Cocconeis subantarctica sp. nov. from Kerguelen Archipelago (Austral Ocean) and comparison with Cocconeis stauroneiformis (W.Smith) Okuno

Figure 2a-i

The re-examination of two samples from Kerguelen Is. with SEM enabled us to examine several isolated specimens, as well as a large monospecific aggregate (clump) of this species (Fig. 1, possibly a fecal pellet of meiofauna, with diverse diatom debris and no macroalgae which would indicate that these specimens are epiphytic).

LM description

The frustules are round-elliptic to subdiscoid (Fig. 2a-b, d-e) and oblong-elliptical in rare, smaller specimens (Fig. 2c, f, i). Valves small (12-18 μm in length, 8-15 μm in width, 12-14 RV striae in 10 μm, 11-14 SV striae in 10 μm, LM and SEM, n = 80, Table 2). SV sternum (axial area) narrow and straight. SV and RV striae with more or less the same stria density. Striae equidistant and radiate. No real stauros present (only one virga larger on one side of both valves (Fig. 2d-e, h arrowheads). SV areolae are not arranged along axial rows (Fig. 2d-e, Fig. 3a), while the RV areolae are often arranged along axial rows (Fig. 2g-h). Note a marginal row of larger SV areolae with a narrow hyaline marginal rim (Fig. 2c-f).

Figure 3a-g

Table 2

Morphometrics and features of 1) C. subantarctica sp. nov. (Kerguelen Is., present study), 2) C. stauroneiformis (cf. Riaux-Gobin & Romero 2003, Kerguelen Is.) and 3) C. stauroneiformis sensu Romero (1996, diverse origin)

	Cocconeis subantarctica sp. nov. present study n = 80	Cocconeis stauroneiformis (W.Smith) Okuno sensu Riaux-Gobin & Romero (2003)	Cocconeis stauroneiformis (Rabenh.) Okuno sensu Romero (1996)	Cocconeis scutellum var. posidoniae M.De Stefano, D.Marino & L.Mazzella (2000)
Valve face	subround to elliptic	elliptic	elliptic to elliptic-lanceolate	elliptic
Length (µm) min-max	12-18	15-32	9.6-36	-
	14.6 ± 1.0
Width (µm) min-max	8-15	8-21	5.5-27	-
	10.3 ± 1.3
SV stria density min-max	11-14	8-9	6-11	12-14
	12.6 ± 0.9
	uniseriate, rotate and reniform hymenate occlusions	wavy transapical rows, externally complex, internally uniseriate	externally tetra- to pentaseriate, internally uniseriate	uniseriate, transapical rows of areolae with axial bar, rotate or not
RV stria density min-max	12-14	8-10	6-12	18-23
	13 ± 0.6
	uniseriate, hymenate reniform occlusions	biseriate in mid-valve, complex on the margin	biseriate, tetra- to pentaseriate at the margin	uniseriate
SV sternum	straight, relatively large	straight, narrow, externally depressed (thickened internally)	straight, narrow, concave	straight, narrow
SV fascia	one virga larger on one side	absent	absent	absent
Marginal SV areolae	transapical to quadrangular group of hymenate reniform occlusions	undifferentiated from the other areolae	-	biseriate to triseriate occlusions
SV marginal rim	often present	absent	absent	absent
SV internal virgae	large, flat	thickened	elevated	narrow, not elevated
SVVC	with short undulations, possibly open at both ends	short fimbriae	reduced fimbriae, open at both ends	digit fimbriae
RV fascia	often assymetrical	more or less wide	present	absent
RV marginal rim	present	robust	present	present
Marginal RV areolae	transapical and complex large group of concentric hymenate reniform occlusions	apically elongated group of hymenate reniform occlusions	group of hymenate reniform occlusions	striae biseriate on the mantle
RVVC	open, without fimbriae	-	short fimbriae, open at one apex	fused fimbriae
Biogeography	Kerguelen Is.	Kerguelen Is.	diverse origin	Ischia I. (Italy)

x ± σ (mean and standard deviation when available) - (not specified)

SEM description

SV: Slightly convex (Fig. 3c), regularly spaced striae (except one mid-valve virga often larger on one side, Fig. 3a, e arrowheads), uniseriate and radiate. Areolae (ca. 21.6 in 10 μm) with complex hymenate pore occlusions (appearing as a pierced rota in the more simple cases, but with hymenes, Fig. 3f arrowhead): one or two round openings are surrounded by two or more reniform openings (Fig. 3f), and all of these openings are closed by internal hymenes with radiate slits (Fig. 4d arrowhead). Each stria ends marginally by a transapically-elongate and more complex areola (Fig. 3g), often separated from the rest of the stria by a marginal hyaline area (Fig. 3b-c arrows). The occlusions appear similar internally and externally (Fig. 4b, d). In the internal view, the virgae are weakly silicified (Fig. 4 d arrow; see also pl. 30, fig. 3 in Riaux-Gobin & Romero 2003). No areolae on apices (Fig. 3a-b, e). Sternum straight and relatively narrow, externally not depressed (Fig. 3c). Sternum valve valvocopula (SVVC) possibly open on both apices (Fig. 3b arrowheads), with low edge’s undulations but no real fimbriae (Fig. 4c arrowhead). Wide cingulum composed of two open valvocopulae and one or two supplementary open bands with ligulae (Fig. 3d arrowhead).

Figure 4a-d

RV: Thin, flat to concave, striae uniseriate (mean 12.9 in 10 μm, n=16), regularly spaced, except for the two median striae, slightly more distant (one virga larger), but often only on one side of the valve (Fig. 5a-b arrowhead), so that there is no real stauros or fascia (see LM, Fig. 2g-i). RV areolae (ca. 21.3 in 10 μm) with a structure slightly more complex than that of the SV (Fig. 5c), with several reniform concentric hymenate occlusions with slits. The marginal row of elongate and complex areolae (complex hymenate perforated plates with marginal pegs, Fig. 5d arrowhead), separated from the rest of the stria by a well-marked hyaline rim (Fig. 5b). In the internal view, the rim has no bumps (Fig. 6a). Areolae are roughly arranged along axial rows (Fig. 6c). No areolae on apices (Fig. 5c framed arrow). Raphe filiform and straight. External proximal raphe endings relatively close to each other and straight (Fig. 5c arrowhead), internally deflected on opposite sides (Fig. 6b arrowheads). Terminal raphe endings well distant from the margin (Fig. 5b), internally terminating in subtle and straight helictogossae (Fig. 6a arrowhead, d). Axial area narrow. Raphe valve valvocopula (RVVC) narrow, open (Fig. 6d arrows) and lacking fimbriae (Fig. 6a twin arrowheads).

Figure 5a-e

Figure 6a-d

Holotype

Specimen on stub ‘180915-1’ sent to NHM (BM 101 806 material). Holotype illustration: SEM Fig. 3a.

Isotypes

Slides mounted with the same material as the stub: BM 101 806 (National History Museum, London, UK), slide SZCZ 16662(10) in the collection of A. Witkowski (the Faculty of Geosciences, Szczecin, Poland) and slide KER11 in the collection of C. Riaux-Gobin (CRIOBE-CNRS, USR 3278, Perpignan, France).

Type locality

‘Pointe Bizet’ (Île Longue, Kerguelen Main Island). Collector C. R.-G., January 18 1991.

Biogeography

intertidal to subtidal sheltered muddy Subantarctic environment.

Etymology

The specific epithet refers to the Subantarctic zone, which comprises the Kerguelen Archipelago.

Taxonomic remarks

The SV of Cocconeis subantarctica sp. nov. shares some similarities with that of Cocconeis scutellum var. posidoniae M.De Stefano, D.Marino & L.Mazzella (De Stefano et al. 2000: figs 72-79), but the marginal areolae of the latter are multiseriate as in C. scutellum var. scutellum, with no hyaline rim. In our taxon, the marginal areolae are a complex arrangement of hymenate reniform structures, often delineated by a hyaline rim (see description). Furthermore, the RV structure of our taxon is different from that of C. scutellum var. posidoniae (see De Stefano et al. 2000: figs 80-86). The re-examination of the Kerguelen material enabled the RV of our new taxon to be described and revealed a high degree of similarity with Cocconeis stauroneiformis (W.Smith) Okuno (see below, history and detailed comparison with C. subantarctica sp. nov.)

Cocconeis scutellum var. stauroneiformis W.Smith (1853/1852: p. 22, ‘β Nodule dilated into a stauros, Supp. Pl. 30. fig. 34 β’, reproduced in Fig. 7a; no type, no locality) is the basionym of C. stauroneiformis (W.Smith) Okuno. For comparison, Fig. 7b refers to Cocconeis scutellum Ehrenberg, as illustrated in Smith (1852: pl. 3, fig. 34). Even if the above-mentioned variety β is not implicitly named and if the diagnosis is reduced to the notification of the presence of a stauros, the illustration (Ref. cit. fig. 34β, Fig. 7a) is considered the holotype (McNeill et al. 2012: art. 8.1). The original nomenclatural name C. scutellum var. stauroneiformis W.Smith is used in Van Heurck (1880: pl. 29, figs 10-11, reproduced in Fig. 7 c, clearly referring to W. Smith, thus not as a homotypic synonym), Cleve (1895), Peragallo & Peragallo (1897-1908: pl. 4, fig. 4, reproduced in Fig. 7d), Hustedt (1933-1959: fig. 792). The name afterwards transferred to C. stauroneiformis (W.Smith) Okuno (1957: p. 217-221, figs 2a-c), used in Witkowski et al. (2000), Riaux-Gobin & Romero (2003) and Lee et al. (2012), while e.g., Rivera (1974), Romero (1996), De Stefano et al. (2000) and Sar et al. (2003) erroneously refer to C. stauroneiformis (Rabenhorst) Okuno. Hustedt (1933-1959), and afterward VanLandingham (1967-1979) erroneously refer to C. paniformis J.-J.Brun. as a synonym of C. scutellum var. stauroneiformis W.Smith [Schmidt et al. 1874-1959: pl. 189, figs 16-20, reproduced in Fig. 7e; Schmidt’s caption refers to C. panifomis Brun and only mentions that for several of these illustrations (ref. cit.: figs 16-19), a stauros is clearly indicated, but the name C. scutellum var. stauroneiformis is not mentioned]. See the illustration in Montgomery (1978: pl. 62, figs E-H; pl. 63, figs 4A-B) and illustration of C. caribensis Romero et Navarro (1999), but C. paniformis remains a valid taxon with priority over C. caribensis. It can be noted that C. tuamotuana Riaux-Gobin, Compère & W.Jordan (2015) has no stauros but belongs to the same group as the above-mentioned tropical diatoms. In addition, Cocconeis scutellum var. stauroneiformis Rabenhorst (1864: p. 101) is a heterotypic synonym, with a short diagnosis (no iconography, no measurements, no locality) referring to Roper (1858: pl. III, fig. 9, C. scutellum var. γ, reproduced in Fig. 7f). Following Roper (ref. cit.)… ‘I think it better to consider it as a variety, though (…) it should be classed as a distinct species’. The latter taxon was later described as C. binotata Grunow (1863: p. 145, pl. 4, fig. 13a, b), Orthoneis binotata (Grunow) Grunow (1867: p. 15) and finally transferred to Mastogloia binotata (Grunow) Cleve [1895: 148, Orthoneis (Stictoneis) binotata Grunow 1863]. Illustrated in Van Heurck (1880-1885: pl. 28, fig. 7) as Orthoneis binotata Grunow, annotated as C. scutellum var. γ Roper. The latter taxon is not a Cocconeis.

Figure 7a-g

None of the available descriptions of Cocconeis stauroneiformis (from the oldest to the most recent one, see below) include observations of a type, instead they all refer to personal materials. The older illustrations refer to small and oblong-elliptical valves (e.g., Van Heurck 1880-1885: pl. 29, figs 10-11, reproduced in Fig. 7c and Peragallo & Peragallo 1897-1908: pl. 4, fig. 4, reproduced in Fig. 7d), with an SV showing axial rows of large quadrangular areolae, and an RV with a well-developed stauros, a marginal rim and a marginal row of larger structures. Several recent studies contributed to a better understanding of C. stauroneiformis, due to SEM examinations: the SV structure is unique with complex areolae, composed of three-dimensional structures (named ‘loculi’ by Okuno 1957: pl. 6, fig. 2b-c). These ‘loculi’ show reniform apertures closed by hymenes. These complex SV areolae open externally by multiple small pores, roughly grouped in quadrats arranged along axial rows (Romero 1996: fig. 29; De Stefano et al. 2000: fig. 88), while internally via quadrangular openings, with each stria bordered by strong virgae (Romero 1996: fig. 28; Riaux-Gobin & Romero 2003: pl. 15, fig. 1; Sar et al. 2003: fig. 54). The RV striae, described by Romero (1996) as biseriate, are composed of complex sub-quadratic areolae with reniform apertures closed by hymenes (Romero 1996: fig. 36; Lee et al. 2012: fig. 37; De Stefano et al. 2000: fig. 92). A stauros is well marked only on the RV (see all quoted references).

Although the ultrastructure of C. subantarctica RV has some similarities with that of C. stauroneiformis, there are several important differences. For example, the RV striae in C. stauroneiformis are biseriate (cf. Romero 1996: table 3, figs 34-35, 37; Table 2) with complex areolae, while in the new taxon, uniseriate and externally less finely ornamented. A fascia is well marked in C. stauroneiformis, while more discrete or asymmetric in C. subantarctica. In C. stauroneiformis, there are always RV areolae on apices (e.g., in De Stefano et al. 2000: fig 91; Lee et al. 2012: fig. 37), while the apical areas in C. subantarctica are void. In C. stauroneiformis, the marginal complex RV areolae, separated from the rest of the stria by a hyaline rim, are short and more or less axially elongate (Romero 1996: fig 36; De Stefano et al. 2000: fig 92), while they are large and transapically elongate in the new taxon.

The most significant difference between the two taxa is their external SV ultrastructure: C. stauroneiformis shows a complex net of small poroids (e.g., Riaux-Gobin & Romero 2003: pl. 16, fig. 2), grouped along a quadratic pattern (Romero 1996: fig. 26; De Stefano et al. 2000: fig. 88; Riaux-Gobin & Romero 2003: pl. 15, fig. 1, 4), with virgae that are almost undifferentiated. The marginal SV areolae are not visibly different from other areolae on the valve. These quadrangular complex areolae are axially arranged and are internally uniseriate. In C. subantarctica, the SV ornamentation (particularly for the external view) is less complex than in C. stauroneiformis: the striae show areolae with more or less simple reniform poroids, as a kind of pierced hymenate rotae (Fig. 3f arrowhead). In C. subantarctica, the virgae are large and unornamented, and the marginal SV areolae are composed of a more complex arrangement of reniform poroids than in C. stauroneiformis. In C. subantarctica, a marginal SV hyaline area (or marginal rim), often clearly identifiable, separates the large and complex proximal areola from the rest of the stria (while this feature is not observed in C. stauroneiformis).

Another distinctive feature between the two latter taxa is the presence of strong and elevated internal SV virgae in C. stauroneiformis (Romero 1996: fig. 28), while in C. subantarctica, the virgae are large and flat (Fig. 4a-b, Table 2). It can also be noted that this taxon is basically smaller than C. stauroneiformis and the stria densities in both valves are higher on C. subantarctica than in C. stauroneiformis (Table 2). Finally, a large percentage of larger C. subantarctica specimens is characterized by subdiscoid shape (Fig. 2a-b, d-e).

Cocconeis subantarctica sp. nov. does not belong to the C. scutellum ‘complex’. Although there are some similarities for smaller and most elliptical specimens, as shown in the illustration of Cocconeis scutellum Ehrenberg f. parva in Peragallo & Peragallo (1897-1908: pl. 4, fig. 3, reproduced in Fig. 7g), the SV sternum is more elliptical in the latter taxon. This observation reflects the important role of SEM in the study of small diatoms. To date, Cocconeis subantarctica sp. nov. has only been found in the type locality (Kerguelen Is.), and further studies are needed to clarify the biogeography of this taxon.