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Evensen 8c Hasle, 1975, p. 158, Figs. 12-14; Rines 8c Hargraves, 1988, p. 45, Fig. 93.

Chains straight, not twisted. Cells isovalvate; apertures elliptical to hexagonal. Setae arising well inside valve margin having a distinct basal part; point of fusion near chain edge, setae often crossing each other. External part of central process a short tube (Evensen 8c Hasle, 1975, EM).

Chaetoceros castracanei Karsten (Plate 39, Table 48)

References: Karsten, 1905, p. 116, Plate 15, Fig. 1; Priddle 8c Fryxell, 1985, p. 26.

Chains straight. Cells isovalvate. Apertures narrow, almost closed by a central valve protuberance. Setae emerging within valve margin, fused outside chain edge, and perpendicular to chain axis in their outer part.

Chaetoceros coarctatus Lauder (Plate 40, Table 48) References: Lauder, 1864b, p. 79, Fig. 8; Hustedt, 1930, p. 655, Fig. 370; Hendey, 1937, p. 293, Plate 5, Figs. 7 and 8; Cupp, 1943, p. 107, Fig. 62; Hernandez-Becerril, 1991b. Chains long and robust. Cells isovalvate. Apertures almost absent. Valve surface flat. Posterior terminal setae large, strongly curved, and shorter than the others and anterior terminal setae less robust. Usually found with a species of Vorticella attached. Eighteen to 22 small, slit-like processes on each valve (Hernandez-Becerril, 1991b, EM).

Chaetoceros concavicornis Mangin (Plate 40, Table 48) References: Mangin, 1917, p. 9, Figs. 5-7; Hustedt, 1930, p. 665, Figs. 376 and 377; Cupp, 1943, p. 109, Fig. 66; Hendey, 1964, p. 122, Plate 9, Fig. 1; Evensen 8c Hasle, 1975, p. 158, Figs. 15-22. Chains straight. Cells heterovalvate; upper valve rounded, setae arising from near center; lower valve flat, setae emerging from inside valve margin. Apertures trapezoid like. Girdle zone narrow (less than one-third of the length of pervalvar axis). Setae thin at base, wider outward, and all bent toward lower end of chain, and outside line concave. External part of central process a short tube (Evensen 8c Hasle, 1975, EM).

Chaetoceros convolutus Castracane (Plate 41, Table 48) References: Castracane, 1886, p. 78; Hustedt, 1930, p. 668, Fig. 378; Cupp, 1943, p. 110, Fig. 67; Fryxell 8c Medlin, 1981, p. 9, Figs. 43-49; Koch 8c Rivera, 1984, p. 67, Figs. 23-35; Rines 8c Hargraves, 1988, p. 47, Fig. 94.

Chains sometimes twisted about pervalvar axis. Cells heterovalvate, upper valve vaulted, setae arising from near center; lower valve flat, setae emerging from well inside valve margin. Setae of the lower valves bent

toward the same end of the chain. Sibling setae often twisted at their basal part thus partialy or wholly covering the apertures. Sibling valves held together by siliceous flaps on setae of upper valve (Fryxell 8c Medlin, 1981; Koch 8c Rivera, 1984, EM). Girdle zone fairly broad (about one-third of the length of pervalvar axis). Setae not increasing in width from base and outward (cf. C. concavicornis). External part of central process short and tubular (Fryxell 8c Medlin, 1981; Koch 8c Rivera, 1984, EM).

Chaetoceros dadayi Pavillard (Plate 41, Table 48) References: Pavillard, 1913, p. 131, Fig. 2b; Hustedt, 1930, p. 658, Fig. 372; Cupp, 1943, p. 109, Fig. 64; Hernandez-Becerril, 1992c, p. 367, Figs. 9-22. Recognized and differentiated from C. tetrastichon by formation and direction of setae (Cupp, 1943). Setae arising from valve margin without a basal part. Setae on one side of the chain short and on the other side very long with some running toward one end of the chain and some toward the other. Usually found with a tintinnid attached.

Chaetoceros densus (Cleve) Cleve (Table 48) Basionym: Chaetoceros borealis var. densus Cleve. References: Cleve, 1897a, p. 20, Plate 1, Figs. 3 and 4; Cleve, 1901a, p. 299; Hustedt, 1930, p. 651, Fig. 368; Drebes, 1974, p. 64, Fig. 48; Rines 8c Hargraves, 1988, p. 50, Figs. 100 and 101. Cells tightly packed together in chains. Cells isovalvate; valve surface slightly convex. Apertures narrowly lanceolate. Setae perpendicular to chain axis or bent slightly toward chain ends, diverging strongly from apical plane.

Chaetoceros eibenii Grunow in Van Heurck (Plate 41, Table 48) References: Van Heurck, 1880-1885, Plate 82, Figs. 9 and 10; Pavillard, 1921, p. 469, Figs. 1-11; Hustedt, 1930, p. 653, Fig. 369; Cupp, 1943, p. 106, Fig. 61; Drebes, 1974, p. 64, Fig. 49; Koch 8c Rivera, 1984, p. 65, Figs. 13-22; Rines 8c Hargraves, 1988, p. 52, Figs. 102-104. Cells isovalvate; valve surface slightly concave with a small central process on each valve (LM). Apertures lanceolate to hexagonal. Setae generally bent toward nearest end of the chain. Resting spores formed inside lateral auxospores ("in need of further study" according to Rines 8c Hargraves, 1988). External part of central process laterally flattened; internally visible as a slit (Koch 8c Rivera, 1984, Figs. 17-20, SEM).

PLATE 40 Chaetoceros coarctatus: chain in broad girdle view with Vorticella. After Cupp (1943). Chaetoceros concavicornis: chain in broad girdle view. After Hustedt (1930). Chaetoceros anastomosans: chain in broad girdle view. After Hustedt (1930). Scale bars = 20 pirn.

PLATE 41 Chaetoceros convolutus: chain in broad girdle view. Chaetoceros dadayi: with tintin-nid. After Cupp (1943). Chaetoceros tetrastichon: chain. After Cupp (1943). Chaetoceros flexuosus: chain. Chaetoceros eibenii: Chain. After Cupp (1943). Scale bars = 20 /xm.

Chaetoceros tetrastichon Cleve (Plate 41, Table 48) References: Cleve, 1897a, p. 22, Plate 1, Fig. 7; Hustedt, 1930, p. 657, Fig. 371; Cupp, 1943, p. 108, Fig. 63; Hendey, 1964, p. 123, Plate 11, Fig. 1; Hernandez-Becerril, 1992c, p. 367, Figs. 1-8.

Chains straight. Cells isovalvate; valve surface flat. Apertures almost absent. Setae emerging from valve margin, curved outward, almost at right angles to chain, toward ends turned nearly parallel to chain axis. One pair of setae generally curve more strongly and may be attached to a tintinnid (Hernandez-Becerril, 1992c).

Distribution:

C. borealis—cosmopolitan?, probably most common in temperate and cold water.

C. concavicornis—cosmopolitan but absent from southern cold water region, common in northern cold water region and temperate waters. C. convolutus and C. densus—cosmopolitan, probably most common in temperate waters.

C. castracanei—southern cold water region. C. coarctatus, C. dadayi, and C. tetrastichon—warm water region. C. eibenii—warm water region to temperate. Remarks: It may be questioned whether C. hendeyi Manguin is a separate species endemic to the southern cold water region or conspecific with C. borealis. It should be noted that specimens including at the same time features specific to C. borealis and C. concavicornis, are present in the northern cold water region (Gran, 1904; Braarud, 1935; Holmes, 1956). Based on these findings, Braarud (1935, p. 92) proposed C. concavicornis to be a form of C. borealis. Chaetoceros convolutus and C. concavicornis are similar in gross morphology as are C. tetrastichon and C. dadayi; distinctive characters have been pointed out previously.

4. Setae not fused. Chaetoceros flexuosus Mangin (Plate 41)

References: Mangin, 1915, p. 45, Fig. 27; Hasle, 1968b, p. 7, Plate 10, Fig. 14, Map 4; Fryxell 8c Medlin, 1981, p. 6, Figs. 1-5 and 16-28. Cells held together in colonies by sibling setae twisted around each other in one or two turns (no fusion of silica, Fryxell 8c Medlin, 1981, EM observations). Apertures narrow but conspicuous. A process located toward one margin from the valve center, externally forms a cone cut at the top (Fryxell 8c Medlin, 1981, SEM). Morphometric data: Pervalvar axis, 9-20 ju.m; apical axis, 9-15 /¿m.

Distribution: Southern cold water region.

Remarks: Chaetoceros flexuosus has been found crowded with chloroplasts extending into the setae (Fryxell 8c Medlin, 1981). Despite the extremely delicate setae it thus belongs in Pbaeoceros, which is a taxononmic position also supported by the presence of a central process on every valve.

B. Subgenus Hyalochaete la. Four to 10 chloroplasts.

Chaetoceros decipiens Cleve (Plate 42, Table 49)

References: Cleve, 1873b, p. 11, Plate 1, Figs. 5a and 5b; Hustedt, 1930, p. 675, Fig. 383; Cupp, 1943, p. 115, Fig. 70; Hendey, 1964, p. 123, Plate 12, Fig. 2; Drebes, 1974, p. 69, Fig. 52a; Evensen & Hasle, 1975, p. 161, Figs. 55-69; Rines & Hargraves, 1988, p. 75, Figs. 148, 149, and 152. Chains straight and stiff. Valve corners touching those of adjacent cell. Apertures slit-like to broadly lanceolate. Sibling setae fused for a length several times their diameter and then diverging in pervalvar direction. Terminal setae thicker than the others, initially divergent, then bent, becoming parallel to chain axis. No resting spores. Central process with no distinct protrusion on the outside and a labiate structure on the inside (Evensen & Hasle, 1975, Figs. 63-66, EM).

Chaetoceros lorenzianus Grunow (Plate 42, Table 49) References: Grunow, 1863, p. 157, Plate 5, Fig. 13; Hustedt, 1930, p. 679, Fig. 385; Cupp, 1943, p. 118, Fig. 71; Hendey, 1964, p. 124, Plate 16, Fig. 1; Rines & Hargraves, 1988, p. 85, Figs. 178-184. Chains straight and stiff. Apertures elliptical to oval. Sibling setae fused only at the point of exit from margin; setae with distinct transverse rows of pores. Terminal setae divergent in broad girdle view for their whole length. Primary valve of resting spores with two conical protuberances which branch repeatedly and dichotomously at the tips; secondary valve smooth.

Chaetoceros mitra (Bailey) Cleve (Plate 42, Table 49) Basionym: Dicladia mitra Bailey. Synonym: Dicladia groenlandica Cleve.

References: Bailey, 1856, p. 4, Plate 1, Fig. 6; Cleve, 1873b, p. 12, Plate 2, Figs. 1 and 2; Cleve, 1896a, p. 8, Plate 2, Figs. 1 and 2; Hustedt, 1930, p. 677, Fig. 384; Hasle & Syvertsen, 1990b, p. 288, Figs. 5-9 and 11. Chains straight. Apertures narrowly lanceolate and slightly constricted centrally. Setae crossing only at the point of exit from margin, setae without distinct structure (LM). Terminal setae parallel or convergent in broad girdle view and divergent in narrow girdle view. Primary valve of resting spores with two conical protuberances, terminating into a stalk which branches dichotomously at the tip; secondary valve smooth. Remarks: Intermediate forms between C. decipiens and C. lorenzianus exist (Rines & Hargraves, 1988). Usually the two species are distinguished by the

PLATE 42 Chaetoceros decipiens: setae fused at base. Chaetoceros lorenzianus: (a) chain; (b) detail of seta. Partly after Cupp (1943). Chaetoceros mitra: (a) partial chain with chloroplasts; (b) cells with resting spores. Chaetoceros teres: (a) chain with chloroplasts; (b) cell with resting spore. After Hustedt (1930). Chaetoceros compressus: chain with one pair of thickened intercalary setae and resting spores. Scale bars = 20 fim.

TABLE 49 Morphometric Data of Chaetoceros spp. with More Than Two Chloroplasts

Species

Apical axis (|xm)

C. decipiens

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