Thalassiosira

organic connecting thread from central strutted process organic connecting thread from central strutted process

Areolae size and array.

Occurrence of occluded processes.

The presence of a central areola or an annulus.

The presence of marginal ribs.

How to identify: For the sake of completeness the shape of the girdle view is included in the description of the single species. It should be kept in mind that the ratio between pervalvar axis and diameter often varies considerably within the single species, and that the cell diameter varies considerably within the single species as illustrated by a clonal culture of T. punctigera (Fig. 12). Information on length of pervalvar axis is lacking for many of the Thalassiosira species. The terms box shaped, cylindrical, drum shaped, discoid, and coin shaped are in frequent use in the literature to describe the shape of Thalassiosira spp. in girdle view. Diatoms in general are box shaped, and the term can thus hardly be regarded as distinctive. Cylindrical, however, as used for "any body having length, a constant diameter, and ends parallel to each other," appears as a convenient descriptive term for diatoms with a pervalvar axis of a certain length but with no reference to the shape of the valves except for the circular outline. Drum shaped may be interpreted as cylindrical cells with flattened valves, and discoid and coin shaped indicate cells with comparatively short pervalvar axis and circular, flattened valves. Only a few Thalassiosira species can be identified when seen in girdle view. Positive identification with LM has to rely, for most of the species, on examination of valves cleaned of organic matter and mounted in a medium of a high refractive index. In this chapter the Thalassiosira species have been grouped according to process patterns, i.e., location of labiate process(es), number and arrangement of central/ subcentral strutted process(es), and number of marginal rings of strutted processes. The objection may be raised that the structure of strutted and labiate processes cannot be distinguished with LM. Although this may be true for many Thalassiosira species, a trained light microscopist can learn to distinguish them by shape and size. Occluded processes are usually readily recognized in water mounts, particularly on cells in girdle view (Fig. 12), and in special cases the labiate process as well as the strutted processes are distinguished in water mounts as soon as the cell loses the chloroplasts (Fig. 12; see Plate 5 for comparison).

A. Labiate process(es) near valve mantle; external tubes usually present.

1. One central or subcentral strutted process.

a. One marginal ring of strutted processes: T. aestivalis, T. allenii, T. angulata, T. binata, T. bulbosa, T. conferta, T. decipiens, T. ferelineata, T. hispida, T. licea, T. mala, T. minuscula, T. nordettskioeldii, T. oceanica, T. pacifica, T. partheneia, T. punctigera, T. subtilis, and T. tenera (Table 7).

one large labiate and many

cell with chloropiasts occluded processes

Od occluded processes one large labiate and many cell with chloropiasts

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