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2-3 in 10 fim

' Mentioned in the text for comparison. d —, No data.

' Mentioned in the text for comparison. d —, No data.

Valve view: Areolation fasciculated, made up of rows of hexagonal areolae or radial ribs. A single strutted process precisely in valve center, a labiate process a little off centered, and long internal parts of marginal strutted processes readily seen at a certain focus. Distinctive features: Cylindrical cells; labiate process close to central strutted process; long internal process tubes. Remarks: Our study material includes Grunow's slides as well as our own samples from the Arctic. Our material from other geographical areas includes diatoms similar to Thalassiosira bioculata in gross morphology. At present we are unable to tell whether they belong to the same taxon as the Arctic T. bioculata. A certain variability of T. bioculata is manifest by the fact that Grunow (1884) described the smaller var. exigua in addition to the nominate variety. Thalassiosira bioculata forms low, heavily silicified semiendogenous resting spores with convex, coarsely areolated primary and secondary valves (Meunier, 1910).

Thalassiosira gracilis (Karsten) Hustedt var. gracilis (Plate 12, Table 14) Basionym: Coscinodiscus gracilis Karsten.

References: Karsten, 1905, p. 78, Plate 3, Fig. 4; Hustedt, 1958a, p. 109, Figs. 4-7; Fryxell & Hasle, 1979a, p. 382, Figs. 12-22; Johansen & Fryxell, 1985, p. 168, Figs. 8, 58 and 59.

Thalassiosira gracilis var. expecta (VanLandingham) G. Fryxell 8c Hasle (Plate 12, Table 14)

Basionym: Thalassiosira expecta VanLandingham.

Synonym: Thalassiosira delicatula Hustedt non Thalassiosira delicatula

Ostenfeld in Borgert.

References: Hustedt, 1958a, pp. 110, Figs. 8-10; VanLandingham, 1978, p. 3994; Fryxell & Hasle, 1979a, p. 384, Figs. 23-28; Johansen 8c Fryxell, 1985, p. 170, Figs. 8 and 60-63. Girdle view: Cells discoid; valve center mainly flat; rest of valve evenly rounded. Connecting thread fairly thin.

Valve view: Areolae in central part large, widely spaced and irregularly arranged, smaller and less spaced on outer part of valve face and on mantle. Central strutted process a little off centered; single labiate process on border between valve face and mantle (EM). Distinctive features: Heavily silicified, rounded cells; larger areolae in central part of valve. Remarks: Thalassiosira gracilis var. expecta differs from T. gracilis var. gracilis mainly by smaller areolae on the central part of valve face. Whereas the processes of heavily silicified specimens of var. gracilis may be difficult to see with LM, those of var. expecta are readily observed when focusing on their longer internal parts (Hustedt, 1958a; Fryxell 8c Hasle, 1979a; Johansen 8c Fryxell 1985).

Thalassiosira oestrupii (Ostenfeld) Hasle (Plate 12, Table 14) Basionym: Coscinosira oestrupii Ostenfeld.

References: Ostenfeld, 1900, p. 52; Hustedt, 1930, p. 318, Fig. 155; Hasle, 1972b, p. 544; Fryxell 8c Hasle, 1980. Girdle view: Pervalvar axis half to twice the diameter; valve face flat or slightly convex; mantle low and rounded; valvocopula striated. Cells in chains united by a thread from the central strutted process. Valve view: Areolae usually larger in central part of valve than closer to the margin, sometimes in sublinear array. One nearly central strutted process; labiate process usually one or two areolae distant. Distinctive features: Striated valvocopula; coarse areolation. Remarks: Thalassiosira oestrupii var. venrickae G. Fryxell 8c Hasle is distinguished from T. oestrupii var. oestrupii by a distinct eccentric areola pattern and by more widely separated marginal processes. The labiate as well as the strutted processes are discernible with LM (Fryxell 8c Hasle, 1980).

Thalassiosira perpusilla Kozlova (Table 14)

References: Kozlova, 1967, p. 60, Figs. 12 and 13; Fryxell 8c Hasle, 1979a, p. 380, Figs. 1-11; Johansen 8c Fryxell, 1985, p. 173, Figs. 5 and 53-55.

Girdle view: Valve evenly rounded. No observations on chain formation. Valve view: Areolae in irregular radial rows. Nearly central strutted process; labiate process eccentric; widely separated marginal processes. Distinctive feature: Few marginal strutted processes. Remarks: The processes are readily seen with LM due to the light silicification of the valves. Thalassiosira rosulata Takano (Takano, 1985, p. 3) is similar to T. perpusilla as well as to small specimens which may belong to T. bioculata. The labiate process of T. rosulata is closer to the valve margin than those of T. perpusilla and T. bioculata, and T. rosulata is similar to T. bioculata regarding valve structure and distance between marginal strutted processes.

Thalassiosira proscbkinae Makarova in Makarova et al. (Plate 13, Table 14)

References: Makarova et al., 1979, p. 922, Plate 1, Figs. 1-7; Makarova, 1988, p. 80, Plate 51, Figs. 13-22; Feibicke et al., 1990; Takano, 1990, pp. 220-221.

Girdle view: Cells low cylindrical; valve face flat; mantle low. A few large chloroplasts. Cells in cultures as well as in natural samples found entangled with threads in large colonies.

Valve view: Areolae in curved tangential rows (eccentric structure). Central strutted process and labiate process close together and marginal strutted processes closely spaced, all readily seen with LM on cleaned valves mounted in a medium of a high refractive index. Distinctive features: Marginal strutted processes not as widely spaced, central strutted and labiate processes closer and areolae smaller than those in T. perpusilla, which is the most similar species. Remarks: Makarova (1988) reduced Thalassiosira spinulata Takano in rank to T. proscbkinae var. spinulata (Takano) Makarova. Our observations on cultures and natural samples showed that T. spinulata is certainly no distinct species. The two varieties can hardly be distinguished with LM.

PLATE 13 Micrographs: single valves. Thalassiosira proscbkinae (a-c): (a) valves with areola and process patterns distinct, LM. Scale bar = 5 ¡im; (b) valves with processes, areolae, and cribra, TEM. Scale bar = 1 (im; (c) external valve surface, SEM. Scale bar = 1 ¿im. Thalassiosira mala (d-f): (d) valves with processes, LM. Scale bar = 5 /um; (e) valve with process pattern, areolae, and cribra, TEM. Scale bar = 1 /¿m; (f) external valve structure, areolae with foramina, SEM. Scale bar = 1 /xm. Thalassiosira oceanica (g-i): (g) valves with process pattern, LM. Scale bar = 5 fim; (h) areolation and processes, TEM. Scale bar = 1 /xm; (i) external view of areolation and processes, SEM. Scale bar = 1 /am.

Distribution:

T. gracilis, and T. perpusilla—southern cold water region. T. oestrupii—warm water region to temperate. T. proschkinae—cosmopolitan (found as far north as 79°N). T. bioculata—northern cold water region? T. rosulata—Japanese waters.

2a. Cluster of central strutted processes.

Thalassiosira ritscheri (Hustedt) Hasle in Hasle 8c Heimdal (Table IS) Basionym: Coscinodiscus ritscheri Hustedt.

References: Hustedt, 1958a, p. 19, Figs. 44-46; Hasle 8c Heimdal, 1970, p. 569, Figs. 39-43; Johansen 8c Fryxell, 1985, p. 176, Figs. 14, 56, and 57.

Girdle view: Valve face slightly convex. Chain formation by numerous central threads.

Valve view: Areolation fasciculated, sublinear or linear. Labiate process a few (two or three?) areolae away from the central cluster of strutted processes.

Distinctive feature: Location of strutted and labiate processes. 2b. One or two rows of central to subcentral strutted processes.

TABLE IS Morphometric Data of Thalassiosira spp. with More Than One Central Strutted Process

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