1 b. Terminal setae on either end of chain different in form and direction, inner setae very long and curved in the same direction along chain axis

B. comosum Pavillard

2a. Bifurcation in the apical plane ( parallel to chain axis), fused part of inner setae short 3

2b. Bifurcation in the valvar plane ( transverse to chain axis), fused part of inner setae long 4

3 a. Terminal setae with arched base, then running outward nearly parallel to chain axis, bell shaped, inner setae usually six on each valve

B. elongatum Cleve

3 b. Terminal setae umbrella shaped, inner setae numerous on each valve . .

B. hyalinum Lauder

4a. Terminal setae of both ends directed toward the chain

B. delicatulum Cleve

4b. Terminal setae first transverse to chain axis, then abruptly obliquely curved toward the chain in their outer part B. furcatum Shadbolt


B. comosum—warm water region. B. delicatulum—temperate waters. B. elongatum—warm water region to temperate.

B. furcatum—poorly known, often recorded under the name B. varians and confused with B. hyalinum (Hustedt, 1930; Cupp, 1943). B. hyalinum—common in temperate waters. How to identify: The species may be identified in water mounts, especially if it is present as intact chains showing the shape of the terminal setae. Some other characters may be used for identification of single cells or part of chains, e.g., for B. comosum, the long, inner setae bent toward the one terminal valve with coarse setae; for B. elongatum, the long cells; for B. hyalinum, the large number of inner setae and the hairy appearance caused by bifurcation of the setae in the apical plane; for B. delicatulum, the slightly curved and wavy branches of the inner setae; and for B. furcatum, the inner part of the terminal setae perpendicular to the end valve surface. Remarks: Bacteriastrum furcatum was found in "near-bloom numbers" in the northwest Gulf of Mexico (Fryxell, 1978c). Bacteriastrum hyalinum seems to be the most common species of the genus in the North Sea and the north Atlantic Ocean, especially in summer (Hendey, 1964; Drebes, 1972).

Genus Chaetoceros Ehrenberg 1844

Lectotype: Chaetoceros tetrachaeta Ehrenberg (vide Boyer, 1927, p. 104).

The genus, Chaetoceros dichaeta and C. tetrachaeta, were described in the same paper based on Antarctic material (Ehrenberg, 1844b, pp. 198 and 200, genus and species, respectively). Chaetoceros tetrachaeta is regarded as a taxo-nomic entry not recommended for use (VanLandingham, 1968), whereas C. dichaeta is frequently recorded from the Antarctic.

Chaetoceros is one of the largest, if not the largest genus of marine planktonic diatoms with approximately 400 species described. Although it is assumed that only a fraction, "no more than one-third to one-half," are still valid (Rines & Hargraves, 1988, p. 19), it leaves us with a high number of species, almost all of which are marine.

Several attempts have been made to impose structure on this large genus by a division into two subgenera and numerous sections (see Hustedt, 1930, p. 630; Cupp, 1943, p. 101; Rines & Hargraves, 1988, pp. 21, and 117). For the sake of simplicity only the names of the subgenera, Phaeoceros and Hyalochaete, will be used here, and species within both subgenera are grouped according to prominent morphological character(s). Hernandez-Becerril (1993a) proposed a new subgenus Bacteriastroidea for Chaetoceros bacterias-troides Karsten and changed the subgenus name Phaeoceros to Chaetoceros. Hernandez-Becerril (1992b; 1993a,b) and Hernandez-Becerril et al. (1993) examined seven warm water Chaetoceros species which are not dealt with in this chapter.

Generic characters:

Cells more or less rectangular in girdle view.

Cells elliptical to almost or rarely circular in valve view.

Opposite setae of adjacent cells (sibling setae) "touch" one another near their origin.

Characters showing differences between species (partially after Rines & Hargraves, 1988): Chloroplasts

The presence/absence in setae.





Coarseness (thickness, spines). Direction.

Terminal setae different from the inner setae in shape and coarseness (e.g., C. affinis) and in direction (e.g., C. atlanticus). Some inner setae different from the others (e.g., C. compressus and G. diversus).

Adjacent (sibling) setae fused for some distance (e.g., C. decipiens). One seta of a cell longer than the three others (e.g., C. socialis). Direction of basal part of setae.

Point of fusion of sibling setae: inside valve or chain margin, at the margin, external to the margin.

Shape and size of aperture determined by Point of origin of setae on valve surface.

Point of fusion of sibling setae (e.g., basal part absent; i.e., apices of sibling valves touching: C. curvisetus and C. constrictus; basal part present and point of fusion of sibling setae at some distance away from the valves: C. debilis and C. diadema).

Height of girdle e.g., in C. convolutus > one-third of pervalvar axis; in C. concavicornis < one-third of pervalvar axis.

Direction of chain

Straight (e.g., C. laciniosus). Curved or helical (e.g., C. debilis). Twisted (e.g., C. tortissimus).

Resting spores

Primary valve (epivalve) and secondary valve (hypovalve) similar or dissimilar. Spiny or smooth. Protuberances with branches.

Paired spores with fused setae of parent hypothecae.

For information on resting spores see Proschkina-Lavrenko (1955) who also includes species not mentioned in this chapter.

A. Subgenus Phaeoceros Gran 1897

Subgeneric characters:

Chloroplasts numerous small granules throughout the whole cell, the setae included.

Large robust forms.

Setae strong, thick, often very long, striated, and armed with conspicuous spines.

One, seldom many, central processes on every valve, often located closer to one side of the valve; no protrusion or thickening around the inner opening (EM).

Valves irregularly perforated with simple holes; more weakly silicified valves with a weak pattern of costae branching out from an annulus (TEM). Mostly oceanic.

Resting spores reported for one species.

1. Cells solitary or in short chains; external part of central process inconspicuous: C. aequatorialis, C. criophilus, C. danicus, C. peruvianus, and C. rostratus (Table 46).

2. Terminal setae differentiated from the others by length and direction, straight chains, wide apertures; external part of central process long and tubular: C. atlanticus and C. dichaeta (Table 47).

3. Terminal setae not distinctly differentiated from the others; setae often diverging in all directions; apertures smaller than cell body; external part of processes inconspicuous: C. borealis, C. castracanei, C. coarctatus, C. concavicornis, C. convolutus, C. dadayi, C. densus, C. eibenii, and C. tetrastichon (Table 48).

4. Setae not fused; apertures narrow in apical and pervalvar directions;

setae delicate; external part of central process inconspicuous: C. flexuosus

B. Subgenus Hyalochaetae Gran 1897

Subgeneric characters:

Chloroplasts, one or a few plates or, more rarely, numerous small granules.

Setae thin, often hair-like; spines and structure seen with LM in some species; no chloroplasts. One, seldom several, central processes on terminal valves and no such processes on intercalary valves; inner opening of processes with or without labiate-shaped protrusions (EM).

Valves with a more or less regular pattern of costae branching out from an annulus; often holes or poroids between the costae (TEM).

Mainly distributed in coastal and inshore waters.

Resting spores in many species.

1. Cells with more than two chloroplasts.

a. Four to 10 chloroplasts; terminal setae more or less differentiated from the others by coarseness and orientation: C. decipiens, C. lorenzianus, and C. mitra (Table 49).

b. Numerous small plate-like chloroplasts; terminal setae scarcely different from the others: C. compressus, C. teres, and C. lauderi (Table 49).

2. Cells with two chloroplasts.

a. Cells with a hemispherical or conical protuberance: C. didymus (Table SO).

b. Cells with deep constriction between valve and girdle band: C. constrictus (Table 50).

c. Apertures high, elliptical, and square to rectangular: C. laciniosus (Table 50).

d. Adjacent cells touching each other in the middle by a central raised region; apertures narrow and divided into two parts: C. similis (Table 50).

e. Setae of adjacent cells not crossing; connected by a pervalvarly directed bridge: C. anastomomosans (Table 50).

3. Cells with one chloroplast.

a. Chains curved or helical; intercalary setae all bent in one direction: C. curvisetus, C. debilis, and C. pseudocurvisetus (Table 51).

b. Chains mostly loose; resting spores united in pairs; resting spore parent cells with fused hypovalvar setae and no apertures: C. cintus, C. furcellatus, and C. radicans (Table 52).

c. Inner setae of two kinds: C. diversus and C. messanensis (Table 53).

d. Valves of adjacent cells touch: C. affittis, C. costatus, C. karianus, C. subtilis, and C. wighamii (Table 54).

e. Valves of adjacent cells do not touch: C. diadema, C. holsaticus, and C. seriacantbus (Table 54).

f. Chains curved and joined in irregular spherical colonies: C. socialis and C. radians.

g. Unicellular species: C. ceratosporus, C. simplex, and C. tenuissimus (Table 55).

4. Cells with one chloroplast; unicellular; two, seldom three, setae per cell: C. minimus and C. throndsenii (Table 56).

A. Subgenus Phaeoceros

1. Cells solitary or in short chains.

Chaetoceros aequatorialis Cleve (Plate 38, Table 46) Synonym:? Chaetoceros pendulus Karsten.

References: Cleve, 1873a, p. 10, Fig. 9; Karsten, 1905, p. 118, Plate 15, Fig. 7; Karsten, 1907, p. 389, Plate 45, Fig. 1; Schmidt, 1920, Plate 325, Figs. 1 and 2; Hendey, 1937, p. 294. Cells heterovalvate; upper valve with central depression or smoothly concave and lower valve with projecting corners and sometimes short tubular extension of the central process. Setae long, very coarse, emerging well inside valve margin, not abutting at point of emergence, proceeding outwards almost at right angles to pervalvar axis, then curving downwards. Setae of upper and lower valves almost parallel and armed with spines.

Chaetoceros criophilus Castracane (Plate 38, Table 46) References: Castracane, 1886, p. 78; Mangin, 1915, p. 34, Figs. 13 and 14; Hendey, 1937, p. 295, Plate 13, Fig. 7; Hasle, 1968b, p. 7, Plate 10, Fig. 12, Map 4; Fryxell, 1989, p. 10, Figs. 33-38. Cells heterovalvate; upper valve almost flat and lower valve curved. Setae long, coarse, and curved. Upper setae emerge at valve margin giving the

PLATE 38 Chaetoceros aequatorialis: cells in broad girdle view. After Schmidt (1920). Scale bar = 50 /um. Chaetoceros criophilus: chain in broad girdle view. Scale bar = 50 /um. Chaetoceros danicus: (a) cell in broad girdle view; (b) cell in valve view. Scale bar = 50 p.m. Chaetoceros peruvianus: (a) cell in broad girdle view. Scale bar = 50 /im; (b) detail of cell in broad girdle view. Scale bar =10 /im.

TABLE 46 Morphometric Data of Chaetoceros spp. Appearing as Solitary Cells or Short Chains

Species Apical axis (|im)

C. aequatorialis ca. 25

C. criophilus 16-50

C. danicus 8-20

C. peruvianas 10-32

C. rostratus 10-37

appearance of being a continuation of valve surface; lower setae emerge inside valve margin. Chains with lanceolate; narrow apertures.

Chaetoceros danicus Cleve (Plate 38, Table 46) References: Cleve, 1889, p. 55; Hustedt, 1930, p. 659, Fig. 373; Cupp, 1943, p. 109, Fig. 65; Hendey, 1964, p. 122, Plate 10, Fig. 5; Drebes, 1974, p. 66, Fig. 50; Koch & Rivera, 1984, p. 71, Figs. 53-56; Rines & Hargraves, 1988, p. 49, Figs. 95-99. Cells isovalvate; valve surface flat. Setae long, stiff, perpendicular to pervalvar or chain axis, and originating at valve margin. Chains with small apertures—terminal setae perpendicular to chain axis, intercalary setae basally directed toward one end of the chain then become parallel to the terminal setae. External part of central process a short, flattened tube (Koch & Rivera, 1984, Fig. 54, SEM).

Chaetoceros peruvianus Brightwell (Plate 38, Table 46) Synonym: Chaetoceros chilensis Krasske.

References: Brightwell, 1856, p. 107, Plate 7, Figs. 16-18; Brightwell, 1858b, p. 155, Plate 8, Figs. 9 and 10; Hustedt, 1930, p. 671, Figs. 380 and 381; Krasske, 1941, p. 266, Plate 4, Fig. 3, Plate 6, Figs. 1 and 2; Cupp, 1943, p. 113, Fig. 68; Hendey, 1937, p. 296, Plate 13, Fig. 6; Hasle, 1960, p. 15, Fig. 2; Koch & Rivera, 1984, p. 69, Figs. 36-47.

Cells heterovalvate; upper valve rounded and lower valve flat. Setae of upper valve arising in pervalvar direction from near valve center, abutting with a groove between them, turning sharply, and running backward in more or less outwardly convex curves. Setae of lower valve originating inside valve margin and slightly convex toward outside. Central process between the bases of the setae, subcentral in location; external part conical (Koch &c Rivera, 1984, EM).

Chaetoceros rostratus Lauder (Table 46)

References: Lauder, 1864b, p. 79, Fig. 10; Hustedt, 1930, p. 660, Fig. 374; Rines 8c Hargraves, 1988, p. 55, Figs. 105-107; Guiffre 8c Ragusa, 1988. Cells isovalvate; valve surface flat or slightly convex. Cells in chains (two to six cells) held together by a central intervalvar connection [not a labiate-like process (Rines 8c Hargraves, 1988)]. Setae perpendicular to the pervalvar axis. Terminal cells heterovalvate i.e. terminal valve lacking the protuberance forming the intervalvar connection. External part of subcentrally located process a laterally compressed tube, slightly cone shaped (Guiffre 8c Ragusa, 1988, SEM).


C. aequatorialis and C. rostratus—warm water region. C. criophilus—southern cold water region. C. danicus—cosmopolitan? C. peruvianus—warm water region to temperate.

2. Terminal setae differentiated from the others by length and direction. Chaetoceros atlanticus Cleve (Plate 39, Table 47)

References: Cleve, 1873b, p. 11, Plate 2, Fig. 8; Hustedt, 1930, p. 641, Figs. 363 and 364; Cupp, 1943, p. 103, Fig. 59; Evensen 8c Hasle, 1975, p. 157, Figs. 6-11; Koch 8c Rivera, 1984, p. 63, Figs. 1-5; Takano, 1990, pp. 282-283.

Cells rectangular in broad girdle view. Apertures hexagonal and smaller than the cells. Setae arising slightly within valve margin; basal part first narrow, then widened, and diagonally oriented. Inner setae almost straight. Terminal setae shorter than others, usually forming a V.

Chaetoceros dichaeta Ehrenberg (Plate 39, Table 47) References: Ehrenberg, 1844b, p. 200; Mangin, 1922, p. 60, Fig. 6; Hendey, 1937, p. 291, Plate 6, Figs. 9 and 10; Evensen 8c Hasle, 1975, p. 157, Figs. 1-5; Koch 8c Rivera, 1984, p. 64, Figs. 6-12. Cells rounded in broad girdle view. Apertures hexagonal to square, usually larger in pervalvar direction than the cell proper. Setae arising far inside valve margin; basal part long, more or less parallel to the chain axis, then bent outward at nearly right angles to chain axis; terminal setae later bent again so they are once more nearly parallel to chain axis.

PLATE 39 Chaetoceros atlanticus: (a) chain. Scale bar = 50 fun; (b) detail of end cell in broad girdle view with external part of process. Scale bar = 10 fim. After Cupp (1943). Chaetoceros dichaeta: partial chain in broad girdle view. After Mangin (1922). Scale bar = 50 /um. Chaetoceros borealis: chain in broad girdle view. Scale bar = 50 pm. Chaetoceros castracanei: partial chain in broad girdle view. After Karsten (1905). Scale bar = 50 fim.

C. castracanei

TABLE 47 Morphometric Data of Chaetoceros spp. with Terminal Setae Differing from Intercalary Setae


Apical axis (|im)

C. atlanticus

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