[Dinoflagellata - Pyrrophyta - Pyrrhophyta] * (Current names in use by various authorities)
The division Pyrrophyta (from the Greek "pyrrhos" meaning flame-colored) comprises a large number of unusual algal species of many shapes and sizes. There are about 130 genera in this group of unicellular microorganims, with about 2000 living and 2000 fossil species described so far.
The name "dinoflagellate" refers to the forward- spiraling swimming motion of these organisms. They are free-swimming protists (unicellular eukaryotic microorganisms) with two flagella, a nucleus with condensed chromosomes, chloroplasts, mitochondria, and Golgi bodies. Biochemically, photosynthetic species possess green pigments, chlorophylls a and c, and golden brown pigments, including peridinin. Dinoflagellates primarily exhibit asexual cell division, some species reproduce sexually, while others have unusual life cycles. Their nutrition varies from autotrophy (photosynthesis; in-nearly 50% of the known species) to heterotrophy (absorption of organic matter) to mixotrophy (autotrophic cells engulf other organisms, including other dinoflagellates).
Free-living dinoflagellates are an ancient and successful group of aquatic organisms. They have adapted to pelagic (free-floating) and benthic (attached) habitats from arctic to tropical seas, and to salinities ranging from freshwater, to estuaries, to hypersaline waters. Many species are found in numerous habitats, living in the plankton or attached to sediments, sand, corals, or to macroalgal surfaces or to other aquatic plants. Some species are present as parasites in marine invertebrates and fish. Some even serve as symbionts, known as zooxanthellae, providing organic carbon to their hosts: reef-building corals, sponges, clams, jellyfish, anemones and squid.
Dinoflagellates exhibit a wide variety in morphology and size (from 0.01 to 2.0 mm). They commonly have a cell covering structure (theca) that differentiates them from other algal groups. Cells are either armored or unarmored. Armored species have thecae divided into plates composed of cellulose or polysaccharides which are key features used in their identification. The cell covering of unarmored species is comprised of a membrane complex. The theca can be smooth and simple or laced with spines, pores and/or grooves and can be highly ornamented.
In systematics, dinoflagellates have been claimed by both botanists and zoologists. Dinoflagellates share features common to both plants and animals: they can swim, many have cell walls, and both photosynthetic and nonphotosynthetic species are known. Botanists have grouped them with the "microalgae" and zoologists have grouped them with the protozoa, and both have produced classification schemes for this diverse and confusing group.
Dinoflagellates have attracted a lot of negative attention from the general public in recent times. For example, blooms (population explosions) of dinoflagellates can cause the water to turn a reddish-brown color known as "red tide". Red tides can have harmful effects on the surrounding sea-life and their consumers. Additionally, certain species of dinoflagellates produce neurotoxins. These toxins are carried up the food chain, ultimately to humans and can, sometimes result in permanent neurological damage or even death. Yet dinoflagellates are important members of the phytoplankton in marine and freshwater ecosystems.
Additional introductory information about this algal group may be found at the University of California - Museum of Paleontology.