Flagellar Shape and Surface Features

Deviations from the cylindrical shape are rare among the algae. Usually the flagellar membrane fits smoothly around the axoneme and a total diameter of 0.25–0.35 µm, excluding scales, hairs, etc., holds for most species. If extra material is present between the axoneme and the flagellar membrane, the flagellum diameter increases either locally as in the case of flagellar swellings, or through almost the entire length as in the case of paraxial rods. Minor deviations from the cylindrical shape are caused by small extensions of the membrane to form one or more longitudinal keels running the length of the flagellum. Greater extension of the membrane forms a ribbon or wing supported along the edge by a paraxial rod. More variations are present in the flagellar tip, because flagella can possess a hairpoint, that is, their distal part is thinner with respect to the rest of the flagellum or blunt-tipped, with an abundance of intermediates between these two types.


Flagellar surface is smooth in many algae, where only a simple plasma membrane envelopes the axoneme. Sometimes, however, a distinct, apparently homogeneous dense layer covers the flagellar membrane throughout (Figure 2.29). One of the two flagella of Heterokontophyta is smooth, and smooth flagella are present in members of the Haptophyta, such as Chrysochromulina parva, and in many Chlorophyta, such as Chlamydomonas reinhardtii.

Flagellar Scales
Flagella may bear a high variety of coverings and ornamentation, which often represent a taxonomic feature. The occurrence of flagellar scale follows that of cell body scales, because they are present only in eukaryotic algae, in the divisions of Heterokontophyta, Haptophyta, and Chlorophyta. As for the cell body scales, they have a silica-based composition in the Heterokonthophyta, a mixed structure of calcium carbonate and organic matter in the Haptophyta, and a completely organic nature in the Chlorophyta.

Members of the Chrysophyceae with flagellar scales (Heterokontophyta) fall into two groups: one possessing exactly the same type of scale on both flagellar and body surface, the other showing flagellar scale different in structure and arrangement from body scales. Example of the first group is Sphaleromantis sp., whose flagella and cell body are closely packed with scales of very peculiar appearance, resembling the branched structure of a tree. Examples of the second group are Mallomonas sp. and Synura sp.; in both genera, flagellar scales are not arranged in a regular pattern, are very small (under 300 nm) and possess different morphological types, the most characteristic being the annular type. As the body scales, flagellar scales are produced in deposition vesicles, extruded from the cell and brought into correct position in relation to the other scales and the cell surface.

As described earlier, flagella of the Haptophyta are usually equal in length and appearance (isokont), however, members of the genus Pavlova possess two markedly unequal flagella, the anterior much longer than the posterior, and carrying small, dense scales in the form of spherical or clavate knobs. These scales are often arranged in regular rows longitudinally, or can be randomly disposed on the flagellum. Scales are formed inside the Golgi apparatus, and then released to the cell surface by fusion of the plasmalemma and the cisternal membrane.

Flagellar scales are known from almost all the genera of the class Prasinophyceae (Chlorophyta). These algae possess non-mineralized organic scales on their cell body and flagella, the same type of scale being rarely present on both surfaces. On the flagella, the scales are precisely arranged in parallel longitudinal rows, sometimes in one layer, two layers, or even three layers on top of each other. Each layer usually contains only one type of scales. The four flagella of Tetraselmis sp. are covered by different types of scales: pentagonal scales attached to the flagellar membrane (Figure 2.30), rod-shaped scales covering the pentagonal scales, and hair scales organized in two rows on opposite sides of the flagellum. A fourth type termed “knotted scales” is present only in some strains, but their precise arrangement is not known. In Nephroselmis spinosa the flagellar surface is coated by two different types of scales arranged in two distinct layers. Scales of the inner layer, deposited directly on the membrane, are small and square, 40 nm across (Figure 2.31); scales of the outer layer are rod-shaped, 30–40 nm long, and are deposited atop the inner scales. As in Tetraselmis, hair scales of at least two different types are also present covering the flagella.


In Pyramimonas sp., the scales are extremely complex in structure and ornamentation, and belong to three different types. Minute pentagonal scales, 40 nm wide, form the layer covering the membrane, which in turn is covered by limuloid scales, 313 nm long and 190 nm wide, arranged in nine rows (Figure 2.32); each flagellum also bears two rows of almost opposite tubular hair scales, 1.3 mm long. Spider web scales with an ellipsoid outline are present in Mamiella gilva, which are ornamented by a radial spoke elongated into a conspicuous spine (Figure 2.33).


The scales are synthesized within the Golgi vesicles. The vesicles then migrate to the base of the flagella and from here are extruded and arranged on the flagella.