Definition
A peridinian is a dinoflagellate belonging to the genus Peridinium or closely related groups. These single-celled algae are characterized by armored cellulose plates (theca) and two flagella for movement. They're found in both marine and freshwater environments. Most are photosynthetic, but some are heterotrophic, feeding on other organisms. They can contribute to harmful algal blooms. Think of them as tiny armored vehicles of the aquatic world.