–36 , 40 ). SV striae 9–13 µm (mean 9.6 µm ± 1.0). SV with regular axial rows of slightly squared areolae, externally concave, opening internally by a round foramina ( Figs 40–41 ). Loculate areolae concave, cross-shaped, delineated by four small pegs in axial and transverse positions that extend towards the center of the areola, but never merging in the center of the areola ( Figs 37, 39 , 41 ). Each peg frequently externally ornamented by one or two round warts ( Figs 37, 39 , arrowheads). Hymenes perforated by small pores ( Fig. 39 , white arrow), with a row of short slits only on the periphery. Marginal pyramidal group of small areolae also occluded by hymenes with pores (not shown). Hymenes strongly internally domed at their center ( Fig. 41 , short arrow). SVVC with digitate fimbriae regularly aligned with SV striae ( Figs 36, 38 ). RV: Concave, with a marginal hyaline rim positioned far from the valve margin ( Figs 42, 45 ), striae uniseriate up to the margin and biseriate with alternate areolae afterwards, with a specific ‘ears of wheat’ pattern ( Fig. 43 , arrowhead). Note one row of round and larger areolae close to the hyaline area ( Figs 43 – 44 , arrows). RV striae 15–18 in 10 µm (mean 17 ± 0.9), composed of regularly spaced, small and round areolae. Areolae present on apices. Areola hymenate occlusions with radiate irregular slits ( Fig. 51 ). Raphe narrow, straight. Central area small ( Fig. 46 ). External proximal raphe endings closely spaced, straight, slightly expanded ( Fig. 46 ), internally deflected in opposite directions ( Fig. 49 ). Helictoglossae almost straight ( Fig. 48 ). Internal sub-marginal raised rim, with small and regular bumps facing each stria ( Fig. 50 ). RVVC with fimbriae irregularly spaced, and joined at their terminus, creating large rectangular fenestrae ( Fig. 52 , arrowhead). RVVC papillae oblong with up to 5 furrows ( Figs 53 – 54 , arrowheads).

Holotype, here assigned: SEM stub BM001222256 ( NHM) from the sample Rapa–1.

Type locality: Intertidal turf, Rapa (Austral Archipelago, South Pacific), sample Rapa–1 ( Table 1). Also found in Nuku-Hiva (Marquesas Archipelago) on intertidal red turf, sample NH4–2.

Etymology: The epithet ‘ santandrea ’ refers to the X-shaped (or decussate) cross of St. Andrew.

Habitat: Epipsammic on coral sand grains and epiphytic on short turf from coral reef environments in the South Pacific.

Remarks: Cocconeis santandrea is most similar to C. nosybetiana sharing a loculate SV areola structure with areola subdivided into sectors by pegs. Cocconeis santandrea can be distinguished from C. nosybetiana by the structure of the loculate SV areolae. The decussate areolae of C. santandrea are split into 4 sectors, while those of C. nosybetiana are transversely split into two parts. Cocconeis santandrea is also slightly larger than C. nosybetiana (16–24 µm versus 9–21 µm, respectively), with SV hymenate occlusions with pores in place of short slits in a rhombic pattern. Cocconeis santandrea also exhibits a greater SV stria density (15–18 in 10 µm) compared to C. nosybetiana (13–16 in 10 µm). The valve outline of C. santandrea is oblong-elliptic versus round-elliptic in C. nosybetiana . When describing C. nosybetiana, Riaux-Gobin et al. (2019b) also observed some rare loculate Cocconeis SV valves that were larger in size than other C. nosybetiana valves and possessed decussate areolae (op. cit., figs 27–30). Those depicted specimens from Madagascar were originally believed to be larger forms of C. nosybetiana , but after examination of a greater number of specimens of C. santandrea observed from the Marquesas and Rapa Archipelagos, those larger specimens (referred to as Cocconeis sp. 2 in Tables 2 & 4, Figs 66 , 76–77 ), are now suspected to be C. santandrea . The SV areola of the Madagascar specimens are slightly smaller in diameter than those specimens described in the present study (600 nm versus 800–1200 nm, respectively). The SV areola morphology of C. nosybetiana is now understood to be limited to those forms with areolae that are partially divided into semi-circular areas delimited by 2 or 3 (when close to the sternum), transapically-located pegs (see figs 12–24 in Riaux-Gobin et al. 2019b). Forms with decussately divided SV areolae with 4 apically and transapically pegs are now considered as C. santandrea .

Figures

FIGURE 76. Dichotomous tree. In red the characters and coding permitting to separate taxa.

FIGURE 77. Cladistic analysis.

FIGURES 34–39. Cocconeis santandrea sp. nov. SEM (Marquesas and Rapa Archipelagos). SV in external views. Valve largely elliptical. Regularly spaced, slightly radiate striae, large squared areolae in regular axial rows (34–35), loculate areolae with regular X-cross (decussate cross), striae multiseriate (up to tetra-seriate) on one quarter of the valve (34). Loculate areolae strongly concave in external side (35). Loculate areolae with short regular pegs, often ornamented with warts (37, arrowheads; 39, black arrowheads). Loculate areola hymen with pores (39, arrow). Broken SV showing the SVVC in advalvar view (36). Detail of Fig. 36, with the digitate SVVC fimbriae with undulated basis, RVVC with oblong papillae with furrows (38, arrow). Scale bars = 7 µm (36), 5 µm (34), 2 µm (35, 38), 1 µm (37), 300 nm (39). Long arrow in Fig. 37 indicates position of valve apex.

FIGURES 40–41. Cocconeis santandrea sp. nov. SEM (Marquesas). SV in internal view. Narrow sternum, round foramina (40). Each loculate areola has an internal domed center with pores (41, arrow). Long arrow indicates the position of the valve apex. Scale bars = 5 µm (40), 300 nm (41).

FIGURES 42–46. Cocconeis santandrea sp. nov. SEM (Rapa Archipelago). RV in external view. Regularly spaced and uniseriate radiate striae, small round areolae and marginal hyaline area (42, 45, arrowhead), becoming biseriate with alternate areolae (in ears of wheat) from the rim to the margin (43–44, arrows). Note the row if bigger and deep areolae close to the rim (44, arrow). Central area reduced and proximal raphe endings close to each other (46). Scale bars = 5 µm (42, 45), 1 µm (43–44, 46).

FIGURES 47–54. Cocconeis santandrea sp. nov. SEM (Rapa Archipelago). RV in internal view, with a marginal raised rim with regularly spaced lateral warts facing each marginal biseriate stria (47–48, 50). Helictoglossa simple (48), central area small elliptic (49). RV hymenate occlusions in internal view, with radiate short slits perpendicular to the frame (51). RVVC still in place with irregularly spaced fimbriae, up to each 4 virgae (52, arrowhead). Detail of the papillae with furrows (53–54, arrowheads). Scale bars = 5 µm (47, 52), 2 µm (54), 1 µm (48–50, 53), 500 nm (51).

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.