Cocconeis tortilis Hide. Suzuki, sp. nov. (Figs 2–35)

Type: — JAPAN. Tsuchikata beach (34°04’20”N, 139°28’41”E), Miyake Island, the Izu Islands, Tokyo, collected from surface of Codium intricatum on 3rd July 2012 (holotype! BM 101683, designated here = Figs 2, 4 ; isotype! MTUF-AL-43010, designated here, the Algae Collections of the Museum of Fishery Sciences, Tokyo University of Marine Science and Technology, Japan .

The frustule is heterovalvate. The valve is small, elliptic to oval, and twisted (Figs 2–9), measuring 10.4–17.2 µm in length, 7.2–11.6 µm in breadth. Striae densities at the centre of the valves are 33–45 in 10 µm for the raphid valve (RV), 43–55 in 10 µm for the araphid valve (ARV). The RV is slightly concave (Fig. 12). The raphe is straight, and inclined from the apical axis (Figs 2, 3, 6, 7). The inner raphe lie in a very narrow, but raised, axial area (Fig. 13). The proximal raphe ends are coaxial and slightly-dilated externally (Fig. 15) but internally they are undilated and deflected in opposite directions (Fig. 17). The distal raphe ends are elliptically-dilated externally (Fig. 14) but, internally, terminate in short helictoglossae (Fig. 16, arrow). The central area is rhombic in shape (Figs 6, 7) and extending laterally to the valve margin in one stria’s width (Figs 12, 13, 18, 19). This area is flat on the outer surface (Figs 15, 18, 19) and slightly raised on the inner surface (Figs 13, 17), forming a stauros (Figs 18, 19, asterisks). The striae consist of small round areolae, and are radiate and uniseriate (Fig. 12). Each areola is circular and occluded by a hymen with perforations arranged in a centric array (Figs 20, 21). A submarginal hyaline area is not recognizable; a narrow marginal hyaline area is visible (Fig. 12). The terminal hyaline area is small and elliptical, and extends to the marginal hyaline one (Figs 12, 14).

The ARV is convex (Fig. 22) and twisted (Figs 11, 34). The ARV (Fig. 23) is thicker than the RV (Fig. 13). The sternum, inclined from the apical axis (Fig. 35, dashed line), is a narrow furrow on the outer surface (Fig. 22), and narrow lanceolate internally (Figs 8, 9, 23). In some valves, a vestigial raphe is observed (Fig. 25, arrows). The striae that consist of several loculate areolae (Fig. 24) are radiate and uniseriate (Fig. 22). Each areola is occluded by a hymen located near the outer surface (Fig. 28, arrows), internally, opens by means of a circular to elliptic foramen (Figs 23, 25). The perforations of the hymen are linear, oblique and arranged in a parallel array (Figs 26, 27).

The mature cingulum consists of three girdle bands (Figs 29, 30): a valvocopula and two bands (the second and the third bands), which are both narrower and thinner than the valvocopula. The valvocopula of each valve is open at one pole (Figs 31–33, arrows) and has no fimbriae and areolae; their inner edges are smooth (Figs 31, 33). The second band, adjacent to the valvocopula, is open at the opposite pole (Fig. 30) and possesses a ligula (Fig. 29). The third band is also open (Fig. 29). The valvocopula of each valve is open at a place slightly shifted from one distal end of the raphe or sternum (Figs 31–35, arrows). This is similar to the second and third bands; the disordered of arrangement of striae, known as the “Voigt fault” (Voigt 1956) can be observed here (Figs 29, 30, arrows).

Etymology:—From the Latin tortilis (twisted), referring to the twisted appearance of the valve plane.

Distribution and ecology:— Cocconeis tortilis has been found in the Izu, Daito, and Okinawa Islands affected by the Kuroshio Current, which is a north-flowing warm ocean current on the west side of the North Pacific Ocean, and grows abundantly on the spherical utricles of Codium intricatum in Miyake Island, Japan (Fig. 10, arrowheads) and on the fine branches of Asparagopsis taxiformis in Hachijo Island, Japan (Fig. 11).

Observations:— Cocconeis tortilis has the following characteristics of the genus (after Round et al. 1990):

1) heterovalvar; one valve (RV) with a raphe, the other (ARV) without;

2) valves elliptical or almost circular;

3) RV concave with a correspondingly convex ARV;

4) areolae occluded by hymens with linear perforations;

5) terminal raphe endings without fissures;

6) central raphe endings externally simple or slightly expanded, internally deflected towards opposite sides; and

7) cingulum consisting of a few narrow, non-areolae bands.

Our study suggests that C. tortilis is most similar to C. molesta Kützing var. crucifera Grunow (in Van Heurck 1880 –1885: pl. 30, figs 20–23; Kobayasi & Nagumo 1985, De Stefano et al. 2000), C. dirupta Gregory (1857: 491; Kobayasi & Nagumo 1985), and C. convexa Giffen (1967: 257; Suzuki et al. 2001, De Stefano & Romero 2005), but can be distinguished from each other by several characters (listed in Table 1).

a This study, b Kobayasi and Nagumo (1985), c De Stefano et al. (2000), d Suzuki et al. (2001), e Sar et al. (2003), f De Stefano and Romero (2005), n.d.: no data.

Cocconeis tortilis has the following unique characters:

1. The ARV plane is twisted; and

2. the raphe in the RV and the sternum in the ARV are inclined from the apical axes.

These characters have not been observed in any other species of Cocconeis, and are thus diagnostic for Cocconeis tortilis . C. tortilis can be readily distinguished from C. molesta var. crucifera in having a central area not forming stauros in the RV, in having an areola occluded by two ‘horseshoe shaped’ hymens and a linear sternum of ARV and in having a lower density of the striae on both valves (30, 28– 30 in 10 µm on the RV, 38, 35– 37 in 10 µm on the ARV; Kobayasi & Nagumo 1985, De Stefano et al. 2000, respectively); it can be distinguished from C. dirupta in having a sigmoid raphe of RV, in having a sigmoid sternum of ARV and in having a lower density of the striae on both valves (22 in 10 µm on the RV, 20 in 10 µm on the ARV; Kobayasi & Nagumo 1985); it can be distinguished from C. convexa in having a central area not forming stauros of RV, in having an alveolate areola of ARV and in having a lower density of the striae on both valves (24–28, 30– 34 in 10 µm on the RV, 36–40, 34– 38 in 10 µm on the ARV; Suzuki et al. 2001, De Stefano & Romero 2005, respectively).