Chains straight. Apertures large and partly filled by the valve protuberance. Setae arising from corners of cells, crossing at their bases or farther out, sometimes far outside chain edge. Each chloroplast with a pyrenoid located in the protuberance. Resting spores smooth; within pairs of resting spore parent cells with short, thick setae on hypovalve. Remarks: Rines & Hargraves (1988) interpreted C. didymus as a complex of taxa including a number of varieties. Based on EM investigations Hernandez-Becerril (1991c) suggested that Chaetoceros protuberans Lauder should be regarded as a separate species and not as a variety of C. didymus. According to the same investigation C. didymus has a centrally located process consisting of a slit-like hollow with a very short projection to the outside; a similar process is located at the center of the typical protuberance of C. protuberans.
Chaetoceros constrictus Gran (Plate 43, Table 50) References: Gran, 1897b, p. 17, Plate 1, Figs. 11-13, Plate 3, Fig. 42; Hustedt, 1930, p. 694, Fig. 395; Cupp, 1943, p. 122, Fig. 76; Hendey, 1964, p. 126, Plate 9, Fig. 2; Rines & Hargraves, 1988, p. 67, Figs. 128-130.
Chains straight. Valve poles drawn up; corners of adjacent cells touching. Apertures lanceolate, slightly narrowing in center. Terminal setae diverging at an acute angle. Constriction between valve mantle and band conspicuous. Resting spores with unequally vaulted, spiny valves.
Chaetoceros laciniosus Schtitt (Plate 43, Table 50) References: SchUtt, 1895, p. 38, Plate 4, Fig. 5; Hustedt, 1930, p. 701, Fig. 401; Cupp, 1943, p. 128, Fig. 80; Hendey, 1964, p. 127, Plate 13, Fig. 2; Evensen & Hasle, 1975, p. 160, Figs. 42-45; Rines & Hargraves, 1988, p. 83, Figs. 167-169. Chains straight and loose. Setae thin; basal part parallel to chain axis and then perpendicular to chain axis; far outer part usually bent toward one chain end. Terminal setae different from the others—almost parallel in broad girdle view and more diverging in narrow girdle view. Apertures high, elliptical, and square to rectangular. Each chloroplast with a central pyrenoid. Resting spore valves smooth; primary valve more or less highly vaulted and secondary valve almost flat to highly vaulted. Central process built as that in C. curvisetus but located near the edge of valve face (Evensen & Hasle, 1975, EM).
Chaetoceros similis Cleve (Plate 43, Table 50)
References: Cleve, 1896b, p. 30, Fig. 1; Hustedt, 1930, p. 720, Fig. 411; Cupp, 1943, p. 135, Fig. 90; Hendey, 1964, p. 130, Plate 15, Fig. 2; Rines &c Hargraves, 1988, p. 94, Figs. 222 and 223. Short and straight chains. Apertures narrow and divided into two parts by a central, raised region of the valve. Setae arising from cell corners, directed diagonally toward chain ends, crossing outside chain margin. Terminal setae parallel to the others. Resting spores pear shaped with small spines.
Chaetoceros anastomosans Grunow in Van Heurck (Plate 40, Table 50) References: Van Heurck, 1880-1885, Plate 82, Figs. 6-8; Hustedt, 1930, p. 743, Fig. 429; Cupp, 1943, p. 140, Fig. 96; Drebes, 1974, p. 81, Fig. 65 as C. externus Gran.
Chains straight or slightly curved, mostly loose. Setae thin, arising from cell corners, variously bent, and not crossing but connected by pervalvarly directed bridges. Apertures wide.
C. anastomosans and C. didymus—warm-water region to temperate. C. constrictus, C. laciniosus, and C. similis—northern cold water region to temperate?
Remarks: Except for the two chloroplasts these species have few characters in common, whereas each of them is readily identified by their specific features. Chaetoceros vanheurckii Gran is very similar to C. constrictus, differing only by some minor features of the resting spores. Chaetoceros pelagicus Cleve and C. brevis SchUtt have both one chloroplast but are otherwise similar to C. laciniosus. Resting spores are unknown for C. pelagicus, those of C. brevis and C. laciniosus are similar. There seems to be a general consensus that positive identification of C. brevis is problematic.
3. Cells with one chloroplast. 3a. Chains curved or helical; intercalary setae all bent in one direction.
Chaetoceros curvisetus Cleve (Plate 44, Table 51) References: Cleve, 1889, p. 55; Hustedt, 1930, p. 737, Fig. 426; Cupp, 1943, p. 137, Fig. 93; Hendey, 1964, p. 133, Plate 17, Fig. 6; Drebes, 1974, p. 79, Fig. 63; Evensen & Hasle, 1975, p. 159, Figs. 23-26; Rines & Hargraves, 1988, p. 71, Figs. 141 and 142. Adjacent cells in chains connected by drawn up poles of the concave valves; basal part of setae short or missing. All setae directed toward the outside of the chain spiral (best seen in narrow girdle view). Apertures a lanceolate slit, elliptical, or nearly circular. Resting spores smooth; primary valve evenly rounded and secondary valve less rounded to almost flat. Central process short, flattened with no protrusion or thickening on the inside (Evensen & Hasle, 1975, Figs. 25 and 26).
Chaetoceros debilis Cleve (Plate 44, Table 51)
References: Cleve, 1894a, p. 13, Plate 1, Fig. 2; Hustedt, 1930, p. 740, Fig. 428; Cupp, 1943, p. 138, Fig. 95; Hendey, 1964, p. 133, Plate 14, Fig. 7; Drebes, 1974, p. 81, Fig. 64; Evensen & Hasle, 1975, p. 159, Figs. 27-32; Rines & Hargraves, 1988, p. 72, Figs. 143-147; Takano, 1990, pp. 286-287. Valves flat or slightly convex. Valves of adjacent cells in chains do not touch. Basal part of setae conspicuous. Setae crossing slightly outside chain edge, extending outward from the spiral. Apertures almost rectangular or slightly constricted in the middle. Primary valve of resting spores with two humps and two setae extending into the corners of the parent cell. Secondary valve smooth or with setae.
Chaetoceros pseudocurvisetus Mangin (Plate 44, Table 51) References: Mangin, 1910, p. 350, Fig. 3, II, Fig. 4, II; Hustedt, 1930, p. 739, Fig. 427; Cupp, 1943, p. 138, Fig. 94; Hendey, 1964, p. 134, Plate 18, Fig. 1; Fryxell, 1978c, p. 68, Figs. 22-26; Rines & Hargraves, 1988, p. 89, Figs. 185-191; Takano, 1990, pp. 290-291. Cells in colonies joined by fusion of sibling setae and at the edges of valves by four elevated projections leaving a large lenticular aperture in the center between cells in broad girdle view.
C. curvisetus—cosmopolitan, mainly temperate and warm waters. C. debilis—cosmopolitan, mainly cooler waters. C. pseudocurvisetus—warm water region.
3b. Resting spores in pairs; resting spore parent cells with fused hypovalvar setae and no apertures.
Chaetoceros cinctus Gran (Plate 45, Table 52)
References: Gran, 1897b, p. 24, Plate 2, Figs. 23-27; Hustedt, 1930, p. 748, Fig. 432; Cupp, 1943, p. 142, Fig. 98; Hendey, 1964, p. 135, Plate 11, Fig. 4.
Apical axis (iim)
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