Key To Genera

la. Raphe system eccentric 2

lb. Raphe system central Bacillaria, p. 293

2a. Cells usually in chains 3

2 b. Cells usually solitary 4

3a. Cells united by valve surfaces into ribbons 5

3 b. Cells united by overlap of valve ends into stepped chains

Pseudo-nitzschia, p. 307

4a. Frustules usually spirally twisted, valves lightly silicified

Cylindrotheca, p. 293

4 b. Frustules not spirally twisted Nitzschia, pro parte, p. 324

Cylindrotheca closterium

Fragilariopsis kergulensis

PLATE 66 Bacillaria paxillifera: (a) part of chain in girdle view. Scale bar = 50 /ira; (b) valve end with raphe and striae. After Cupp (1943). Scale bar = 10 /urn. Cylindrotheca closterium: (a) cell in girdle view; (b) valve with fibulae visible. Scale bar =10 /um. Fragilariopsis kerguelensis: (a) ribbon in girdle view, curved valve faces, strong interstriae. From Hasle (1968b). Scale bar = 10 /urn. (b) larger specimen heteropolar, medium sized and smaller specimens isopolar, and transverse striae with two distinct rows of poroids. Scale bar = 10 p.m.

5a. Valve interior with transapical pseudosepta Neodenticula, p. 305 5b. Valve interior without pseudosepta Fragilariopsis, p. 295

Genus Bacillaria J. F. Gmelin 1791

Type: Bacillaria paradoxa J. F. Gmelin.

Correct name: Bacillaria paxillifera (O. F. Müller) Hendey.

Bacillaria paxillifera (O. F. Müller) Hendey (Plate 66) Basionym: Vibrio paxillifer O. F. Müller.

Synonyms: Bacillaria paradoxa J. F. Gmelin; Nitzschia paradoxa (J. F. Gmelin) Grunow in Cleve 8c Grunow.

References: Müller, 1783, pp. 277 and 286; Gmelin, 1791, p. 3903; Cleve 8c Grunow, 1880, p. 85; Cupp, 1943, p. 206, Fig. 159; Hendey, 1951, p. 74; Hendey, 1964, p. 274, Plate 21, Fig. 5; Drum 8c Pankratz, 1966; Round et al., 1990, p. 608. Girdle view: Cells rectangular; cells in colonies sliding along one another to form a linear array to retract into a tabular array. Valve view: Valves linear lanceolate with produced ends. Raphe system slightly keeled. Raphe continuous from pole to pole. Fibulae strong. Valve surface with transverse parallel striae. Morphometric data: Apical axis, 70-115 /tm; transapical axis, 5-6 /xm; 7-9 fibulae and 20-21 striae in 10 /xm. Distribution: Probably cosmopolitan.

How to identify: Bacillaria may consist of more than one species (Round et al., 1990). Bacillaria paxillifera seems to be the only one recorded from plankton, however. Due to the unique type of motile colonies the species may be identified in water mounts. Permanent mounts may be needed for identification of single valves.

Genus Cylindrotheca Rabenhorst 1859 Type: Cylindrotheca gerstenbergeri Rabenhorst.

Correct name: Cylindrotheca gracilis (Brebisson in Kützing) Grunow (vide Van Heurck, 1880-1885, p. 186). The usual characterization of the genus Cylindrotheca is Frustules cylindrical, fusiform, twisted about the apical axis, and rotating when in motion. Very weakly silicified, girdle bands narrow and numerous, and the valves appear to be hyaline in LM.

Reimann 8c Lewin (1964) redefined the genus and transferred N. closterium (Ehrenberg) W. Smith to Cylindrotheca probably based on a similarity with Cylindrotheca in raphe structure (TEM) and the weakly silicified valves.

Cylindrotheca closterium (Ehrenberg) Lewin 8c Reimann (Plate 66) Basionytn: Ceratoneis closterium Ehrenberg. Synonym: Nitzschia closterium (Ehrenberg) W. Smith. References: Ehrenberg, 1841a, p. 144, Plate 4, Fig. 7; Smith, 1853, p. 42, Plate 15, Fig. 120; Cupp, 1943, p. 200, Fig. 153; Hendey, 1964, p. 283, Plate 21, Fig. 8; Hasle, 1964, p. 16, Text Figs. 1-10, Plate 7, Figs. 1-12, Plate 9, Figs. 1-9, Plate 10, Figs. 1-4; Reimann 8c Lewin, 1964, p. 289, Plate 124, Figs. 1-4, Plate 125, Figs. 1-4, Plate 126, Figs. 1-3; Takano, 1990, pp. 320-321; Hasle 8c Medlin, 1990a, p. 177, Plate 23.1, Figs. 1-4.

Emended description (Reimann 8c Lewin, 1964, p. 289) Frustules in the fusiform part not twisted, in the rostra not or only slightly twisted about the apical axis. Valve face weakly silicified, almost imperforate, traversed by more or less transapical silicified thickenings. Raphe traversed by a series of fibulae, joined directly to the valve face. One of the edges of the valve bordering the fissure minutely serrate. Fissure interrupted at the center. Two chromatophores.

It should be kept in mind that the valves of the other Cylindrotheca species examined by the same authors consisted of a canal raphe solely, and the frustules were twisted two to three times about the apical axis. The lack of, or the insignificant twisting of the C. closterium cell, together with the presence of a striated valve face, indicates a rather isolated position of this diatom within Cylindrotheca. There are, therefore, still reasons for not using Reimann 8c Lewin's combination but instead to retain the name Nitzschia closterium until more thorough studies of marine planktonic species of the "N. closterium" shape from various types of habitats have been performed. It is true, however, that the species in question does not belong to Nitzschia sensu stricto (see also Medlin, 1990 for rRNA molecule studies of C. closterium and Nitzschia spp.).

Morphometric data: Apical axis, 30-400 /¿m; transapical axis, 2.5-8 ju,m; fibulae, 10-12 in 10 /tm; interstriae, 70-100 in 10 /u,m (TEM; Hasle, 1964). Distribution: Cosmopolitan?—planktonic and common on seaweeds and polar ice.

How to identify: The identification of C. closterium may have caused more problems and more confusion than the identification of any other diatom enountered in marine plankton (see Lennoxia faveolata and Phaeodactylum tricornutum). When working with coastal material the most common problem is distinguishing between C. closterium and the coarser N. longissima; examination of water mounts is usually not sufficient. Not too thoroughly acid-cleaned material is better since this often shows the two delicate valves and the many bands of C. closterium lying together, seemingly twisted around each other in the rostrate ends.

Genus Fragilariopsis Hustedt in Schmidt emend. Hasle 1993 Type: Fragilariopsis antarctica (Castracane) Hustedt in A. Schmidt. Basionym: Fragilaria antarctica Castracane.

Correct name: Fragilariopsis kerguelensis (O'Meara) Hustedt (vide Hustedt, 1952, p. 294).

Glezer et al. (1988) and Round et al. (1990) regarded Fragilariopsis as a separate genus. Since this is also done in this chapter, the new names and combinations introduced by Hasle (1972c, 1974, see the synonomy list) are inappropriate.

Fragilariopsis has a canal raphe which justifies the placement in the family Bacillariaceae. The raphe system as seen with EM is simple in construction compared to that of N. sigmoidea and several other Nitzschia species (Mann, 1986). The raphe is not raised above the valve surface, the external canal wall is not poroid, and the fibulae are small and not extending across the valve (Hasle, 1965a, 1968c, 1972c; Mann, 1978). In further contrast to the subgenus Nitzschia, Fragilariopsis has no conopea (flaps of silica extending out from near the raphe), most species are narrow, not sigmoid in girdle view, and the valve striae mostly have two rows of poroids.

The little information available from EM observations on the Fragilariopsis girdle demonstrates a certain unconformity that may to some extent be related to the silicification of the cell wall. The lightly silicified F. oceanica has several distinctly striated intercalary bands, whereas those of the more heavily silicified F. cylindrus and F. curta, and especially F. kerguelensis, have one or perhaps two bands with one row of perforations (Hasle, 1965a, 1972c; Medlin & Sims, 1993).

All Fragilariopsis species examined by Hasle (1965a) are present in polar waters and are dealt with in the handbook "Polar Marine Diatoms" (Medlin &c Priddle, 1990) in which a key to species is constructed (Hasle & Medlin, 1990b), as was also done by Hasle (1965a). In this chapter we include only the species more commonly recorded from plankton. For the examination of samples collected from or near sea ice Hasle & Medlin (1990b) should be consulted.

Generic characters:

Cells rectangular in girdle view.

Cells in ribbons united by the entire or the greater part of the valve surface.

Raphe strongly eccentric.

Raphe not raised above the general level of the valve.

Approximately equal numbers of interstriae and fibulae.

Fibulae often more distinct than interstriae (LM).

Central larger interspace lacking in most species.

Valve face more or less flattened and not undulated.

Valves narrowly elliptical to lanceolate to broadly elliptical or subcircular, or linear to sublinear.

Apical axis often heteropolar.

Valve poles usually bluntly rounded.

Striae parallel except near poles.

Striae with two rows of poroids, seldom one or more than two. Chloroplasts—two plates lying along the girdle, one on either side of the median transapical plane. Resting spores rare (one species).

Characters showing differences between species: Valve outline. Polarity of apical axis.

The presence or absence of central larger interspace. Structure of striae, discernible or not discernible with LM.

A. Valves elliptical to lanceolate.

1. Apical axis often heteropolar; apices generally rounded: F. kerguelensis, F. ritscheri, and F. atlantica (Table 73).

2. Apical axis isopolar; apices more or less rounded: F. oceanica and F. pseudonana (Table 73).

3. Apical axis isopolar; apices generally pointed: F. rhombica and F. separando (Table 73).

B. Valves linear to sublinear.

1. Apical axis isopolar: F. cylindrus and F. cylindriformis (Table 73).

2. Apical axis heteropolar: F. curta (Table 73).

C. Valves semilanceolate: F. doliolus (Table 73).

A. Valves elliptical to lanceolate.

1. Apical axis often heteropolar, apices generally rounded.

Fragilariopsts kerguelensis (O'Meara) Hustedt (Plate 66, Table 73) Basionym: Terebraría kerguelensis O'Meara.

Synonyms: Fragilariopsis antarctica (Castracane) Hustedt in A. Schmidt; Nitzscbia kerguelensis (O'Meara) Hasle.

References: O'Meara, 1877, p. 56, Plate 1, Fig. 4; Castracane, 1886, p. 56, Plate 25, Fig. 1; Schmidt, 1913, Plate 299, Figs. 9-14; Hustedt, 1952, p. 294; Hustedt, 1958a, p. 162, Figs 121-127; Hasle, 1965a, p. 14, Plate 4,

TABLE 73 Morphometric Data of Fragilariopsis spp.

Striae and fibulae in

Species Apical axis (n.m) Transapical axis (p.m) 10 p.m

Group A

Species Apical axis (n.m) Transapical axis (p.m) 10 p.m

Group A

F. kerguelensis

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