Phytoplankton is another term scientists use for microalgae that float in the sea. One kind of phytoplankton are called Dinoflagellates. Even though they are plants, they can also swim using a long whip-like extension called a flagellum. They are very small, about 0.02 to 0.04 millimetres in diameter. A human hair is about 0.05 mm in diameter, by comparison.
Dinoflagellates come in many shapes.
Some are shaped like a soccer ball top with a groove around the equator. Dinoflagellates have a propelling longitudinal flagellum at the base of the cell and a transverse flagellum around the cell which controls rotation. Many shellfish, such as oysters, mussels, and cockles, feed on the phytoplankton. Of the thousands of species of dinoflagellates, only a few kinds produce poisons, some of which can kill other species of marine life. When shellfish eat toxic dinoflagellates, the poisons become concentrated in them.