Key To Species

la. Cells heteropolar, one end with two winged spines, the other with no spines, smoothly rounded marginal spines in the middle of the vela. . 2 lb. Cells isopolar, either end with serrated protrusions, solitary, slightly curved, marginal spines at the lower edge of the vela

T. longissima Cleve & Grunow

2a. Cells in radiating colonies, joined by bent foot poles and a shorter or longer part of the cell length, sigmoid in girdle view

T. antarctica Schimper ex Karsten

2b. Cells solitary, usually straight, inflated in the middle

T. gibberula Hasle

Distribution:

T. antarctica—southern cold water region. T. gibberula—warm water region.

T. longissima—northern cold water region to temperate. Remarks: The colony-forming T. antarctica is readily recognized in water mount by the bent foot pole, and T. longissima, is recognized by the smooth curvature of the cell. As cleaned valves in permanent mounts T. longissima and T. antarctica are distinguished by the shape of the valve ends and the difference in location of the marginal spines (Hasle & Semina, 1987, Figs. 2-12, T. longissima; Figs. 34-43, T. antarctica). The heteropolarity and the location of the marginal spines in the middle of the vela make T. gibberula morphologically closer to T. antarctica than to T. longissima. The apparent lack of colony formation, the straight cells, and the inflation in the middle (Hasle, 1960, Fig. 6) characterize T. gibberula. The sternum of T. lanceolata Hustedt is narrower and the marginal spines are more widely spaced than those in T. gibberula (Simonsen, 1987, Plate 667, Figs. 7-11). A warm water species under description (Hasle, manuscript in preparation) differs from the Thalassiothrix species mentioned by the shape of the foot pole. Thalassiothrix gibberula was described from water samples of the equatorial Pacific (Hasle, 1960). The description was accompanied by a figure and a Latin diagnosis but no holotype was indicated (see Taxonomic Appendix).

Genus Trichotoxon F. M. Reid & F. E. Round 1988 (Plate 58) Type: Trichotoxon reinboldii (Van Heurck) Reid &c Round. Monospecific genus.

Trichotoxon reinboldii (Van Heurck) Reid &c Round Basionym: Synedra reinboldii Van Heurck. Synonym: Synedra pelagica Hendey.

References: Van Heurck, 1909, p. 23, Plate 3, Fig. 35; Hendey, 1937, p. 335; Hasle & Semina, 1988, p. 189, Figs. 67-75; Reid & Round, 1988. Cells solitary or in pointed, ovoid colonies formed by cells attached at either end. Cells bow shaped and not twisted. Valves expanded in central part and less at the ends. Cell ends isopolar with no apical spines. No marginal spines. Internal openings of areolae much smaller than the external openings (SEM). External vela reticulate. Morphometric data: Apical axis, 800-3500 pan; transapical axis of midsection, 5-8 fim, and of the ends, 3.5-6.6 fim (Reid & Round, 1987). Distribution: Southern cold water region.

How to identify: Diatoms of the family Thalassionemataceae in intact colonies, occasionally also as single whole cells, may be identified in water mounts. Permanent mounts of material cleaned of organic matter may be needed in critical cases, e.g., to distinguish between T. bacillare and T. frauenfeldii.

Suborder Bacillariineae—Raphid pennate diatoms

Terminology specific to raphid pennate diatoms (Mann, 1978; Ross et al., 1979; Round et ah, 1990): (Fig. 18)

Raphe system—one or two longditudinal slits through the valve wall. Central raphe ending—central end of the raphe slit when the raphe system consists of two slits.

Central pore—a pore-like expansion of the central raphe ending.

Central nodule—bridge of silica separating the two raphe slits, often thicker than the rest of the valve.

Stauros—a central nodule that is expanded transapically and reaches or almost reaches the margin of the valve. Terminal nodule—a thickening at the apical end of a raphe. Helictoglossa—an inwardly projecting lipped structure terminating the raphe on the inner side of the valve.

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