Fig. 7.57 Scanning electron micrographs of both sides of a vegetative cell of Ceratium cornutum. (G) Girdle; (S) sulcus. (From Happach-Kasan, 1982.)

forms always have their horns crowded with chloroplasts.

Ceratium horridum has a life cycle that is more or less similar to the known life cycle of other Dinophyceae. Von Stosch (1972) (Fig. 7.60) has shown that the vegetative cells of C. horridum are haploid. The male gametes are smaller than the female gametes; thus fusion in this organism is anisogamous (Fig. 7.58). The male gamete attaches by its ventral side to the ventral side of the female gamete. The cell wall of the male then breaks up into its individual plates, which are taken up into the female cytoplasm. The naked male gamete fuses with the female gamete to produce a zygote. The zygote remains motile (planozygote) through all of the following steps. The planozygote grows for several days, ending up as a large cell with unusually long horns and a thick cell wall. Finally, the nucleus enlarges, and nuclear cycloses (movement) begins. At its highest speed the chromosomal mass circulates once every 30 seconds. After some hours the motion slows down and stops, and approximately 12 hours later meiosis commences. The first meiotic division halves the chromosome number and gives rise to one flagellate with normally sized antapical, and abnormally long apical

horns and a second flagellate with normally sized apical, and abnormally long antapical horns. After 2 to 3 days the second meiotic division takes place to yield haploid vegetative cells.

Gymnodiniales

The dinoflagellates in this order are similar to those in the previous order, the Peridiniales, except that they have thin or no thecal plates.

The life cycle of Gymnodinium pseudopalustre is a representative of the order. Vegetative multiplication results in daughter cells that remain attached to each other for at least 12 hours (Fig. 7.59). Gametes are formed when an actively growing culture at 21 °C is subjected to a short-day treatment (10 hours light) at 15 °C (von Stosch, 1973). Gamete differentiation occurs by divisions that give rise to cells lower in mass and poorer in plastids and pigments than the vegetative cells. Apart from their smaller size and lighter color, the gametes are not obviously different from vegetative cells. They are incapable of living for long as vegetative cells and die if fusion does not occur. The gametes are homothallic; thus gametes from a single strain will fuse.

Initially, small groups of two to ten gametes swarm around each other in a weaving motion. A copulation pair occurs between two gametes, with the pair swimming rapidly while turning slowly on a common axis. Copulation is isogamous although