These dinoflagellates produce a toxin that is concentrated by invertebrates, including shellfish. The toxin is dangerous to humans consuming shellfish and causes the condition known as paralytic shellfish poisoning. Most are unicellular, some are colonial. Cell surface is reinforced by cellulose plates with flagella in perpendicular grooves, creating its whirling movement and resulting in a characteristic shape. Some live as photosynthetic symbionts of the cnidarians that build coral reefs.
Food is stored as starch.
Chromosomes lack histones and are always condensed. Has no mitotic stages. Kinetochores are attached to the nuclear envelope and chromosomes distributed to daughter cells by the splitting of the nucleus. Dinoflagellates are probably more closely related to zooflagellates than to any phylum of the algae. The phylum Bacillariophyta includes the diatoms which are yellow or brown in color due to the presence of brown plastids.