KINGDOM PROTISTA - Division Dinoflagellata

Although some systematists include Dinoflagellates in the protozoan phylum Mastigophora, most place them in their own division, Dinoflagellata, sometimes called Pyrrhophyta. There are about 2100 species of dinoflagellates, inhabiting both freshwater and marine environments. Most have two flagella undulating along grooves in the organism's cell wall: one groove encircles the cell like a belt, and the other runs perpendicular to the "belt." The beating flagella within the grooves propel the dinoflagellate through the water like a tiny, spinning top.

Many dinoflagellates are bizarre in shape and appearance, with hornlike protrusions and armor-like plates surrounding the living cell. Unlike most algae, which have a cell wall, the dinoflagellate "armor" consists of cellulose plates lying just beneath the plasma membrane.

The primary photosynthetic pigments are chlorophylls a and c, whose color is often obscured by bright red, orange or yellow carotenoid pigments. One of these carotenoids, known as peridinin, is chemically similar to fucoxanthin, found in both diatoms and brown algae (which you will meet later). Some biologists hypothesize that dinoflagellate chloroplasts may be remnants of diatoms ingested by a heterotrophic flagellate which was the ancestor of modern dinoflagellates. Excess fuel is stored as oil or polysaccharides.

Dinoflagellates love it when surface ocean waters are warm and calm, nutrients are abundant, and salinity is low. These conditions are often met in late summer, and the result may be the notorious "red tides," so-called because the density of dinoflagellates becomes so great that the water looks red. Unfortunately, the algae do more than color the water. Some (Gymnodinium spp. and Gonyaulax tamarensis) also produce extraordinarily powerful nerve toxins.

Fish and filter-feeding invertebrates such as clams and oysters which eat dinoflagellates may not be killed outright, but when predators higher in the food web--carnivorous fish, dophins, humans--eat dinoflagellate-fed prey, the toxins are concentrated, and poisoning may result. In southern Florida and elsewhere, it is inadvisable to eat large, predatory fish (such as barracuda) which may have concentrated neurotoxins in their tissues.