Cocconeiopsis juandenovensis 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 : 46-51

Cocconeiopsis juandenovensis Riaux-Gobin & Witkowski , sp. nov. ( Figs 2–20 , SEM; Table 1)

Frustula solitaria. Valvae minutae, rotundatae ad ellipticae et planae. Striae radiantes (42.4 + 5.7 in 10 µm), aequidistantes per totam valvam, cum intercalaribus brevibus striis in media valva ad marginem. Virgae moderate elevatae. Areolae minutae, irregulariter rotundatae, externe plano hymenio 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 et longinqu ō ab apicibus posita. Area centralis rotundata et mediocris. Copulae complures et angustissimae. Habitat coralloides sedimentum, ad oras Juan de Nova Insulae.

Cells (frustules) solitary, ellipsoid to disc-like, always lying in valve views. Valves generally elliptical ( Figs 2, 3, 5 ) to almost circular ( Fig. 4 ), very shallow, without proper mantle.

External view: Uniseriate transapical striae, strongly radiate (avg. 48 degree angle to raphe), alternatively shortened near central area ( Figs 6, 7 ), with one marginal row of horse-shoe shaped areolae ( Figs 11–13 ), separated from rest of stria by a marginal hyaline rim ( Fig. 11 , arrow). Virgae very slightly elevated ( Fig. 9 ). Areola hymen very slightly (or not) depressed, with short regular radial slits ( Fig. 13 ). Areolae very distinct near raphe ( Fig. 9 , arrows) and central area ( Fig. 8 ), but more indistinct on remainder of valve surface ( Fig. 12 , arrows). Central area round and small ( Fig. 7 ), axial area narrow and straight. External central raphe endings well apart, hooked in opposite directions ( Figs 6 , 8 ), terminal raphe endings simple, expanded and well apart from apex. Girdle (average 500 µm high) composed of several narrow copulae, probably open ( Figs 14, 15 , arrows), apparently without perforations ( Figs 14, 15 ).

Internal view: raphe slightly elevated, helictoglossae very slightly salient ( Fig. 16 , arrows; Figs 17–19 ), marginal row of horse-shoe shape areolae depressed, elevated virgae ( Fig. 17 ). A slightly elevated rim separates horse-shoe shape areolae from remainder of valve ( Fig. 17 , long arrow). Valvocopula margin not fully observed, but appears undulated ( Fig. 20 , arrows) or ornamented with short fimbriae ( Fig. 5 , arrows). A

reticulate structure, geometric and regular, covers several valves ( Figs 18, 19 , short arrows: artefact due to the cytoplasm dehydration?).

Dimensions: 6.4–9.4 (8.2 + 0.6) µm long; 4.6–7.7 (6.3 + 0.5) µm wide; length/width: 1.3 + 0.08; stria density: 35–53 (42.4 + 5.7) striae in 10 µm on median part of valve; 32.6–48.8 (36.9 + 4.0) striae in 10 µm near valve margin; 41 areolae in 10 µm (SEM observed specimens n = 39).

Type:— SCATTERED ISLANDS. Juan de Nova : Juan de Nova lagoon, Holothuria atra scraping, 17° 02.797' S, 42° 43.811' E, (holotype Fig. 5 , original picture “SEM 240212-12” (in collection of C. R.-G.); isotypes BM 101632 BRM ZU8 / 34, AW18110 (collection A.W.), and JDN1 (collection C. R.–G.) .

Habitat and distribution:—Marine, shallow lagoon of Juan de Nova Island, relatively rare on Holothuria atra teguments, not found on nearby sediment or macroalgae of the site.

Etymology:—The specific epithet refers to Juan de Nova Island, where the species was discovered.

Observations:—Several characteristics of Cocconeiopsis juandenovensis match those for the genus Cocconeiopsis : symmetry of the valve, raphe straight with endings simple and bulbous, terminal raphe endings well apart from the margin, numerous cingular bands, shallow mantle, valve elliptic to disk—shape, except it was not possible to observe the perforations on the girdle bands. There are some similar features that are shared with the genus Cavinula ( Round et al. 1990) , i.e. Cavinula kerguelensis Riaux-Gobin & Compère (2004: 61) , from Kerguelen Islands (Indian Ocean), can be compared to Cocconeiopsis juandenovensis : the valves of C. kerguelensis are larger, stria density is lower (average 25 in 10 µm), a row of perforations is easily identified on one girdle band, the raphe is irregular and the areola structure is completely different ( Riaux-Gobin & Compère (2004).

The valves of Cocconeiopsis juandenovensis bear some similarity to the raphe valve (RV) of Anorthoneis minima Foged (1975: 12) , described and illustrated from the Tanzania coast ( Foged 1975: pl. 7, figs 14 & 15; Table 3). In contrast to A. minima , Cocconeiopsis juandenovensis has a well-defined valve symmetry, higher stria density (RV: 35–53 in 10 µm as compared to 20–22 respectively), and its terminal raphe endings are well apart from the margin. A few other small species of Anorthoneis , such as A. pulex Sterrenburg (1987: 16) , A. vortex Sterrenburg (1987: 16) and A. tenuis Hustedt (1955: 15) have RV with a large central area that is not comparable to Cocconeiopsis juandenovensis . Furthermore, the species of Anorthoneis observed in "JDN2-3" (which has yielded a new taxon to be described elsewhere), have RV with biseriate marginal areolae which are not horse-shoe shaped as in Cocconeiopsis juandenovensis .

Figures

FIGURES 2–7: Cocconeiopsis juandenovensis sp. nov. (SEM) from "JDN2 Holothuria atra". Figs 2–5: Fig. 5: illustration of holotype specimen). External views of valve. Figs 6, 7: details of central area. Note areolae well identifiable at the end of each stria (long arrows in Fig. 7) and difficult to observe, or absent, on stria (Fig. 7, short arrows). Scale bars = 3 µm (Figs 2, 4–5); 2 µm (Fig. 3); 1 µm (Fig. 7); 0.4 µm (Fig. 6).

FIGURES 8–15: Cocconeiopsis juandenovensis sp. nov. (SEM) from "JDN2 Holothuria atra". Fig. 8: External view of central raphe endings, curved in opposite side. Fig. 9: Note elevated virgae and well identifiable areola at each end of stria. Fig. 10: External view of one apex and terminal raphe ending. Fig. 11: marginal row composed of one areola toped by a horse-shoe shaped marginal areola. This marginal group of areolae is well separated from the rest of the stria by a hyaline rim (arrow). Fig. 12: Note short slits randomly scattered on stria with no well identifiable areolae. Fig. 13: Detail of horse-shoe shaped marginal areolae (arrow). Figs 14, 15: cingulum composed of several narrow open bands (arrows). Scale bars = 0.6 µm (Fig. 15); 0.5 µm (Figs 8–12); 0.4 µm (Fig. 14); 0.3 µm (Fig. 13).

FIGURES 16–20: Cocconeiopsis juandenovensis sp. nov. (SEM) from "JDN2 Holothuria atra". Internal views. Fig. 16: Complete valve with helictoglossae (arrows). Fig. 17: Tilted apex of the valve illustrated in Fig. 16. Note virgae (short arrows), marginal rim (long arrow), and marginal row of areolae. Figs 18, 19: Note reticulate structure (short arrows) and marginal rim (long arrow). Fig. 20: Detail of edge of valvocopula, slightly undulated (short arrows). Scale bars = 2 µm (Fig. 16); 0.7 µm (Fig. 17); 0.4 µm (Figs 18–20).