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The marine Achnanthales (Bacillariophyceae), with a particular focus on Cocconeidaceae, were investigated in a variety of habitats from the South Province of New Caledonia (Melanesia, Pacific Ocean). Species diversity ranged from species-poor and scarce assemblages on mangrove mud flats to highly diverse and abundant assemblages on short turfs and scrapings of aerial pneumatophores of Avicennia marina. Macroalgae (in place or as sea leash), as well as feces of Acanthopleura gemmata (invertebrate grazer, living on intertidal rocky environments), contained few diatoms, with low diversity. The New Caledonia assemblages show some taxa previously found in the tropical Indian Ocean. Several noteworthy taxa, listed as rare from Polynesia, are not uncommon in New Caledonia, such as Vikingea gibbocalyx and Cocconeis pseudodiruptoides. The notion of endemism concerning marine eukaryotes is approached.

Amphicocconeis ruatara Riaux-Gobin sp. nov. SEM Figs 49–61. Table 1 Description:— SEM. Valves oblong-elliptic to elongate, with slightly rostrate apices (Fig. 51). Axially bent in cingular view. Valve length 15 ± 2 µm, 14–17 µm. Valve width 6 ± 2 µm, 6–8 µm; L/ W 6.2 ± 0.8; n = 54 (SEM) (Table 1). SV convex with a concave narrow sternum (Figs 49–50). One row of large and transapically elongated areolae on each part of the sternum, one marginal row of dense and short areolae and, in between these two sectors, short areolae arranged more or less in quincunx. Often a low crest (or crista marginalis) delineating the marginal row of dense and short areolae, as a sort of short mantle (Figs 49–50, arrow). SV striae parallel in mid-valve, slightly radiate and denser on apices, uniseriate. SV striae 22 ± 3 in 10 µm. SVVC open, narrow (or sometimes larger), composed of short fused fimbriae (27 in 10 µm), separated from each other by an irregularly biseriate row of pores (Figs 52–54), festooned edge. Head pole of the SVVC with very short fimbriae. RV flat, dense striae composed of one narrow macro-areola, short supplementary marginal striae on mid-valve (Fig. 55). Raphe filiform. Proximal raphe endings externally simple and relatively close to each other, central nodule slightly elevated externally and unique for the genus (Figs 55–56, 59). Terminal raphe endings largely hooked on the mantle (Fig. 58). Raphe sternum somewhat elevated internally, with raphe slit slightly undulating. Proximal raphe endings simple and coaxial internally (Fig. 57). Central area small. RV striae 51 ± 6.5 in 10 µm; 42–60 in 10 µm. RVVC open, 30–50% valve width, 51 rows of uniseriate pores in 10 µm (Figs 60–61). Holotype:—Stub12 12/02/2020, BM001222887. Holotype specimen illustrated in Fig. 49. Syntypes:—Whole slide from the sample RAI 20, deposited at NHM (BM 81917 material). Etymology:—The specific epithet ruatara refers to the Ruatara rock (Raivavae Isl.) on the intertidal coastal environment where was first found the new taxon. Locality:—Ruatara rock (‘Rocher de l’Homme’), Raivavae (Austral Islands, South Pacific), RAI 20 and RAI 18 (intertidal red macroalgal turfs on fissures of the rock). Geo localization: S 23° 51.274’; W 147° 39.595’; T°C 23°.7, salinity Remarks:—Some similarities of A. ruatara sp. nov. with A.discrepans (A.W.F.Schmidt) Riaux-Gobin, Witkowski, Ector & Igersheim (Riaux-Gobin et al. 2018: figs 15–16), but with some differences: The axial rows of SV areolae on each part of the sternum are larger on A. ruatara than on A. discrepans, along with (in most cases) the presence of a crista marginalis delineating a narrow mantle on the new taxon. The SVVC in A. ruatara has short fused fimbriae with irregularly uni- to biseriate rows of pores, in contrast with a marginal row of one small pore in A. discrepans. The RVVC of A. ruatara shows dense uniseriate rows of pores (51 in 10 µm), while showing one marginal and elongate pore in A. discrepans (30–31 in 10 µm; Riaux-Gobin et al. 2018: figs 17–20)., Published as part of Riaux-Gobin, Catherine, Garcia, Marinês, Witkowski, Andrzej, Saenz-Agudelo, Pablo, Coste, Michel & Daniszewska-Kowalczyk, Genowefa, 2021, New Amphicocconeis (Bacillariophyta) from Raivavae and Tahiti Islands (South Pacific) and Porto Belo (Brazil), with re-examination of Psammococconeis, pp. 30-54 in Phytotaxa 513 (1) on pages 49-52, DOI: 10.11646/phytotaxa.513.1.2, http://zenodo.org/record/5306935, {"references":["De Stefano, M., Sacchi, U., Totti, C. & Romero, O. E. (2006) Cocconeis distans Gregory and Amphicocconeis debesi (Hustedt) De Stefano comb. nov. (Bacillariophyta), an intricate taxonomical history. Botanica Marina 49: 438 - 449. https: // doi. org / 10.1515 / BOT. 2006.055","Riaux-Gobin, C., Ector, L., Witkowski, A. & Igersheim, A. (2018) Achnanthales from historical Grunow collection in Porto Subzanski, Croatia. Botanica Marina 61 (6): 573 - 593. https: // doi. org / 10.1515 / bot- 2018 - 0045","De Stefano, M. & Marino, D. (2003) Morphology and taxonomy of Amphicocconeis gen. nov. (Achnanthales, Bacillariophyceae, Bacillariophyta) with considerations on its relationship to other monoraphid diatom genera. European Journal of Phycology 38: 361 - 370. https: // doi. org / 10.1080 / 09670260310001612646","Ehrenberg, C. G. (1838) Atlas von Vier und Sechzig Kupfertafeln ze Christian Gottfried Ehrenberg uber Infusionsthierchen. pp. pls I- LXIV. Leipzig: Verlag von Leopold Voss."]}

Amphicocconeis catharinensis Riaux-Gobin & Garcia. SEM, Figs 7–17. Table 1 Description:— SEM.Valves robust, oval-elliptic, with round apices. Valve length 18 ± 3 µm, 14–22 µm. Valve width 11 ± 2 µm, 9–13 µm; n = 10 (SEM) (Table 1). SV: surface convex, striae radiate and regularly spaced, uniseriate, composed of large rectangular areolae. On the valve face, each areola is positioned on the bottom of a depression with opening bordered by digit flaps (Figs 8–9). Areolae are internally closed by hymenes (Figs 10–12). On the mantle, each oblong areola is separated from the next by a granular and concave plain structure of triangular to rectangular shape (Figs 8–9, arrows). Short marginal striae not observed. 11 ± 1 SV stria in 10 µm. SVVC relatively narrow with big pores corresponding to each areola. RV: surface flat, striae radiate, composed of macro-areolae (large along the margin, short along the raphe, with short areolae dispersed in between, Fig. 13). Terminal-distal raphe endings strongly curved on same side (Figs 13, 15). Proximal raphe endings slightly enlarged and curved on same side (opposite to the terminal raphe endings, Fig. 16). 26 ± 2.5 RV stria in 10 µm. RVVC 1/4 of the valve width, with marginal transapical oblong pores corresponding to each macro-alreola (Fig. 17, twin arrowheads). Remarks:—The SV of Amphicocconeis catharinensis was illustrated as being the SV of Psammococconeis disculoides in Garcia (2001: figs 21, 23). The specific ornamentation of the SV mantle, with a concave structure in between each oblong areola (Figs 8–9) seems unique. The RV of A. catharinensis has some similarities with that of A. disculoides and A. debesi (large area with short radiate macro-areolae), but these short areolae in median-area are in very low density in A. catharinensis (Fig. 13), furthermore the SV of A. catharinensis is highly dissimilar to that of Amphicocconeis disculoides (Hustedt) De Stefano & Marino (figs 2–3, 6– 7 in De Stefano & Marino 2003; Figs 1). Type locality:—Zimbros, Peninsula do Porto Belo, Santa Catarina State, Brazil. Holotype:—Stub 4 19/06/2020, BM001222889. Holotype specimen illustrated in Fig. 7. Etymology:—The epithet refers to the location of the type habitat of this taxon in Brazil (locality Santa Catarina State).

Amphicocconeis cf. mascarenica Riaux-Gobin & Compère, 2011 (Figs 62–67). Table 1 Remarks:—The measurements and features of the specimens from the Pacific islands (Raivavae, Tahiti and Moorea Isls, Table 1) are close to those from the Mascarenes (Riaux-Gobin et al. 2011: tabl. 2), i.e., with valvocopulae with regularly biseriate rows of simple pores. This taxon is dominant on the Society Archipelago (Tahiti and Moorea Isls) as well as in the Austral Archipelago (Raivavae Isl.). In Tuamotu and Marquesas Archipelagos, Amphicocconeis are less abundant, but with, here again, A. cf. mascarenica as dominant taxon (pers. obs.)., Published as part of Riaux-Gobin, Catherine, Garcia, Marinês, Witkowski, Andrzej, Saenz-Agudelo, Pablo, Coste, Michel & Daniszewska-Kowalczyk, Genowefa, 2021, New Amphicocconeis (Bacillariophyta) from Raivavae and Tahiti Islands (South Pacific) and Porto Belo (Brazil), with re-examination of Psammococconeis, pp. 30-54 in Phytotaxa 513 (1) on page 52, DOI: 10.11646/phytotaxa.513.1.2, http://zenodo.org/record/5306935, {"references":["Riaux-Gobin, C., Compere, P. & Al-Handal, A. Y. (2011) New Amphicocconeis species off Mascarenes (Western Indian Ocean) and related taxa. Diatom Research 26 (2): 175 - 188. https: // doi. org / 10.1080 / 0269249 X. 2011.597958"]}