Cocconeiopsis discoides Riaux-Gobin & Witkowski, 2012

Riaux-Gobin, Catherine & Witkowski, Andrzej, 2012, Small-sized and discoid species of the genus Cocconeiopsis (Bacillariophyta) on Holothuria atra (Juan de Nova, Mozambique Channel), Phytotaxa 54 (1), pp. 43-58 : 51-55

Cocconeiopsis discoides Riaux-Gobin & Witkowski , sp. nov. ( Figs 21–28, 29–31; Table 1)

Frustula solitaria. Valvae minutae, rotundatae et planae. Striae radiantes (43 + 8.3 in 10 µm), aequidistantes per totam valvam, cum intercalaribus brevibus striis in media valva ad marginem. Virgae elevatae. Areolae minutae, rotundatae, externe hymenio modice concavo clausae, cum brevibus marginalibus radiantibus rimulis. Una marginalis series areolarum hippocrepicarum ("horse-shoe shaped"). Area axialis regularis, angusta et recta. Externe visa, extrema centralia raphis approximata et in oppositis directionibus deflecta. Extrema terminalia raphis simplicia, rotundata et longinqu ō ab apicibus posita. Area centralis mediocris. Habitat coralloides sedimentum, ad oras Juan de Nova Insulae

Cocconeiopsis discoides is very similar to C. juandenovensis except in the following features: 1) slightly larger dimensions; 2) Length/width ratio is lower (more disk-shaped); 3) it has a higher stria density (average 42 in 10 µm as compared to 37.4, on valve margin); 4) striae are less strongly radiate (average. 55 degree angle with raphe, as compared to 48 degree); 5) areola hymen are strongly depressed ( Figs 26–28) are situated on one side of the virgae ( Figs 24, arrows); and 6) the horse-shoe shaped marginal areolae are very irregular and not separated from the rest of the stria by a hyaline area ( Figs 26–28). The cingulum width is relatively low and composed of several bands, apparently un-perforated ( Fig. 32 ) and quasifract ( Figs 28, 31). All other features are similar to those of Cocconeiopsis juandenovensis , in particular the bilateral valve symmetry (even if a certain asymmetry can be noticed in LM, Fig. 29).

On the unique internal view observed ( Figs 32–33 ), helictoglossae are very slightly salient ( Fig. 33 , arrow), central raphe endings are slightly curved in opposite direction ( Fig. 33 ), the virgae are marginally elevated, with sort or pearls. One pearl ( Fig. 32 , long arrows) is also present in between each first areolae of the stria ( Fig. 32 , short arrows). Only 13 specimens photographed. Not found in "JDN2-3 sediments and macroalgae".

Dimensions: 7.5–9.7 (8.8 + 0.7) µm long; 6.6–9.5 (8.1 + 0.7) µm wide; length/width: 1.1 + 0.04; stria density: 45–52.9 (43 + 8.3) striae in 10 µm on median part of valve; 32.5–44.7(40.6 + 3.3) striae in 10 µm near valve margin (SEM observed specimens n = 13).

Type:— SCATTERED ISLANDS. Juan de Nova : Juan de Nova lagoon, Holothuria atra scraping, 17° 02.797' S, 42° 43.811' E, (holotype Fig. 21, original picture SEM "160710-15" (collection C. R.-G.); isotypes BM 101632, BRM ZU8 / 34, AW 18110 (collection A. W.) and JDN1 (collection C. R.-G.) .

Habitat and distribution:—Marine (shallow lagoon of Juan de Nova Island), found on Holothuria atra teguments, very rare (during average 70 hours SEM devoted to "JDN2-3" samples, upon which "JDN2 Holothuria atra ", only 13 valves were found and examined), only found on "JDN2 Holothuria atra ".

Etymology:—The specific epithet discoides (from the Greek ƖσκοεƖδήσ: disk-shaped) refers to the shape of the valve.

Observations:—There are certain similarities of Cocconeiopsis discoides and C. orthoneioides ( Yamaoka et al. 2012) ; the latter is larger, with less radiate striae, lower stria density (18–20 in 10 µm), well identifiable perforations on the girdle bands and areola hymen with marginal short slits ( Yamaoka et al. 2012), different from the structure observed in Cocconeiopsis discoides .

(?) Cocconeiopsis sp. ( Figs 35, 43–47; Table 1)

Cells solitary, disc-like, always lying in valve views. Frustule thin, very delicate. Valves circular, very shallow, without distinct mantle. Striae radiate, alternatively shortened near central area, becoming biseriate near margin ( Figs 34–38, 40, 42). A marginal reticulate silica structure underlines proximal part of striae, with a kind of digitiform thin irregular fimbriae ( Figs 37, 38, 42). The latter structure may be part of valvocopula but that is yet unsubstantiated. Axial area narrow, straight. Silica thickenings present along raphe slits on axial area and along virgae near centre of valve ( Figs 39, 41). Central area not expanded ( Figs 39, 41). Areola hymen details not discernible ( Fig. 39), possibly macroareolae with plain hymenes. External central raphe endings set apart, very slightly hooked in opposite direction ( Fig. 39, arrows); terminal raphe endings simple and distant from margin ( Fig. 37, arrow). Numerous narrow copulae ( Figs 37, 38, 40, 42), several, all open ( Fig. 38, short arrows indicated copula endings; long arrow indicates ligula). Internal view ( Figs 45–47 ), helictoglossae very slightly salient ( Fig. 47 , arrows), marginal rim ( Fig. 46 , long arrow), and possibly a row of regular minute pores on girdle band ( Fig. 46 , short arrows).

Measurements: 10.4–14.2 (12.7 + 1.4) µm long; 9.2–13.8 (11.9 + 1.9) µm wide length/width: 1.1 + 0.08; stria density: 32.5–40.7 (36.4 + 4.5) striae in 10 µm on median part of valve; 32.5–36.6 (33.9 + 1.8) striae in 10 µm on valve margin (n = 12).

Habitat and distribution: — Very rare on Holothuria atra teguments, in the shallow lagoon of Juan de Nova Island. Not found on adjacent sediments or macroalgae from the same site.

Observations:—Rare in sample "JDN2 Holothuria atra " (only 12 specimens photographed). Specimens are slightly larger than C. juandenovensis ( Table 1) and more discoid (length/width 1.1 as compared to 1.3). There are some similar features to C. discoides (e.g. the same length/width) but the valves have larger dimensions (12.6 µm as compared to 8.6 µm) and lower stria density (36.4 as compared to 45). Furthermore, a very specific marginal structure and indistinct striae characterise? Cocconeiopsis sp. Some specimens are slightly asymmetrical ( Fig. 36, 43–44, 47 ), as with the RV of Anorthoneis . Nevertheless, no SV was found matching the RV valves, justifying the temporary classification of this taxon under? Cocconeiopsis . Formal description requires more details, particularly concerning the areola (or macroareola) occlusion.

Figures

FIGURES 32–33: Cocconeiopsis discoides sp. nov. (SEM) from "JDN2 Holothuria atra". Internal views. Fig. 32: detail of Fig. 33, cingulum composed of several low bands, apparently un-perforated, virgae marginally elevated and ornamented by pearls, presence of a pearl (long arrows) between each marginal areola (short arrows). Fig. 33: Complete valve with helictoglossae (arrow) and radiate striae. Scale bars = 3 µm (Fig. 33); 1 µm (Fig. 32).

FIGURES 45–47: (?)Cocconeiopsis sp. (SEM) from "JDN2 Holothuria atra". Internal views. Fig. 45: Valve broken along the raphe (arrows). Fig. 46, detail of Fig. 45, marginal rim (long arrow) with fimbriate ornamentation, minute pores on a cingular band (small arrows). Fig. 47: Valve slightly asymmetrical with helictoglossae (arrows) and marginal fimbriate ornamentation. Scale bars = 4 µm (Fig. 47); 3 µm (Fig. 45); 0.5 µm (Fig. 46).