Cocconeis churalis Hide. Suzuki, 2012

Suzuki, Hidekazu, Nagumo, Tamotsu & Tanaka, Jiro, 2012, Cocconeis churalis: a new marine diatom (Bacillariophyta, Cocconeidaceae) from Japan, Phytotaxa 68 (1), pp. 36-44 : 37-41

Cocconeis churalis Hide. Suzuki , sp. nov. ( Figs 1–32 )

Frustules heterovalvate. Valve small, elliptical to oval, 8.5–20.0 µm long, 6.0–15.0 µm wide. Raphid valve (RV) concave, raphe slightly sigmoid, central area small, elliptic to orbicular. Striae uniseriate, radiate, 29–36 in 10 µm. Submarginal hyaline area absent. Araphid valve (ARV) convex. Striae alveolus, radiate, 32–38 in 10 µm. Alveolus opens by means of circular to elliptical foramen, viewed internally. Alveolus occluded by hymen. Cingulum consisting of four girdle bands, valvocopula non fimbriate.

Type:— JAPAN. Okinawa Pref., Ishigaki-shi, the Southwest Islands, Ishigaki Island, Ibaruma (24˚31’16.20” N, 124˚16’48.72” E). Sample collected from surface Murrayella periclados (J. Agardh) Schmitz ( Rhodomelaceae , Rhodophyceae) on 4 March 2005. Holotype material and slide; BM 101651 (holotype represented by valves on Figs 1, 5 ).

Distribution and ecology: — Cocconeis churalis has been found only from the type locality and was not recorded in the previous study of the Nansei Islands (including Ishigaki Island and other tropical and subtropical regions) ( Takano 1962, Nagumo & Hara 1990, Nagumo & Tanaka 1990, Witkowski et al. 2000, Hein et al. 2008).

Cocconeis churalis grows abundantly on Murrayella periclados in Ishigaki Island, Japan ( Fig. 10 , arrows). Tabularia parva (Kütz.) D.M. Williams & Round and Gomphonemopsis pseudexigua (Simonsen) Medlin were found growing epiphytically ( Fig. 10 , arrowheads).

Etymology: —The specific epithet “chura” is derived from the Okinawa dialect meaning ‘beautiful’.

Observations: —Frustules with a single flat plastid, C-shaped, which is either simple or elaborately lobed ( Fig. 9 ) as in some other species of Cocconeis (cf. Suzuki et al. 2001b –d, 2008, Suzuki & Nagumo 2003b).The frustules are composed of very small and elliptic to oval valves ( Figs 1–8 ), measuring 8.5–20.0 µm in length, 6.0–15.0 µm in breadth. Striae densities at the centre of the valves are 29–36 in 10 µm for the raphid valve (RV), 32–38 in 10 µm for the araphid valve (ARV). The RV is concave ( Fig. 11 ); the curvature of the valve face is concave at the transapical axis not at the apical axis, as in Cocconeis convexa Giffen (1967: 257 , see Suzuki et al. 2001c). The raphe appears simple, with a slightly sigmoid slit ( Fig. 11 ) as in Cocconeis subtilissima Meister (1935: 99 , see Suzuki et al. 2008) but not more curved than that found in Cocconeis heteroidea Hantzsch (in Rabenhorst 1863: 21, see Suzuki et al. 2001a). The inner raphe fissures lie in a very narrow, but raised, axial area ( Fig. 12 ). The proximal raphe ends are coaxial and somewhat dilated externally ( Fig. 14 ) but internally they are undilated and deflected in opposite directions ( Fig. 16 ). The distal raphe ends are dilated externally ( Fig. 13 ) but, internally, terminate in short, narrow and small helictoglossa ( Fig. 15 ). The central area is round and small ( Fig. 14 ). The striae consist of small round areolae and are radiate and uniseriate ( Fig. 11 ). Each areola is circular and occluded by a hymen with perforations arranged in a centric array ( Figs 17, 18 , see Mann 1981). These hymenes are circular in outline and located near the internal surface of the valve ( Fig. 18 ). A submarginal hyaline area is not recognizable; a narrow marginal hyaline area is visible ( Fig. 11 ).

The ARV is convex and corresponds to the curvature of the RV. The axial area is a narrow, slightly sigmoid furrow on the outer surface ( Fig. 19 ) and, internally, lanceolate ( Fig. 20 ). In some valves, a vestigial raphe is observed ( Fig. 26 , arrows). The striae that consist of several alveoli ( Fig. 21 ) are radiate and uniseriate ( Fig. 19 ). Internally, each alveolus opens by means of a circular to elliptic foramen ( Figs 20, 22 ). Each alveolus is occluded by a hymen located near the outer surface ( Fig. 24 , arrows). The perforations of the hymen are linear, oblique and arranged in a parallel array ( Fig. 23 , see Mann 1981). The structure of the alveolus of Cocconeis churalis is essentially the same as that of Cocconeis dirupta Gregory (1857: 491 , see Kobayasi & Nagumo 1985), C. pellucida Grunow (1863: 145 , see Kobayasi & Nagumo 1985 and Suzuki et al. 2008), C. heteroidea ( Suzuki et al. 2001a) , C. shikinensis Hide. Suzuki ( Suzuki et al. 2001b: 138), C. convexa ( Suzuki et al. 2001c) , C. pseudomarginata Gregory var. intermedia Grunow (1867: 13 , see Suzuki et al. 2001d and Suzuki et al. 2008), C. nagumoi Hide. Suzuki (Suzuki in Suzuki & Tanaka 2006: 282) and C. subtilissima ( De Stefano & Romero 2005, Suzuki et al. 2008). The external face of an immature ARV illustrates the structure of the internal perforations of alveoli ( Fig. 25 ).

The mature cingulum consists of four girdle bands ( Figs 27, 28 ): a valvocopula and three bands (the second, the third and the fourth bands), which are all narrower and thinner than the valvocopula. The valvocopula of each valve is open at one pole ( Figs 29, 30 , arrows) and has no fimbriae; their inner edges are smooth ( Fig. 32 ). The second band, adjacent to the valvocopula, is open at the opposite pole and possesses a ligula ( Fig. 31 , L). The third and fourth bands are also open ( Figs 27, 28 ). The structure of the cingulum is like that of: C. dirupta ( Kobayasi & Nagumo 1985) , C. pellucida ( Kobayasi & Nagumo 1985) , C. heteroidea ( Suzuki et al. 2001a) , C. convexa ( Suzuki et al. 2001c) , C. pseudomarginata var. intermedia ( Suzuki et al. 2001d) and C. subtilissima ( Suzuki et al. 2008) .

The RV morphology of Cocconeis churalis is similar to that of C. subtilissima ( De Stefano & Romero 2005, Suzuki et al. 2008) but Cocconeis churalis can be readily distinguished by the following characters:

i) The curvature of the RV is concave at the transapical axis;

ii) The slightly sigmoid raphe slits terminate in short, narrow and small helictoglossa, not hooked;

iii) The terminal hyaline area is small and round, not an arrowhead type; and

iv) The density of striae is higher than that of C. subtilissima (26–28 in 10 µm; Suzuki et al. 2008).

Cocconeis churalis is similar to C. dirupta ( Kobayasi & Nagumo 1985) but can be readily distinguished from it as C. dirupta lacks a stauros in the central area of the RV, has a broad, sigmoid lanceolate axial area on the ARV, and a higher density of the areolae on both valves (22 in 10 µm on the RV, 20 in 10 µm on the ARV; Kobayasi & Nagumo 1985).

Figures

FIGURES 1–10: Cocconeis churalis. LM images of holotype and other specimens. Figs 1–8: Raphid valves (RV) (Figs 1–4) and araphid valves (ARV) (Figs 5–8) of the same frustule. Figs 1, 5: Specimens from holotype slide. Figs 9, 10. Specimens from fixed sample. Fig. 9. Cell showing a C-shaped and elaborately lobed plastid. Fig. 10. C. churalis (arrows) and Gomphonemopsis pseudexigua (arrowheads), epiphytic on Murrayella periclados.

FIGURES 11–18: Cocconeis churalis. SEM images of raphid valves (RV). Figs 11, 13, 14, 17: External views of RV. Figs 12, 15, 16, 18: Internal views of RV. Figs 11, 12: Whole valves. Figs 13, 15: Distal raphe ends in terminal area. Figs 14, 16: Proximal raphe ends in central area. Figs 17, 18: Areolae occluded by the hymenes with linear perforations in a centric array.

FIGURES 19–24: Cocconeis churalis. SEM images of araphid valves (ARV). Figs 19, 21, 23: External views of ARV. Figs 20, 22, 24: Internal views of ARV. Figs 19, 20: Whole valves. Figs 21, 22: Marginal areas. Fig. 23: Areolae occluded by the hymenes with perforations in a parallel array. Fig. 24: Alveoli occluding by hymenes (arrows) located near the outer surface.

FIGURES 25, 26: Cocconeis churalis. SEM images of araphid valves (ARV). Fig. 25: External face of immature ARV showing alveoli, each with one foramen. Fig. 26: Early stage of ARV formation, with rudimentary raphe (arrows).

FIGURES 27–32: Cocconeis churalis. SEM images of cingula. Fig. 27: Valvocopula (VC), and second (S), third (T) and fourth (F) band of ARV. Fig. 28: Opposite pole. Figs 29, 30: Internal views of the terminal areas of RV and ARV with valvocopula (VC), respectively. Arrows indicate the open parts of valvocopula. Fig. 31: Valvocopula (VC) and the second band (S) with ligula (L), remaining on RV. Fig. 32: Valvocopula (VC) and second band (S) of araphid valve (ARV).