annulus

Dinoflagellates are single-celled aquatic organisms that possess two distinct flagella, enabling them to move. These organisms exhibit characteristics of both plants and animals, making their classification complex. They are a vital part of phytoplankton in most seas and play a crucial role in the food chain.1

The dinoflagellate cell features a groove called the annulus, which encircles the cell and contains one flagellum. A second flagellum is attached at the posterior end, extending from a longitudinal groove known as the sulcus. Some dinoflagellates have a protective covering of cellulose plates, referred to as the theca, which can have spiny extensions. These organisms can reproduce rapidly, sometimes leading to water blooms or red tides that can be harmful to aquatic life.12