Cross-section circular. Valve shallow. Process swollen basally; distal part tubular. Otaria extending along the basal part of the process; distal margin straight to weakly concave.
Rhizosolenia fallax Sundstrom (Plate 29, Table 35) Reference: Sundstr6m, 1986, p. 89, Figs. 38 and 227-233.
Cross-section circular to slightly elliptical. Valve obliquely conical. Process swollen basally, narrowing abruptly into the distal tube. Otaria small, extending along the swollen part of the process.
Rhizosolenia striata Greville (Plate 29, Table 35) References: Greville, 1864, p. 234, Plate 3, Fig. 4; Sundstrom, 1986, p. 84, Figs. 36 and 209-217. Cross-section elliptical. Valve shallow. Process triangular in outline with lateral edges usually weakly concave. Otaria small, extending along the lower part of the process.
Rhizosolenia decipiens Sundstrom (Plate 29, Table 35) Reference: Sundstrom, 1986, p. 92, Figs. 39a, 39b, and 234-240. Cross-section usually circular. Valve obliquely conical. Basal part of the process conical, gradually narrowing into a distal tube of roughly equal length. Otaria narrow, extending along the conical part of the process.
R. imbricata—widely distributed although not in polar regions. R. fallax—temperate to tropical waters.
R. decipiens, R. ostenfeldii, and R. striata—warm water region. Remarks: The bands of R. imbricata and R. striata have a similar striation pattern (Fig. 15) but differ in the shape of the valves and the basal part of the processes. The bands of R. ostenfeldii and R. decipiens have a similar striation pattern (Fig. 15) but these species also differ in the shape of the valves and the basal part of the processes. The bands of R. fallax have a striation pattern different from that of the other species (Fig. 15), whereas the valve process is similar to those of R. imbricata and R. ostenfeldii. Although the structure may be too fine for the striae to be counted using LM, the striation pattern is visible with LM in all species of this section. Rhizosolenia chuttii Karsten was also allocated to the section Imbricatae by Priddle et al. (1990, p. 118, Plate 15.3, Fig. 2).
C. Incertae sedis (Rhizosolenia) Rhizosolenia setigera Brightwell (Plate 30)
References: Brightwell, 1858a, p. 95, Plate 5, Fig. 7; Hustedt, 1930, p. 588, Fig. 336; Cupp, 1943, p. 88, Fig. 49; Drebes, 1974, p. 52, Fig. 40; Sundstrom, 1986, p. 104, Figs. 286-288; Priddle et al., 1990, p. 120, Plate 15.4, Fig. 5.
Cell wall weakly silicified; structure not resolved with LM. Areolae poroid (SEM). Two dorsiventral columns of bands. Valves conical. External process long, almost straight along the whole length, slightly wider for some distance from the base, and gently tapering toward the tip. No otaria. Labiate structure present (G. Hasle, personal observations).
Rhizosolenia pungens Cleve-Euler (Plate 30)
References: Cleve-Euler, 1937, p. 43, Fig. 10; Drebes, 1974, p. 52, Fig. 34a. Basal part of external process narrow, abruptly swollen for about half its length. Otherwise, same as R. setigera. Morphometric data: Rhizosolenia setigera—4-25 /¿m in diameter R. pungens—8-14 /am in diameter.
R. pungens—mainly brackish water (Swedish and Danish coastal waters, Kiel Bay, Brazil, Japan).
R. setigera—cosmopolitan, probably absent from polar waters. Remarks: Rhizosolenia setigera and R. pungens would fit fairly well into Pseudosolenia except for the different shape of the valves and processes. Some species in Rhizosolenia sensu stricto have no otaria (Table 34) like R. setigera and R. pungens. Unlike R. setigera and R. pungens they have more than two columns of segments and also often have a large diameter. In addition, R. setigera forms resting spores in pairs distinctly different from the vegatative cells—a feature which, together with the poroid areolae of the bands (SEM), Sundstrom (1986) regarded as decisive for excluding this species from R. sensu stricto. The reason for listing R. pungens as a separate species and not a form or variety of R. setigera is the unique shape of the process.
PLATE 30 Rhizosolenia pungens: characteristic process, lower part swollen. After Cleve-Euler (1951). Scale bar = 20 /u.m. Rhizosolenia setigera: long, tapering process. Scale bar = 20 (im. Rhizosolenia robusta: frustule with characteristic valves and bands. After Cupp (1943). Scale bar = 100 p.m. Proboscia alata: proboscis. After Brightwell (1858a). Scale bar = 20 p.m. Pseudosolenia calcar-avis: parts of thecae. Scale bar = 20 p.m.
Rhizosolenia robusta Norman in Pritchard (Plate 30) References: Pritchard, 1961, p. 866, Plate 8, Fig. 42; Hustedt, 1930, p. 578, Fig. 330; Cupp, 1943, p. 83, Fig. 46; Hasle, 1975, p. 110, Figs. 42-47; Sundström, 1986, p. 104, Figs. 289 and 290. Cells cresent shaped or S shaped. Two columns of typically collar-shaped segments. Areolae loculate (SEM). Valves deeply convex or conical, curved, and with longitudinal lines. External process consisting of a needle-shaped part (?) extending from a short, wider tube (SEM). Morphometric data: Diameter, 48-400 pm; cell length, 0.5-1 mm; valve areolae, 19-20 in 10 /xm; band areolae, 24-26 in 10 fim.
Distribution: Warm water region.
Remarks: Sundström's (1986) argument for not including this species in R. sensu stricto, despite the presence of areolated segments, was the shape of the valve apex and the tubular process.
Genus Proboscia Sundström 1986
Type: Proboscia alata (Brightwell) Sundström.
Valves subconical, terminating in a proboscis. No process. Auxospores terminal. Claspers usually present.
Proboscia alata (Brightwell) Sundström (Plate 30) Basionym: Rhizosolenia alata Brightwell.
References: Brightwell, 1858a, p. 95, Plate 5, Fig. 8; Drebes, 1974, p. 57, Figs. 39c and 39d; Sundström, 1986, p. 99, Figs. 258-266; Jordan et al., 1991, p. 65, Figs. 1-9. Bands in two columns, numerous, rhomboidal, with pores (LM) scattered between loculate (SEM) areolae. Proboscis, tip truncate, short longitudinal slit just below tip. Morphometric data: Diameter, 2.5-13 ¡xm.
Distribution: "The biogeographical limits cannot be determined without further research" (Sundström, 1986, p. 101).
Remarks: Sundström (1986, p. 99) wrote: "Proboscia comprises the generic type P. alata and an undetermined number of species commonly referred to in the literature as Rhizosolenia alata, Rh. arafurensis, Rh. indica, Rh. inermis, Rh. truncata, etc." Jordan et al. (1991) examined Antarctic phytoplankton and made two new combinations, Proboscia inermis (Castracane) Jordan & Ligowski and Proboscia truncata (Karsten) Nöthig Sc Ligowski. Proboscia inermis has moderately prolonged valves with stout, wedge-shaped, markedly truncate proboscis (Jordan et al., 1991, p. 66, Figs. 10-18). Proboscia truncata has either somewhat rounded valves with short, truncate, oblique proboscis, or tapered valves with long, straight or slightly oblique proboscis somewhat wider near its tip (Jordan et al., 1991, p. 70, Figs. 19-29). Proboscia eumorpha Takahashi, Jordan & Priddle and P. subarctica Takahashi, Jordan & Priddle were described from subarctic waters (Takahashi et al., 1994), and the genus thus includes five modern species.
Genus Pseudosolettia Sundstrom 1986
Type: Pseudosolettia calcar-avis (Schultze) Sundstrom.
Pseudosolettia calcar avis (Schultze) SundstrOm (Plate 30) Basionym: Rhizosoletiia calcar avis Schultze. References: Schultze, 1858, p. 339; Schultze, 1859, p. 19, Figs. 5-8; Hustedt, 1930, p. 592, Fig. 339; Cupp, 1943, p. 89, Fig. 51; Sundstrom, 1986, p. 95, Figs. 40-46 and 247-257. Bands in two, or a multiple of two, columns. Areolae poroid (SEM). Valves subconical. Process claw like. No otaria. Contiguous area roughly sigmoid. Labiate structure different from that in Rhizosolenia but similar to the two larger labiate processes in Coscinodiscus (SEM). Morphometric data: Diameter, 4.5-190 fim; process, 28-52 ¡j,m; areolae on bands 28-32 in 10 /xm measured with TEM. Distribution: Warm water region, occasionally in temperate waters. Remarks: The shape of the valve and the external as well as the internal parts of process and the poroid areolae distinguish the genus from Rhizosolenia sensu stricto.
How to identify: Most of the Rhizosolenia species as well as Proboscia and Pseudosolenia may be identified in girdle view in water mounts. In critical cases in which information on the otaria is urgent, valves cleaned of organic matter and mounted in a medium of a high refractive index may be examined in valve view.
Genus Guinardia H. Peragallo 1892
Lectotype: Guinardia flaccida (Castracane) H. Peragallo (vide Round et al., 1990, pp. 326 and 691).
Low, open ligulate bands usually distinct with LM.
Band structure composed of regular rectangular poroids (EM).
Valve structure composed of faint ribs radiating from the single process (occasionally revealed with LM).
External process tube.
External depression for process of sibling valve.
Characters showing differences between species: Shape of chains (loose or close set).
Shape of cells (straight or curved).
Shape of valves (flat or convex).
Location of process (central or marginal).
Shape of external part of process.
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