Tabellaria flocculosa

Valves are linear with an inflation in the center of the valve and capitate to slightly capitate apices. The medial inflation of the valve is wider than the apices. Striae are usually parallel, but can be slightly radiate in the medial inflation. The striae are alternate. The axial area is narrow and linear. The central area is small and rhombic. Short marginal spines are present, although the spines are difficult to see in LM. The girdle bands are closed and have large septa. In addition to large septa, much smaller rudimentary septa can be observed in the girdle bands. The number of girdle bands varies per frustule; there can be any number from two to many. One rimoportula per valve is present, usually located near or within the central area. Cells are joined in colonies, usually in zig-zag or stellate formations. Frustules are rectangular in girdle view.

This taxon is highly variable in its morphology and its variability has been treated differently by different authors. Knudson (1952) described four varieties, based primarily on colony morphology. Koppen (1975), who many have followed for US taxonomy, separated T. flocculosa into three ‘strains’, based on size range and autecology. In Diatoms of the US, I include strains III, IIIp, and IV together in the concept of T. flocculosa.

For those interested in following Koppen (1975), strain III is distinguished by ends that are not distinctly capitate. The medial inflation widens gradually, usually to a width of 4.5-6.5 µm. Valves of strain III can be as long as 83 µm. Cells of strain III grow attached to surfaces, usually on rocks or macrophytes.

Strain IIIp appears nearly identical to strain III, except that valves can reach up to 122µm in length. Strain IIIp valves can also be slightly twisted. In addition, strain IIIp is planktonic.

Strain IV has capitate ends and the medial inflation widens abruptly to 7.5-8.5 µm. Strain IV is smaller than strain III, with a maximum length of 39 µm.