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2a. Valve margins generally straight or slightly convex. Valve width approximately the same along the whole valve length except near the apices 4

2b. Whole valve almost narrowly clavate: one end acutely club shaped, the other end pointed and ending in a spine. Valve narrow near the club-shaped end, wider in the middle and close to the tapering apex. Marginal structure visible with LM as short ribs between less silicified interspaces T. javanicum22 (Grunow in Van Heurck) Hasle comb. nov. 3a. Valves linear to narrowly lanceolate in outline, presence of apical spine variable, marginal structure visible with LM as ribs

T. nitzschioides (Grunow) Mereschkowsky

3b. Valves more or less expanded in the middle and less often at the ends, areolae visible with LM as circular or subcircular holes, a structure sometimes visible within the holes

T. bacillare22 (Heiden in Heiden & Kolbe) Kolbe

4a. Valves linear in outline, one end more or less tapering and usually with a spine, the other end broader, rounded, and sometimes slightly expanded, areolae crossed by a simple strongly silicified bar, discernible with

LM T. frauenfeldii22 (Grunow) Hallegraeff

4b. Valves linear in outline except for the one tapering end, usually with a spine, both apices smoothly rounded, marginal structure visible with LM

as short ribs

. ... T. pseudonitzschioides22 (Schuette & Schrader) Hasle comb. nov.

Distribution:

T. bacillare—warm water region.

T. frauenfeldii—warm water region to temperate.

T. javanicum—warm water region to temperate.

T. nitzschioides—cosmopolitan but not in the high Arctic and

Antarctic.

T. pseudonitzschioides—warm water region? Remarks: Thalassiothrix pseudonitzschioides was said to differ from T. nitzschioides "by having heteropolar apices" (Schuette & Schrader, 1982). Hallegraeff (1986) regarded the two species as conspecific, probably interpreting the heteropolarity as the presence of one apical spine per valve.

22 Basionyms: Thalassiothrix frauenfeldii var. javanica Grunow in Van Heurck, Spinigera bacillaris Heiden in Heiden & Kolbe; Asterionella frauenfeldii Grunow and Thalassiothrix pseudonitzschioides Schuette & Schrader, respectively. Synonym: Thalassiothrix (fauenfeldii [sic!] var.) javanica (Grunow) Cleve and Thalassiothrix frauenfeldii (Grunow) Grunow in Cleve & Grunow, respectively.

Based on the areola structure T. pseudonitzschioides certainly belongs to Thalassionema. The valve outline with one apex more pointed and the other more rounded exhibits only inconsiderable variation from the smallest to the largest specimens (Schuette 8c Schrader, 1982; G. Hasle and E. Syvertsen, personal observations), and distinguishes T. pseudonitzschioides as a separate species. Grunow's drawings of T. frauenfeldii and T. javanicum (Van Heurck, 1880-1885, Plate 37, Figs. 12 and 13) point out the difference in the marginal structure which refers to the density of areolae as well as the velum structure seen with EM. There is a certain possibility that Asterionella synedraeformis (Greville, 1865a) is an earlier synonym of T. frauenfeldii var. javanica. The variety was raised in rank by Cleve (1990c) which antedates T. javanica Hustedt in Meister, 1932 (see VanLandingham, 1967-1979, p. 4001). It should be noted that, in agreement with Hallegraeff (1986), weight here has been put on the presence and absence of marginal spines and the areola structure and not on the polarity of cells as the distinction between Thalassiothrix and Thalassionema.

Genus Thalassiothrix Cleve 8c Grunow 1880 (Plates 58 and 59, Table 67) Type: Thalassiothrix longissima Cleve 8c Grunow (conservation proposed by Silva 8c Hasle, 1993).

Synonym: Synedra thalassiothrix Cleve ["Thalassothrix"]. References: Cleve, 1873b, p. 22, Plate 4, Fig. 24; Cleve 8c Grunow, 1880, p. 108; Karsten, 1905, p. 124, Plate 17, Fig. 12; Cupp, 1943, p. 184, Fig. 134; Hustedt, 1959, p. 247, Fig. 726; Hasle, 1960, p. 19, Fig. 6, Plate 5, Figs. 46-48; Hallegraeff, 1986, pp. 64 and 66-69; Hasle 8c Semina, 1987; Silva 8c Hasle, 1993.

Generic characters:

Cells solitary or in radiating colonies.

Cells straight or slightly curved or sigmoid.

Cells usually strongly twisted.

Cells isopolar or heteropolar.

Valves more or less inflated in the middle and near the apices. Sternum wide and sometimes narrower near the apices. One marginal row of areolae.

External openings of areolae elongate, with LM appearing as short marginal striae.

Internal openings (foramina) of areolae smaller than the external openings (Plate 59, Fig. la; SEM).

Thalassiothrix longissima

Trichotoxon reinboldii

Thalassiothrix antarctica

PLATE 58 Thalassiothrix antarctica: (a) colony in girdle view. Scale bar = 100 /xm; (b) valve showing foot and head poles, marginal spines, and areolation. After Heiden & Kolbe (1928). Scale bar = 10 /xm. Thalassiothrix longissima: (a) single cell. Scale bar = 100 /im; (b) valve end. Scale bar = 10 /xm. After Hustedt (1959). Trichotoxon reinboldii: (a) whole cell. After Van Heurck (1909). Scale bar = 100 /xm; (b) parts of valve with marginal structure. Scale bar = 10 /xm.

PLATE 59 Scanning electron micrographs. (1) Thalassiothrix antarctica: (a) external view, head pole with two heavy apical spines, external opening of labiate process, and marginal spines in the middle of the vela; (b) partial internal view, one marginal row of small foramina, and marginal spines; (c) internal view of rounded, spineless foot pole with labiate process. (2) Thalassiothrix longissima: (a) internal view, valve end with serrated protrusions, and labiate process; (b) external side view showing vela and one marginal spine below the vela. Scale bar = 1 /J.m.

External openings of areolae covered by reticulate vela (SEM) divided more or less distinctly into two compartments by a longitudinal bar running parallel to valve margins just discernible with LM.

TABLE 67 Morphometric Data of Thalassiothrix spp.

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