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The Class Dinophyceae was earlier (until ca. 1950) placed in Phylum Pyrrophyta, along with Class Cryptophyceae according to some authors.  More recently Pyrrophyta has been abandoned and the two classes are not grouped together (Smith 1950).  However the chloroplasts of the photosynthetic dinoflagellates are apparently derived from Cryptophyceae by phagocytosis and endobiosis.  Approximately 50% of dinophyceans are colorless heterotrophs.

Dinoflagellates are heterokont (two dimorphic flagella) unicells.  Desmokonts have two leading flagella, while dinokonts have one trailing flagellum and the other wrapped around the cell; both flagella arise from grooves called the cingulum or girdle (equatorial or transverse) and the sulcus (longitudinal).  Some genera such as Peridinium and Ceratium have heavy cellulose and polysaccharide plates on the outside of the cell membrane (the “armored dinoflagellates”), others have little armor as in Gymnodinium and Prorocentrum. An excellent review of dinoflagellate evolution is by Hackett et al. (2004).

Some dinoflagellates have complicated life cycles involving flagellated, amoeboid, and immobile cyst stages, as well as both asexual (vegetative) and sexual stages with diploid zygotes, such as Pfeisteria.  Except for the zygote dinoflagellates are haploid (“haplontic” life cycle).  Ecdysis (shedding armor) occurs prior to sexual fusion, and in some cases during asexual cell division.

Dinoflagellates are classified as Myzozoans, a group of protists capable of piercing the cytoplasmic membrane of prey protists and sucking out the contents as if through a soda straw. However, nutrition varies from photoautotrophy (chl. a and c) to mixotrophy and heterotrophic phagotrophy using a specialized extendible “peduncle” located near the flagellar pores to capture prey.  Nearly half of dinoflagellate species are colorless (Steidinger and Jangen 1996).  Some genera have ejectile organelles (“nematocysts”, “trichocysts”, and “mucocysts”) associated with pores in the outer wall, to aid in prey capture or self-defense. One order (Syndiniales) is endoparasitic in copepods (Skovgaard et al. 2005).

Identification to “morphospecies” of dinokonts requires consideration of cell shape, size, position of cingulum and sulcus, presence or absence of armor plates, shape and number of plates if present, various horns, spines or wings on armored forms, and ultrastructural features.  Complex life cycles complicate identification of non-flagellate stages.


Toxic marine genera are responsible for massive fish kills and paralytic shellfish poison. The most common dinoflagellate toxin is saxitoxin, a neurotoxin 105 times more potent than cocaine. The most notorious producer of saxitoxin on the west coast of North America is Protogonyaulax catenella, and on the east coast Gessnerium monilatum.  Both have been known to cause paralytic shellfish poisoning (PSP).

A second toxin, found in the dinoflagellate Ptychodiscus brevis, is brevitoxin, an assemblage of various polyether alcohols that produce fish kills, and may also cause poisoning in humans when it accumulates in the tissues of shellfish.

Toxicity occurs when blooms form.  Even when non-toxic species such as Ceratium tripos (a.k.a. synonym Tripos muelleri) bloom, diminished dissolved oxygen levels may cause high fish mortality.  Such a bloom is relatively common, such as occurred in the Gulf of Maine, August 2023.  Photography by Joe Vallino, Woods Hole Oceanographic Institute. Concentrated with a plankton net.


Hackett, J.D., D.M. Anderson, D.L. Erdner and D. Bhattacharya  2004.  Dinoflagellates:  A remarkable evolutionary experiment.  American Journal of Botany 91(1):1523-1534.

Skovgaard, A.; Massana, R.; Balagué, V.; Saiz, E.  2005.  Phylogenetic position of the copepod-infesting parasite Syndinium turbo (Dinoflagellata, Syndinea).  Protist156(4): 413-423.

Smith, G.M.  1950. The Freshwater Algae of the United States. McGraw-Hill Book Company, Inc. (719 pp).

Steidinger, K.A., and K. Jangen  1996.  Dinoflagellates.  In:  Identifying Marine Phytoplankton.  C.R. Tomas, ed. (pp387-584).