The name pyrrophyte (meaning “fire plant”) refers to the bio-luminescence of many dinoflagellates. Dinoflagellates (meaning “terrible whip bearer”) refers to the fact that some species are poisonous. When they multiply explosively (20 to 40 millions per ml), they kill fish by millions.
Dinoflagellates occur in countless numbers in marine plankton, and a number of them are abundant in freshwater also. Under ideal environmental conditions, populations of certain species (genera – Gymnodinium, Gonyaulax, etc.) increase to enormous numbers and turn the sea red, yellow or brown.
Dinoflagellates are common marine and freshwater flagellates. They contain two flagella. One flagellum is equatorial and the other is situated posteriorly. The flagella lie in grooves. Their body is either naked or covered with cellulose plates or valves or a cellulose membrane (pellicle or theca). They contain brown or yellow chromoplasts. These are autotrophic. Some are colourless and heterotrophic. Some like Ceratium, are both auto-and heterotrophic. Some forms are parasitic and symbionts. The symbionts help reef-building corals in extracting calcium from sea water.
A few dinoflagellate species contain stinging capsules like the nematocysts of coelenterates. This indicates that the nematocyst forming cells (cnidoblasts) of coelenterates (corals, anemones, etc.) may have evolved from dinoflagellates.
Dinoflagellates are different from other eukaryotes:
1. The nucleus of dinoflagellates is different from other eukaryotes. Their nuclear membrane is formed by a single lipid bilayer, not double.
2. The chromosomes are like those of prokaryotes. It has no centromere and no or few histone proteins. The chromosomes are attached to the nuclear membrane as found in bacteria. However, the chromosomes remain in supercoiled state as in eukaryotes.
3. Cell division is bacteria like. Spindles are not formed. The chromosomes separate as the nuclear membrane elongates and pinches into two.
4. Meiosis is also unusual. It consists of only one division.
The dinoflagellates are unlike all other eukaryotes in many respects. Their inclusion in the phylum Protozoa is not justified. They deserve a separate phylum, if not a kingdom.