Cocconeis santandrea Riaux-Gobin, Witkowski & Bemiasa 2021, sp. nov.
Creators: name: Riaux-Gobin, Catherine
name: Frankovich, Thomas
name: Witkowski, Andrzej
name: Agudelo, Pablo Saenz-
name: Esteve, Peter
name: Ector, Luc
name: Bemiasa, John
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Chromista
Bacillariophyta
Bacillariophyceae
Achnanthales
Cocconeidaceae
Cocconeis
Cocconeis santandrea

description: 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.
description: Published as part of 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, pp. 145-169 in Phytotaxa 484 (2) on pages 152-155, DOI: 10.11646/phytotaxa.484.2.1, http://zenodo.org/record/5421470