Ethmodiscus gazellae

Ethmodiscus rex

Palmeria hardmaniana

PLATE 18 Coscinodiscus radiatus: girdle view, flattened valves. Ring of processes close to margin. Scale bar = 20 |xm. Coscinodiscus marginatus: girdle view. Flattened valve, ring of processes close to junction between valve mantle and valve face. Scale bar = 20 /im. Ethmodiscus spp.: (a) girdle view. Scale bar = 200 /um.; (b) E. gazellae and (c) E. rex, valve center with processes. After Kolbe (1957). Scale bar = 10 /xm. Palmeria hardmaniana: valve view. From Allen 8c Cupp (1935). Scale bar = 10 /tm.

pp. 310 and 312, Figs. 8-32 and 35-39; Sancetta, 1987, p. 234, Plate 2, Figs. 1-10; Fryxell 8c Ashworth, 1988. Girdle view: Cells discoid, flat, coin shaped. Numerous cocciform chloroplasts.

Valve view: Indistinct central rosette of slightly larger areolae. Areolae in radial rows, sometimes in indistinct decussating arcs (Figs. 6d and 6e). Small labiate processes at the points of origin of incomplete striae (interstitial meshes). No hyaline lines. One marginal ring of processes, including two slightly larger processes visible with LM as indentations of the valve margin, ca. 135° apart. The smaller marginal processes barely discernible.

Distinctive features: Flat cells, large areolae. Distinguished from C. argus by areolae of uniform size throughout the entire valve face and from C. marginatus by smaller areolae in more regular radial rows.

C. Frustules with a stepped elevated valve face.

Coscinodtscus bouvet Karsten (Plate 17, Tables 20a and 20b) References: Karsten, 1905, p. 83, Plate 3, Fig. 9; Hendey, 1937, p. 244, Plate 13, Figs. 3 and 4; Priddle 8c Thomas, 1989; E. Syvertsen, unpublished observations.

Girdle view: Cells about as high as wide, octagonal in outline due to an elevated area of the valve face, about one-half to two-thirds of the valve diameter, and occasionally with no elevation. Sometimes in chains (up to 18 cells). Numerous irregular stellate chloroplasts.

Valve view: Central rosette of larger areolae. Radial rows of areolae. One ring of processes, including two larger processes ca. 125° apart, close to the valve margin, visible with LM (water mounts). Distinctive features: The shape of the cell in girdle view. In chains.

D. Frustules wedge shaped.

Coscinodiscus grattii Gough (Plate 17, Tables 20a and 20b) References: Gough, 1905, p. 338, Fig. 3B; Hustedt, 1930, p. 436, Fig. 437; Boalch, 1971, Plate 1; Brooks, 1975; Karayeva 8c Dzhafarova, 1984; Takano, 1990, pp. 244-245; Hasle 8c Lange, 1992, p. 60, Figs. 63-76. Girdle view: Cells asymmetric, one side much higher than the other. Valvocopula wedge shaped, widest opposite to the opening. Greatest convexity of the valve not in the center of the valve but nearest the widest part of the valvocopula. Chloroplasts discoid and smooth in outline. Valve view: Central rosette of larger areolae. Radial areolation, incomplete striae, and decussating arcs in the central part of the valve. Cribra barely discernible with LM. One ring of marginal processes including two larger processes ca. 135° apart, readily seen with LM; the larger processes seen as deep indentations of the valve mantle. Hyaline lines from the marginal processes toward the valve center more or less distinct.

Distinctive features: Cells wedge shaped in girdle view. Eccentric convexity of the valve, manifest with LM in valve view by only a part of the valve in focus. Valvocopula of uneven width, a second narrow band with rectangular, tall ligula positioned ca. 90° apart from the band opening (Hasle & Lange, 1992, Figs. 63 and 67).

It should be noted that the illustrations (Plates 15-18) show cells consisting of epitheca and incomplete hypotheca. The epithecae have, when known, the number of bands listed (Table 20a) with valvocopula included. The formation of all the bands of a hypotheca is usually first completed when the cell starts to divide.


C. bouvet—southern cold water region.

C. alboranii, C. concinniformis, and C. jonesianus—warm water region. C. wailesii—warm water to temperate (recently introduced to North Atlantic waters).

The remaining species have a wide distribution according to the literature and may be cosmopolitan with a wide temperature tolerance. How to identify: When not specifically mentioned, the morphological details are discernible with LM. Coscinodiscus wailesii, C. bouvet, and C. granii can be identified in water mounts in girdle view because of their special shape. To ensure a correct identification the remaining species should, in addition to water mounts, be examined as valves cleaned of organic matter and mounted in a medium of a high refractive index.

Genus Ethmodiscus Castracane 1886 (Plate 18, Table 21) Lectotype: Ethmodiscus gigas Castracane selected by F. E. Round (vide Round et al., 1990, p. 206).

Correct name: Ethmodiscus gazellae (Janisch ex Grunow) Hustedt (vide Hustedt, 1930, p. 375).

TABLE 21 Morphometric Data of Ethmodiscus spp.


Diameter (nm)

Areolae in 10 ftm

Size of processes (fim)'

E. gazellae E. rex

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