Peridinium

The Dinophyceae are the main class of dinoflagellates. They include all species where the nucleus remains a dinokaryon throughout the entire cell cycle, which is typically dominated by the haploid stage. All the “typical” dinoflagellates, such as Peridinium and Gymnodinium, belong here. Others are more unusual, including some that are colonial, amoeboid, or parasitic.

Dinoflagellates are classified by morphology. Species with a theca are divided into four orders, based on the arrangement of the armor plates:

Peridiniales – e.g. Peridinium
Gonyaulacales – e.g. Ceratium, Gonyaulax
Dinophysiales – e.g. Dinophysis
Prorocentrales – e.g. Prorocentrum
The Peridiniales are probably paraphyletic to the others, and on rRNA trees they are mixed with the species that lack thecae. The other three orders are probably monophyletic, with the Dinophysiales and Prorocentrales as close relatives, united by the presence of a saggital suture dividing the theca in two. However, on rRNA trees the Prorocentrales are split up.