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18, Fig. 3; Drebes, 1974, p. 75, Fig. 58; Evensen & Hasle, 1975, p. 161, Figs. 46-54; Rines & Hargraves, 1988, p. 59, Figs. 113 and 114. Chains straight. Poles of adjacent cells touch. Apertures narrow. Inner setae thin and without basal part. Terminal setae large, strongly divergent, and different from the inner setae. Resting spores with more or less vaulted valves; secondary valve often higher and bulging in the middle. Both valves with small spines; secondary valve with some longer and stouter spines. Remarks: Lauder (1864b, p. 78) described the resting spores of C. affinis "with unequal, convex, hirsute valves," whereas later descriptions reported a slight dissimilarity between the two valve surfaces (e.g., Hendey, 1964). Chaetoceros willei Gran has very narrow apertures, thin setae, and terminal setae "shorter, slightly thickened in the middle, diverging at an acute angle" (Gran, 1897b, p. 19). Resting spores were unknown at the time of the description of the species and have apparently never been reported. Hustedt (1930) regarded Gran's species as a variety of C. affinis based on a certain variability of the shape of the terminal setae and the resting spore spines. Hendey (1964) retained C. willei as a separate species. Evensen & Hasle (1975) compared one culture from the Trondheimsfjord, Norway, probably identical to C. affinis var. willei, and a culture from California identified as C. affinis. Investigations (EM) showed certain differences regarding process and valve structure. The central process of both diatoms had a true labiate structure on the inside (Evensen & Hasle, 1975, Figs. 51 and 54). However, the external parts differed.

Chaetoceros costatus Pavillard (Plate 46, Table 54)

References: Pavillard, 1911, p. 24, Fig. lb; Hustedt, 1930, p. 699, Fig. 399;

Cupp, 1943, p. 127, Fig. 79; Hendey, 1964, p. 126, Plate 19, Fig. 3; Drebes, 1974, p. 77, Fig. 59; Rines & Hargraves, 1988, p. 69, Figs. 139 and 140.

Chains straight. Adjacent valves touch by two symmetrical valve protuberances at a short distance inside chain edge. Apertures small, elliptical, and shorter than apical axis. Setae thin and at nearly right angles to chain axis. Girdle bands conspicuous. Primary valve of resting spores evenly vaulted, with short spines; secondary valve smaller, centrally vaulted, and smooth.

Chaetoceros karianus Grunow (Table 54)

References: Grunow, in Cleve & Grunow, 1880, p. 120, Plate 7, Fig. 135; Hustedt, 1930, p. 736, Fig. 424. Chains straight and short. Poles of adjacent valves touch. Apertures narrowly lanceolate to elliptical. Setae thin and without basal part. Some of the inner setae perpendicular to chain axis, others curved around the chain. Inner part of terminal setae U shaped and outer part bent and outward divergent. Resting spores not known.

Chaetoceros subtilis Cleve (Plate 46, Table 54)

References: Cleve, 1896b, p. 30, Fig. 8; Hustedt, 1930, p. 723, Fig. 413; Hendey, 1964, p. 130, Plate 10, Fig. 2; Rines & Hargraves, 1988, p. 96, Figs. 204-206.

Chains short. Valves flat adjacent valves fitting tightly together. Apertures missing. Setae thin, arising at valve edge, straight, and all directed toward one end of chain. Resting spores with spines; the two valves unequally vaulted.

Chaetoceros subtilis var. abnormis Proschkina-Lavrenko is characterized by having only one terminal seta (Proschkina-Lavrenko, 1955 as Chaetoceros abnormis).

Chaetoceros wighamii Brightwell (Plate 46, Table 54) References: Brightwell, 1856, p. 108, Plate 7, Figs. 19-36; Hustedt, 1930, p. 724, Fig. 414; Cupp, 1943, p. 136, Fig. 91; Hendey, 1964, p. 131, Plate 11, Fig. 3.

Chains delicate and straight. Poles of adjacent valves touch. Apertures narrowly lanceolate to lanceolate. Setae thin, arising from valve margin, without basal part. Inner setae running very irregularly being perpendicular, bowed, or parallel to chain axis. Terminal setae not thicker than the others, often nearly parallel to chain axis. Primary valve of resting spores rounded, with fine spines, secondary valve constricted at base and blunt cone shaped in the middle.

3e. Valves of adjacent cells do not touch at the valve poles.

Chaetoceros diadema (Ehrenberg) Gran (Plate 46, Table 54) Basionym: Syndendrium diadema Ehrenberg (= resting spore). Synonyms: Chaetoceros distans var. subsecunda Grunow in Van Heurck, Chaetoceros subsecuttdus (Grunow) Hustedt. References: Ehrenberg, 1854, Plate 35a, Fig. 18/13; Van Heurck, 1880-1885, Plate 82bis, Figs. 6 and 7; Gran, 1897b, p. 20, Plate 2, Figs. 16-18; Hustedt, 1930, p. 709, Fig. 404; Cupp, 1943, p. 130, Fig. 83; Hendey, 1964, p. 128, Plate 10, Fig. 1; Drebes, 1974, p. 77, Fig. 60; Rines 8c Hargraves, 1988, p. 76, Figs. 150, 151, and 153. Chains slightly twisted about central axis. Setae arising inside valve margin; basal part extending outward in valvar plane, crossing at chain edge, generally perpendicular to chain axis but running fairly irregularly. Terminal setae diverging at an acute angle. Apertures rather wide and elliptical with a slight central constriction. Primary valve of resting spore topped with 4-12 dichotomously branching spines; secondary valve centrally inflated and smooth.

Chaetoceros holsaticus Schiitt (Table 54)

References: Schiitt, 1895, p. 40, Figs. 9a and 9b; Hustedt, 1930, p. 714, Fig. 407; Cupp, 1943, p. 131, Fig. 85; Hendey, 1964, p. 128, Plate 15, Fig. 4.

Chains straight and sometimes slightly twisted. Setae thin, arising from inside valve margin, basal part running diagonally outward, crossing at chain edge; outer part of setae perpendicular to chain axis or bent toward chain ends. Terminal setae diverging at an acute angle. Apertures wide and hexagonal. Resting spores with small spines; primary valve larger than secondary valve.

Chaetoceros seiracanthus Gran (Plate 46, Table 54) References: Gran, 1897b, p. 21, Plate 3, Figs. 39-41; Hustedt, 1930, p. 711, Fig. 405; Cupp, 1943, p. 131, Fig. 84; Hendey, 1964, p. 129, Plate 15, Fig. 1; Rines 8c Hargraves, 1988, p. 92, Fig. 199. Setae thin. Otherwise as described for C. diadema. Primary valve of resting spores smoothly vaulted and covered with small spines, secondary valve rounded to capitate, also covered by spines.

Distribution:

C. affinis and C. diadema—cosmopolitan.

C. costatus and C. seiracanthus—warm water region to temperate. C. karianus—northern cold water region.

C. holsaticus—cold water (Cupp, 1943), common in brackish water. C. subtilis and C. wighamii—probably restricted to brackish water.

3f. Chains curved and in irregular, spherical colonies. Chaetoceros socialis Lauder (Plate 47)

References: Lauder, 1864b, p. 77, Plate 8, Fig. 1; Hustedt, 1930, p. 751, Fig. 435; Cupp, 1943, p. 143, Fig. 100; Hendey, 1964, p. 136, Plate 15, Fig. 3; Drebes, 1974, p. 82, Fig. 68; Evensen 8c Hasle, 1975, p. 160, Figs. 33-39; Rines 8c Hargraves, 1988, p. 95, Fig. 207; Takano, 1990, pp. 292-293.

Chains short. Poles of adjacent valves not touching one another. Three setae of two adjacent valves short, the fourth one straight, elongated, and serving in formation of the more or less spherical secondary colonies by being entwined in the colony center with the elongated setae of other chains. Resting spores, both valves rounded and smooth. The central process is similar to that of C. curvisetus (Hasle 8c Evensen, 1975, EM). Morphometric data: Apical axis, 2-14 /xm.

Distribution: Probably cosmopolitan, very important in plankton close to the ice in the northern cold water region and, according to Cupp (1943), also one of the most prominent species in the Gulf of California. Remarks: Chaetoceros radians Schutt, also appearing in spherical colony, has usually been regarded as a separate species distinguished from C. socialis by spiny resting spores (Plate 47) but was reduced in rank to a variety of C. socialis by Proschkina-Lavrenko (1953).

3g. Unicellular species.

Rines 8c Hargraves (1988) mention seven unicellular Chaetoceros species commonly reported in the literature, one of them has now been transferred to Attheya and is discussed here. Of the other six Rines 8c Hargraves regarded C. ceratosporus Ostenfeld, C. muelleri Lemmermann (inland waters), C. simplex Ostenfeld, and C. tenuissimus Meunier as adequately described species possible to recognize. According to the same authors "names such as C. gracilis Schutt and C. calcitrans have most likely been applied to many different, not necessarily related forms which happen to have a similar appearance" (Rines 8c Hargraves, 1988, p. 99).

Chaetoceros ceratosporus Ostenfeld (Plate 47, Table 55) References: Ostenfeld, 1910, p. 278; Hustedt, 1930, p. 760, Fig. 442; Hendey, 1964, p. 138, Plate 17, Fig. 7; Rines 8c Hargraves, 1986, p. 104, Figs. 1, 2, 22, and 23. Cells most commonly single or in pairs. Valves drawn up at the poles, each valve with one central process, usually visible with LM. Setae, thin, originate at poles of apical axis and bend sharply outward. Primary valve

PLATE 47 Chaetoceros socialis and Chaetoceros radians: (a) chains and secondary colonics; (b) cells with resting spores; (c) cell in valve view. Chaetoceros simplex: (a) cell with chloroplast; (b) cell with resting spore. Chaetoceros ceratosporus: Cell with resting spore. After Hustedt (1930). Chaetoceros minimus: (a) whole cell; (b) detail. After Hustedt (1930). Chaetoceros tenuissimus: two cells. After Hustedt (1930). Chaetoceros throndsenii: part of cell with characteristic setae. After Marino et al. (1987). Attheya septentrionalis: three cells with characteristic setae. Scale bars = 20 ftm.

TABLE 55 Morphometric Data of Unicellular

Chaetoceros

spp.—Solitary Cells

Species

Apical axis (|xm)

C. ceratosporus

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