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Chains straight or weakly curved. Setae thin, arising somewhat inside valve margin; basal part short, diagonal, and conspicuous. Setae crossing outside chain edge, perpendicular to chain axis. Apertures fairly large and rectangular. Resting spore setae curved to surround the cell like a girdle.

Chaetoceros furcellatus Bailey (Plate 45, Table 52) References: Bailey, 1856, Plate 1, Fig. 4; Hustedt, 1930, p. 749, Fig. 433. Chains straight or weakly curved. Setae thin, arising slightly inside valve margin; short and diagonal basal part. Setae of vegetative cells crossing slightly outside chain edge, irregularly oriented toward chain axis. Apertures rectangular, slightly compressed in center. Resting spores smooth, within resting spore parent cells with coarse hypovalvar setae (E. Syvertsen, personal observations), which are often fused for a fairly long distance, perpendicular to chain axis, twisting, branching, and diverging at a low angle.

Chaetoceros radicans Schutt (Plate 45, Table 52) References: Schutt, 1895, p. 48, Fig. 27; Hustedt, 1930, p. 746, Fig. 431; Cupp, 1943, p. 141, Fig. 97; Hendey, 1964, p. 134, Plate 14, Fig. 4; Drebes, 1974, p. 82, Fig. 66; Fryxell & Medlin, 1981, p. 8, Figs. 9-15, and 29-35; Rines & Hargraves, 1988, p. 90, Figs. 192-198.

Chains straight or slightly curved, twisted about the chain axis. Setae arising from just inside the valve margin, all bent out transversely. Intercalary setae with hair-like siliceous spines; terminal setae without spines. Apertures narrow and elliptical with central constriction. Hypovalvar setae of resting spore parent cell thick and smooth and separate after the fused space to surround the cell like a girdle.

PLATE 44 Chaetoceros neglectus: part of chain with resting spores. Chaetoceros curvisetus: (a) narrow girdle view; (b) broad girdle view; (c) valve view. Chaetoceros debilis: (a) spiralled chain; (b) part of chain with resting spores. Partly after Cupp (1943). Chaetoceros pseudocurvisetus: partial chain. After Cupp (1943). Scale bars = 20 p.m.

C. cinctus

C. tortissimus

C. tortissimus

TABLE 52 Morphometric Data of Chaetoceros spp. with One Chloroplast and Paired Resting Spores

Species Apical axis (p.m)

C. ductus 5-15

C. furcellatus 8-20

C. radicans 6-25

C. tortissimus 11-20

Chaetoceros tortissimus Gran (Plate 45, Table 52)

References: Gran, 1900, p. 122, Plate 9, Fig. 25; Hustedt, 1930, p. 751, Fig. 434; Cupp, 1943, p. 142, Fig. 99; Hendey, 1964, p. 135, Plate 11, Fig. 2; Drebes, 1974, p. 82, Fig. 67. Chains straight or slightly bent, very strongly twisted about chain axis. Setae thin, arising somewhat inside the valve margin, perpendicular to chain axis but going in all directions. Apertures apparent only at corners; slightly convex valve surfaces of sibling cells touching in the middle. Gross morphology similar to that of the three species previously described. Resting spores unknown.

Distribution:

C. cinctus, C. radicans, and C. tortissimus—cosmopolitan. C. furcellatus—northern cold water region. Remarks: Chaetoceros furcellatus and C. cinctus can hardly be distinguished without resting spores. The typical C. radicans is conspicuous by the spiny setae and the twisted chains, but Rines &C Hargraves (1988) found forms with few or no spines on the setae. Also, the shape of the C. radicans chains and apertures varied during the season, and a similarity with C. cinctus and C. tortissimus was therefore striking. Chaetoceros furcellatus is one of the most common species of this genus in the Arctic.

PLATE 45 Chaetoceros radicans: slightly twisted chain with two resting spores. Chaetoceros furcellatus: chain with two resting spores. Chaetoceros cinctus: (a) partial chain with resting spore; (b) resting spore in valve view. After Cupp (1943). Chaetoceros tortissimus: twisted chain. After Cupp (1943). Chaetoceros messanensis: part of chain with characteristic intercalary setae. After Hustedt (1930). Scale bars = 20 fim.

3c. Short rigid chains; two kinds of intercalary setae. Chaetoceros diversus Cleve (Table S3)

References: Cleve, 1873a, p. 9, Plate 2, Fig. 12; Hustedt, 1930, p. 716, Fig. 409; Cupp, 1943, p. 132, Fig. 87; Hendey, 1964, p. 130, Plate 17, Fig. 4. Chains straight and usually short. Setae arising from cell corners, no basal part. Apertures slit like. One type of intercalary setae thin, more or less curved, and usually turned toward chain ends; and the other type heavy, almost club shaped, first straight and at a sharp angle from chain axis, then turning and running almost parallel to chain axis in outer part. Terminal setae thin and differ from the intercalary setae in position, being first U shaped, then nearly parallel to chain axis in outer part.

Chaetoceros messanensis Castracane (Plate 45, Table 53) References: Castracane, 1875, p. 394, Plate 1, Fig. la; Hustedt, 1930, p. 718, Fig. 410; Cupp, 1943, p. 133, Fig. 89; Hendey, 1964, p. 129, Plate 12, Fig. 3; Evensen 8c Hasle, 1975, p. 162, Figs. 70-74. Chains straight. Cells connected by drawn up poles of concave valves. Apertures wide and linear six sided to almost round. Setae thin; no basal part. Terminal setae strongly diverging, unlike, and usually one directed backward from the chain. Some intercalary setae thicker than the others, first fused, then forked. Central process as in C. curvisetus (Evensen 8c Hasle, 1975, Figs. 71b).

Distribution: C. diversus, and C. messanensis—warm water region. Remarks: Chaetoceros laevis Leuduger-Fortmorel is listed as a separate species in Cupp (1943). Hustedt (1930) suggested that C. laevis and C. diversus are conspecific.

3d. Valves of adjacent cells touch at valve poles. Chaetoceros affinis Lauder (Plate 46, Table 54)

References: Lauder, 1864b, p. 78, Plate 8, Fig. 5; Hustedt, 1930, p. 695, Figs. 396-398; Cupp, 1943, p. 125, Fig. 78; Hendey, 1964, p. 127, Plate

TABLE S3 Morphometric Data of Chaetoceros spp. with One Chloroplast and Two Types of Intercalary Setae

Species

Apical axis (|un)

C. diversus

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