Motile stage

Dinoflagellate cells range in size from 5 to 2000 | m (Fig. 10.1). These organisms are amongst the most primitive of the eukaryotes and have been regarded as intermediates between prokaryotes and eukaryotes.

Fig. 10.2 (opposite) Dinoflagellate motile stage. (a) Schematic section through the wall of an unarmoured dinoflagellate. (b) Schematic section through the wall of an armoured dinoflagellate. (c), (d) Tabulation of a hypothetical peridinialean motile stage: (c) ventral side; (d) dorsal side. (e), (f) Motile cell of a Recent Peridinium approx. x505. (g), (h) Motile cell of a Recent Gonyaulax, approx. x750. ((e)—(h) Based on Sarjeant 1974.)

Unarmoured

Armoured

Cavity.

Plate

Pellicle o OO O o O o 00O 0000

Pellicle '

Apex

Transverse flagellum-

In cingulum

Sulcus

Longitudinal flagellum

Pellicle

Apex

Transverse flagellum-

In cingulum

Sulcus

Pellicle Cell Membrane Position

Epitheca

Hypotheca

Sulcus plate

■ Posterior intercalary

Epitheca

Hypotheca

Sulcus plate

The cell wall may be either, flexible and unarmoured or rigid and armoured (Fig. 10.2). In the former case it comprises a proteinaceous envelope (pellicle) containing flattened cavities near the surface (Fig. 10.2a). In the armoured cell wall these cavities are filled by plates of fibrous cellulose to form a closely fitting theca (Fig. 10.2b). The mode of arrangement of these plates, known as tabulation, is consistent within a species.

The cell contains eukaryote organelles such as a single large nucleus, the endoplasmic reticulum, Golgi apparatus and mitochondria (Fig. 10.1). But, as in the prokaryotes, the chromosomes remain condensed throughout life and the nuclear spindle which forms during meiosis lies external to the nuclear membrane. Within the cell, several fluid-filled vessels (pusules) are connected to the exterior via canals. Photosynthetic pigments, where present, are contained in round chloroplasts at the cell margins. Light sensory eye spots may also be present.

The two flagella arise either from pores at the anterior end or from the ventral surface (Fig. 10.2c,d). Two furrows, each of which bears a flagellum, generally traverse the cell surface. One occupies a more or less equatorial position in a transverse furrow called the cingulum, the other lies in a longitudinal furrow called the sulcus. That half of the cell anterior to the cingu-lum is called the epitheca and that posterior to it is termed the hypotheca (Fig. 10.2c,d). The side bearing the sulcus is ventral (Fig. 10.2c,e,g), whilst the opposite side is dorsal (Fig. 10.2d,f,h). Many cells and cysts are dorso-ventrally compressed so that these two views are the ones usually illustrated.

The sulcus extends in a posterior direction and may terminate in a depression flanked by one or two antapical horns (Fig. 10.2c,d). The other anterior, or apical, end is often rounded, pointed or produced into an apical horn (Fig. 10.2e). Overall cell shape can be very varied even within a single genus, but includes spherical, subspherical, ovoid, biconical, fusiform, rod-shaped, rectangular, polygonal, discoidal and peridinioid outlines.

Tabulation refers to the arrangement of plates in the armoured motile cells of the class Peridinea. In these, five plate series are found to encircle each cell, each plate being numbered for reference in a counter-clockwise direction using the Kofoidian System (Fig. 10.2c-h).

This system of nomenclature is objective and purely descriptive and does not normally imply homology between plates in different taxa. Around the epitheca occur the apical and precingular series. In the cingu-lum lie the cingular series whilst the postcingular and antapical series occur on the hypotheca. Additional anterior and posterior intercalary plates may also develop at sites between the series, and the sulcus bears small sulcal plates that can also be of taxonomic value.

The functional significance of cell shape and tabulation is little understood. As the planktonic forms maintain their position in the water by active flagellar propulsion rather than by passive floating, the cells tend to be streamlined. Nevertheless, the long horns of certain genera may serve to retard sinking.

Cyst stage

Only about 10-20% of living species are known to encyst following sexual reproduction, yet almost all fossil dinoflagellates are preserved as cysts. Three basic kinds of cyst are recognized, termed proximate, proxi-mochorate and chorate, depending upon the relative length of any ornament, although intergradations between these exist. Proximate cysts resemble the theca in both size and shape and presumably formed in close contact with the thecal wall (Fig. 10.3a,b). The tabulation, cingulum and sulcus are all reflected in the surface sculpture of proximate cysts. Proximochorate cysts are an intermediate type between proximate and chorate cysts. They have processes that are between 10 and 30% of the overall diameter (Fig. 10.3d,e) and an elaborate ornament. The tips of the processes were in contact with the thecal wall and in some species were plate centred and can be numbered in a similar fashion to proximate cysts. The tips of the processes may be joined by thin, filamentous trabeculae giving the impression of an additional layer. Chorate cysts (Fig. 10.3c,d) usually exhibit no traces of a reflected cingulum or sulcus.

The cyst is formed within the motile cell and contains the same organelles. The cyst wall (phragma), built of organic material resistant to bacterial decay and called dinosporin, may consist of one or multiple layers (Fig. 10.3a,f). An autocyst has a single layer and

(a) Proximate

Theca

(a) Proximate

Theca

Epicyst

Hypocyst

Epicyst

Hypocyst

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