Key To Species

la. Both valves of cells in girdle view evenly curved, pili usually arising at some distance from the elevations on the convex valve, crossing each other in girdle view, smaller cells in ribbons

M. polymorphus (Hargraves 8c Guillard) Hasle, von Stosch, 8c Syvertsen lb. Pilus valve convex in the middle and concave distally, the other valve slightly convex, pili arising close to the elevations, more or less parallel in girdle view, diverging from the apical axis when seen in valve view, solitary M. scriptus Hasle, von Stosch, 8c Syvertsen

Distribution:

M. polymorphus—probably cosmopolitan, marine, planktonic. M. scriptus—known only from Helgoland and Bremerhaven, Germany, probably planktonic.

Genus Plagiogrammopsis Hasle, von Stosch, 8c Syvertsen, 1983 Type: Plagiogrammopsis vanheurckii (Grunow) Hasle, von Stosch, 8c Syvertsen.

Monospecific genus.

Plagiogrammopsis vanheurckii (Grunow) Hasle, von Stosch, 8c Syvertsen (Plate 36)

Basionym: Plagiogramma vanheurckii Grunow in Van Heurck. References: Van Heurck, 1880-1885, Plate 36, Fig. 4; Hasle et al., 1983, p. 30, Text Fig. 4, Figs. 104-131; Gardner 8c Crawford, 1994, Figs. 20-29. Girdle view: Ribbons, often twisted around their long axis. Cells in girdle view convex in the middle and constricted near the elevations. Valve view: Valve outline narrowly lanceolate with rostrate apices or broadly lanceolate, rhombic or subcircular. Fascia with a pseudoseptum. Labiate process submarginal in the fascia. Coarse areolation. Morphometric data: Apical axis 3-50 ¿im; transapical axis, 4 /xm; valve areolae, 12 in 10 /xm. Distribution: Cosmopolitan.

How to identify: Brockmanniella, Cymatosira, and Plagiogrammopsis may be identified in water mounts in girdle as well as in valve view. Arcocellulus and Minutocellus may easily be overlooked in water mounts, especially the smaller specimens. The larger and medium sized specimens with pili are characteristic and readily observable. The extreme variation in valve outline depending on cell size, present in all genera, also complicates the identification when cleaned material mounted in a medium of a high refractive index or in EM is examined.

Remarks: The labiate process of Brockmanniella, Cymatosira and Plagiogrammopsis has a tubular external part (EM) and is discernible with

LM. The tubular process of Arcocellulus and Minutocellus is short externally and internally and is visible with EM.

Incertae sedis (Cymatosiraceae):

Genus Lennoxia Thomsen 8c Buck 1993 Type: Lennoxia faveolata Thomsen 8c Buck. Monotypic genus.

Lennoxia faveolata Thomsen 8c Buck Reference: Thomsen et al., 1993, p. 279, Figs. 1-16. Girdle view: Cells bipolar and rostrate. Approximately eight weakly silicified bands without ornmentation. One chloroplast. Valve view: Spindle shaped, apical axis curved; middle part roughly triangular. Valve face with honeycomb pattern of flat, hexagonal chambers (TEM). Reminiscent marginal tubular process on one valve (TEM).

Morphometric data: Apical axis, 10-22 /im; pervalvar axis, 1.5-2 ju,m; hexagonal chambers, 0.15-0.2 ju,m in diameter.

Distribution: South America, central California, West Greenland, and Denmark. Cell counts published by Thomsen et al. (1993) characterize the species as an important, frequently and abundantly occurring species in marine plankton.

How to identify: When examined with LM L. faveolata is confusingly similar to certain species of the subgenus Nitzschiella, and if ever observed, it has most probably been identified as Nitzschia closterium/Cylindrotheca closterium.

Family Chaetocerotaceae Ralfs in Prichard 1861

Bacteriastrum and Chaetoceros belong in this family (Simonsen, 1979; Glezer et al., 1988; Round et al., 1990). For the sake of convenience Attheya is also mentioned here since two former Chaetoceros species, C. armatus and C. septentrionalis, are now transferred to Attheya (Crawford et al., 1994). Attheya is usually not classified under Chaetocerotaceae, and it is not primarily planktonic. Simonsen (1979) placed Attheya in Hemiauloideae, subfamily of Biddulphiaceae. The family Hemiaulaceae is, in this chapter, used for diatoms with valve outgrowths, called elevations or horns, structured like the rest of the valve and not projecting laterally beyond the valve margin. The valve outgrowths of Hemiaulaceae thus differ from the setae of Chaetocerotaceae in structure and orientation (see below). The valve outgrowths of Attheya spp. are structured very much like the rest of the valve, and they do not project outside the valve margin in the generitype A. decora but they do in the former C. armatus and C. septentrionalis (Plate 47; Round et al., 1990, p. 334). Round et al. (1990) solved the problem concerning the classification of Attheya by

BROAD GIRDLE VIEW FIGURE 16 Schematic illustration of Chaetoceros sp. in broad girdle view with terminology.

describing a new monotypic family Attheyaceae Round & Crawford. The new family and Chaetocerotaceae were placed in the order Chaetocerotales Round & Crawford in the subclass Chaetocerotophycidae Round & Crawford.

Terminology specific to Chaetocerotaceae (partialy after Rines & Hargraves, 1988) (Fig. 16):

Aperture—foramen—open space between adjacent (sibling) cells/valves in chains.

Seta—hollow outgrowth of valve projecting outside the valve margin, with structure different from that of the valve.

Inner setae—intercalary setae—setae occurring within the chain.

Terminal setae—setae of end valves of a chain (= separation valves).

Basal part—portion of a terminal seta closest to valve face, portion of an inner seta between its point of origin on valve face and its point of fusion or crossing with its sibling seta.

Family characters:

Valves with long setae.

Cells solitary or, in chains, mostly inseparable, formed by fusion of silica between setae.

Resting spores common, always endogenous and distinctly different from the vegetative cells.

Because of the fusion of silica between setae, sibling valves from cells within a chain are usually held together by their setae when organic material has been removed, as in Skeletonema costatum, by the external parts of the strutted processes.

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