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" Mentioned in the text for comparison.

" Mentioned in the text for comparison.

Thalassiosira minima Gaarder (Table 9)

Synonyms: Coscinosira floridana Cooper; Thalassiosira floridana (Cooper) Hasle.

References: Gaarder, 1951, p. 31, Fig. 18; Cooper, 1958; Hasle, 1972b, p. 544; Hasle, 1976a, Figs. 44 and 45; Hasle, 1980, p. 167, Figs. 1-17; Rivera, 1981, p. 90, Figs. 226-245. Girdle view: Cells rectangular; pervalvar axis less than half to slightly shorter than cell diameter; valve face flat and slightly depressed in the center; mantle low and beveled. Connecting thread as long as or longer than pervalvar axis.

Valve view: Areolation barely visible with LM (cleaned, embedded material). Processes distinct although with short external tubes; two central processes, sometimes one, seldom three; labiate process close to one marginal strutted process. Distinctive feature: The two central processes. Remarks: Cells in girdle view are similar to small specimens of T. nordenskioeldii as well as to T. conferta, T. binata, and T. curviseriata. The specific distinctions are evident especially in the process shape and pattern when examined as cleaned, embedded material.

Distribution:

T. curviseriata, and T. minima—cosmopolitan, exclusive polar regions. T. dichotomica—southern cold water region. T. tealata—Japanese warm water (Takano, 1980), English waters, Oslofjord, Norway (unpublished observations).

2b. Cluster of central strutted processes.

These species are very similar in girdle view, and despite a fairly large size and mostly coarse valve structure, reliable identification requires cleaned mounted valves.

Thalassiosira antarctica Comber (Plate 8, Table 10) Synonyms: Thalassiosira antarctica var. borealis G. Fryxell, Doucette, 8c Hubbard: Thalassiosira fallax Meunier (see Syvertsen, 1979, p. 59); Takano, 1990, pp. 184-185.

References: Comber, 1896; Meunier, 1910, p. 268, Plate 30, Figs. 1-4; Hustedt, 1930, p. 327, Fig. 162; Hasle 8c Heimdal, 1968; Syvertsen, 1979, p. 52, Figs. 15-59; Fryxell et al., 1981; Hasle 8c Syvertsen, 1990b, p. 289, Figs. 25-27.

Girdle view: Cells rectangular; valve face flat or slightly convex; mantle slightly sloping. Connecting thread thin, usually as long as or longer than pervalvar axis.

Valve view: Areolation radial, bifurcate, occasionally lightly fasciculate. Two to several central processes; two or three rings of marginal strutted processes, located in valve mantle and hardly distinguishable with LM in intact valves; occluded processes present (usually not seen with LM); large labiate process located near valve mantle.

Distinctive feature: Valve mantle with more than one ring of processes. Remarks: Fryxell et al. (1981) distinguished between T. antarctica var. antarctica from the southern hemisphere and T. antarctica var. borealis from the northern hemisphere. The valves of the vegetative cells are essentially the same but there are some differences in the bands (EM). Both varieties form heavily silicified resting spores, morphologically different from the vegetative cells. Three types of resting spores, endogenous, semiendogenous, and exogenous, have been found together in clonal culture of the northern variety (Oslofjord, Norway; Syvertsen, 1979). Semiendogenous resting spores were observed by Hasle 8c Heimdal (1968, Fig. 4) on a slide prepared of Comber's original material of T. antarctica (British Museum mount No. 59677). The primary valve of the exogenous/ semiendogenous resting spores is flattened and the secondary valve is highly vaulted in both varieties. The main differences between the resting spore of var. antarctica and var. borealis seem to be the overall coarseness of the areolation and the more stable presence of strong, prominent marginal occluded processes, visible with LM, in the nominate variety. The flattened primary resting spore valve of var. borealis may easily be confused with

PLATE 8 Thalassiosira antarctica var. borealis: (a) chain in girdle view; (b) valve with process pattern, open circles indicate occluded processes. From Hasle & Syvertsen (1990a). Thalassiosira gravida: (a) chain in girdle view; (b) valve with process pattern. From Hasle & Syvertsen (1990a). Thalassiosira hyalina: (a) chain in girdle view; (b) valve with process pattern. From Hasle 8c Syvertsen (1990a). Thalassiosira baltica: (a) chain in girdle view; (b) valve with process pattern. From Hasle 8c Syvertsen (1990a). Thalassiosira rotula: (a) chain in girdle view; (b) valve with process pattern. Scale bars =10 fim.

var. borealis

var. borealis

T. gravida

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