Oq

T. minuscula

T. nordenskioeldii

T. punctigera

PLATE 5 Thalassiosira minuscula: (a) mucilage colony of four cells, three in girdle and one in valve view; valve with processes, labiate process with adjacent strutted process. Thalassiosira nordenskioeldii: (a) chain in girdle view; (b) valve view showing processes. From Hasle 8C Syvertsen (1990a). Thalassiosira pacifica: (a) chain in girdle view; (b) valve with process pattern. Thalassiosira punctigera: (a) chain in girdle view with evident occluded processes on one cell; (b) valve with marginal ribs, ring of close set strutted processes, and occluded processes (open rings). Scale bars =10 fim.

Valve view: Areolation barely visible with LM. Strutted processes lacking external tubes (SEM), labiate process with a long external tube located close to one or two strutted processes at some distance from valve margin. Distinctive features: In water mounts cells in colonies inside capsules. On cleaned, mounted valves the location of the labiate process and the associated strutted process(es). Remarks: Due to the lack of external process tubes and the location of the labiate process at some distance from the valve margin T. minuscula could just as well be placed into group B as in group A. Unlike species of group B, T. minuscula has short internal tubes, a feature shared with T. subtilis, T. diporocyclus, and T. fragilis, all appearing in mucilage colonies (Hasle, 1972a; Fryxell et al., 1984)

Thalassiosira nordenskioeldii Cleve (Plate 5, Table 7) References: Cleve, 1873b, p. 7, Plate 1, Fig. 1; Hustedt, 1930, p. 321, Fig. 157; Cupp, 1943, p. 46, Fig. 8; Hendey, 1964, p. 85, Plate 1, Fig. 8; Hasle, 1978a, p. 79, Figs. 1 and 5-20; Syvertsen, 1979, p. 49, Figs. 6-14; Takano, 1990, pp. 214-215. Girdle view: Cells octagonal, pronounced concavity around valve center; valve mantle high; oblique. Connecting thread as long as pervalvar axis or shorter. Long slender external tubes of marginal processes with bases on the distinct bend between valve face and^mantle. Valve view: Areolation delicate but discernible with LM, more so in resting spores (semiendogenous or endogenous) than in vegetative cells. Central strutted process close to central areola (annulus). Marginal processes prominent; location of labiate process variable within the ring of strutted processes.

Distinctive features: Shape of cell, especially the slanting valve mantle; long external tubes of processes. Remarks: Thalassiosira aestivalis Gran, T. binata G. Fryxell, T. hispida Syvertsen and T. conferta are all more or less octagonal in girdle view. Thalassiosira aestivalis is distinguished from T. nordenskioeldii in girdle view by a lower mantle and usually a longer distance between cells in colonies, and in valve view mainly by the more closely spaced marginal processes (Hasle, 1978a). Thalassiosira binata is generally smaller with much finer areolation (Hasle & Fryxell, 1977a), and T. conferta is distinguished by the small labiate and the adjacent long strutted processes. Thalassiosira hispida is generally smaller than T. nordenskioeldii, with smaller mantle areolae than on valve face, and as conspicuous with EM, "knobs and hairs on valve surface" (Syvertsen, 1986, Table 1).

Thalassiosira oceanica Hasle (Plate 13, Table 7)

Synonym: Cyclotella nana Guillard clone 13-1 in Guillard 8c Ryther.

References: Guillard 8c Ryther, 1962, Plate 1, Fig. ID; Hasle, 1983a, p. 220, Figs. 1-18. Girdle view: Cells rectangular; pervalvar axis somewhat shorter than diameter.

Valve view: Valve structure (radial ribs or poroid areolae, EM) not resolved with LM. Processes distinct; marginal processes with short external tubes located close to valve margin; labiate process close to one strutted process.

Distinctive features: On cleaned mounted valves the widely spaced marginal processes and the location of labiate process. Remarks: An undulated marginal ridge, spinules and sometimes a finely perforated top layer covering the radial ribs are revealed with EM (Plate 13; Hasle, 1983a; Hallegraeff, 1984).

Thalassiosira pacifica Gran & Angst (Plate 5, Table 7) References: Gran 8c Angst, 1931, p. 437, Fig. 12; Hasle, 1978a, p. 88, Figs. 42-69; Rivera, 1981, p. 105, Figs. 281-307; Makarova, 1988, p. 57, Plate 28, Figs. 1-9; Takano, 1990, pp. 218-219. Girdle view: Cells rectangular with rounded low mantle; valve face flat or slightly concave. Connecting thread about as long as the pervalvar axis. Valve view: Loculate areolae in linear, eccentric or fasciculate patterns depending on the diameter of the cell. Central process adjacent to a central areola (annulus), marginal strutted processes with distinct coarse external tubes, labiate process positioned as for a marginal strutted process.

Distinctive features: Mantle areolae smaller than those on valve face, valve margin ribbed. Distinguished from T. angulata by the ribbed margin; the more closely spaced marginal processes with shorter external tubes and the location of the labiate process.

Thalassiosira partbeneia Schrader (Table 7)

References: Schrader, 1972; Hasle, 1983a, p. 223, Figs. 19-36; Fryxell et al., 1984, p. 143, Figs. 2-16. Girdle view: Pervalvar axis equal to or larger than cell diameter; valve evenly curved; occurring in large gelatinous masses containing numerous threads extruded from strutted processes.

Valve view: Valve structure (radial ribs or poroid areolae in radial rows, EM) not revealed with LM. Processes distinct; internal parts longer than inconspicuous external parts; labiate process almost equidistant between two strutted processes.

Distinctive features: In water mounts the colonies with cells are entangled in threads. On cleaned mounted valves the shape of the strutted processes and the location of labiate process (as distinct from Thalassiosira oceanica).

Thalassiosira punctigera (Castracane) Hasle (Figs. 6f and 6g and 12, Plate 5, Table 7)

Basionym: Ethmodiscus punctiger Castracane.

Synonym: Thalassiosira angstii (Gran) Makarova; for further synonomy see Hasle, 1983b, p. 602.

References: Castracane, 1886, p. 167, Plate 3, Fig. 1; Makarova, 1970, p. 13; Fryxell, 1978a, p. 133, Figs. 9-20; Hasle, 1983b; Makarova, 1988, p. 67, Plate 38, Figs. 1-10; Takano, 1990, pp. 224-225. Girdle view: Cells low cylindrical; valve surface convex, evenly curved, usually with long tubes (occluded processes). Connecting thread thin. Valve view: Areolation fasciculate, valve margin ribbed. Small, densely spaced strutted processes with short external tubes close to valve margin; larger widely spaced occluded processes more away from valve margin. Distinctive features: Occluded processes; structure of valve margin. Remarks: Thalassiosira puntigera is recognized by the ribbed margin and the presence of occluded processes. Thalassiosira licea G. Fryxell and T. lundiana G. Fryxell are closely related to T. punctigera; T. licea has more widely spaced marginal strutted processes, and T. lundiana has strutted processes scattered on the valve face in two marginal rings and a finer areolation (Fryxell, 1978a). Occluded processes may or may not be present in the three species.

Thalassiosira subtilis (Ostenfeld) Gran (Table 7) Basionym: Podosira (?) subtilis Ostenfeld.

References: Ostenfeld, 1899, p. 55; Ostenfeld, 1900, p. 54; Gran, 1900, p. 117; Ostenfeld, 1903, p. 563, Fig. 119; Hasle, 1972a, p. 112, Figs. 1-20; Rivera, 1981, p. 123, Figs. 359-377. Girdle view: Pervalvar axis and diameter of about the same size, valve surface curved; marginal zone usually more than valve center. Embedded in mucilage without any particular order.

Valve view: Areolation fasciculate; areola rows parallel to central row. Strutted processes scattered on valve face (between one subcentral process and one marginal ring of processes). Strutted and labiate processes lacking external tubes (SEM). Labiate process located at some distance from valve margin.

Distinctive features: On cleaned mounted valves, the scattered strutted processes and the labiate process are from valve margin. See remarks to T. minuscula.

Remarks: Thalassiosira subtilis was the first species of this genus to be described as being embedded in mucilage (Ostenfeld, 1899, p. 55). Other, more recently described species occurring in this type of colony (Hasle, 1972a; Fryxell et al., 1984) may therefore have been misidentified as T. subtilis.

Thalassiosira tenera Proschkina-Lavrenko (Table 7) References: Proschkina-Lavrenko, 1961, p. 33, Plate 1, Figs. 1-4, Plate 2, Figs. 5-7; Hasle & Fryxell, 1977b, p. 28, Figs. 54-65; Takano, 1990, pp. 232-233.

Girdle view: Cells rectangular; valve face and mantle forming an almost right angle; pervalvar axis about the same length or longer than cell diameter. Chloroplasts 5-9.

Valve view: Hexagonal areolae in straight tangential rows (lineatus structure); a slightly larger central areola (encompassing the central strutted process, SEM). External parts of marginal strutted processes wedge shaped.

Distinctive features: The linear areola array and the shape of marginal strutted processes. Remarks: Thalassiosira ferelineata Hasle & Fryxell is slightly larger and coarser in structure but with the same areola array and process pattern as T. tenera (Hasle &c Fryxell, 1977b). The main difference is the lack of external process tubes in T. ferelineata.

Distribution:

T. bulbosa—northern cold water region.

T. hispida and T. nordenskioeldii—northern cold water region to temperate.

T. ferelineata, T. licea, T. oceanica, and T. partheneia—mainly warm water region.

T. aestivalis, T. allenii, T. binata, T. mala, T. minuscula, T. punctigera, and T. subtilis—warm water region to temperate? T. conferta, T. pacifica, and T. tenera—cosmopolitan, exclusive polar regions.

T. angulata—North Atlantic Ocean (see Hasle, 1978a). It should be noted that T. hibernalis Gayoso, described from Argentina, is morphologically very close to T. angulata.

lb. One central or subcentral strutted process. More than one marginal ring of strutted processes.

Thalassiosira delicatula Ostenfeld in Borgert, 1908 (Plate 6, Table 8), non Thalassiosira delicatula Hustedt. Synonym: Thalassiosira coronata Gaarder.

References: Borgert, 1908, p. 16, Figs. A and B; Gaarder, 1951, p. 30, Fig. 17; Hasle, 1980, p. 170, Figs. 18-34; Rivera, 1981, p. 53, Figs. 63-107. Girdle view: Pervalvar axis as long as or longer than cell diameter; valve face with a central cavity; valve mantle high and smoothly rounded. Connecting thread thin and as long as the pervalvar axis. Single cells or infli

a b

T. diporocyclus

T. delicatula

T. eccenlrica

TABLE 8 Morphometric Data of Thalassiosira spp. with One Central or Subcentral Strutted Process and More Than One Marginal Ring of Strutted Processes

Species

Diameter (|tm)

Valve areolae in 10 |i.m

Marginal proc. in 10 |im

T. delicatula

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