What characteristic defines a dinoflagellate?

The defining characteristic of a dinoflagellate is indeed the presence of a pair of flagella located in perpendicular grooves. This unique arrangement allows dinoflagellates to move through water in a spinning motion, which is essential for their survival as they are often suspended in aquatic environments. The two flagella provide different types of movement: one is used for forward propulsion while the other effectively steers the organism. This specialized structure not only aids in mobility but also plays a significant role in their ability to thrive in various aquatic habitats where they can photosynthesize or feed on other microorganisms.