Cocconeis tortilis Hide. Suzuki, 2014

Cocconeis tortilis Hide. Suzuki , sp. nov. ( Figs 2–35 View FIGURES 2–11 View FIGURES 12–21 View FIGURES 22–28 View FIGURES 29–35 )

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 View FIGURES 2–11 GoogleMaps ; isotype! MTUF-AL-43010 , designated here, the Algae Collections of the Museum of Fishery Sciences, Tokyo University of Marine Science and Technology, Japan GoogleMaps .

The frustule is heterovalvate. The valve is small, elliptic to oval, and twisted ( Figs 2–9 View FIGURES 2–11 ), 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 View FIGURES 12–21 ). The raphe is straight, and inclined from the apical axis ( Figs 2, 3, 6, 7 View FIGURES 2–11 ). The inner raphe lie in a very narrow, but raised, axial area ( Fig. 13 View FIGURES 12–21 ). The proximal raphe ends are coaxial and slightly-dilated externally ( Fig. 15 View FIGURES 12–21 ) but internally they are undilated and deflected in opposite directions ( Fig. 17 View FIGURES 12–21 ). The distal raphe ends are elliptically-dilated externally ( Fig. 14 View FIGURES 12–21 ) but, internally, terminate in short helictoglossae ( Fig. 16 View FIGURES 12–21 , arrow). The central area is rhombic in shape ( Figs 6, 7 View FIGURES 2–11 ) and extending laterally to the valve margin in one stria’s width ( Figs 12, 13, 18, 19 View FIGURES 12–21 ). This area is flat on the outer surface ( Figs 15, 18, 19 View FIGURES 12–21 ) and slightly raised on the inner surface ( Figs 13, 17 View FIGURES 12–21 ), forming a stauros ( Figs 18, 19 View FIGURES 12–21 , asterisks). The striae consist of small round areolae, and are radiate and uniseriate ( Fig. 12 View FIGURES 12–21 ). Each areola is circular and occluded by a hymen with perforations arranged in a centric array ( Figs 20, 21 View FIGURES 12–21 ). A submarginal hyaline area is not recognizable; a narrow marginal hyaline area is visible ( Fig. 12 View FIGURES 12–21 ). The terminal hyaline area is small and elliptical, and extends to the marginal hyaline one ( Figs 12, 14 View FIGURES 12–21 ).

The ARV is convex ( Fig. 22 View FIGURES 22–28 ) and twisted ( Figs 11 View FIGURES 2–11 , 34 View FIGURES 29–35 ). The ARV ( Fig. 23 View FIGURES 22–28 ) is thicker than the RV ( Fig. 13 View FIGURES 12–21 ). The sternum, inclined from the apical axis ( Fig. 35 View FIGURES 29–35 , dashed line), is a narrow furrow on the outer surface ( Fig. 22 View FIGURES 22–28 ), and narrow lanceolate internally ( Figs 8, 9 View FIGURES 2–11 , 23 View FIGURES 22–28 ). In some valves, a vestigial raphe is observed ( Fig. 25 View FIGURES 22–28 , arrows). The striae that consist of several loculate areolae ( Fig. 24 View FIGURES 22–28 ) are radiate and uniseriate ( Fig. 22 View FIGURES 22–28 ). Each areola is occluded by a hymen located near the outer surface ( Fig. 28 View FIGURES 22–28 , arrows), internally, opens by means of a circular to elliptic foramen ( Figs 23, 25 View FIGURES 22–28 ). The perforations of the hymen are linear, oblique and arranged in a parallel array ( Figs 26, 27 View FIGURES 22–28 ).

The mature cingulum consists of three girdle bands ( Figs 29, 30 View FIGURES 29–35 ): 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 View FIGURES 29–35 , arrows) and has no fimbriae and areolae; their inner edges are smooth ( Figs 31, 33 View FIGURES 29–35 ). The second band, adjacent to the valvocopula, is open at the opposite pole ( Fig. 30 View FIGURES 29–35 ) and possesses a ligula ( Fig. 29 View FIGURES 29–35 ). The third band is also open ( Fig. 29 View FIGURES 29–35 ). The valvocopula of each valve is open at a place slightly shifted from one distal end of the raphe or sternum ( Figs 31–35 View FIGURES 29–35 , 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 View FIGURES 29–35 , 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 View FIGURES 2–11 , arrowheads) and on the fine branches of Asparagopsis taxiformis in Hachijo Island, Japan ( Fig. 11 View FIGURES 2–11 ).

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 View TABLE 1 ).

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).