The batoid disc can be driven in two modes. In undulation, several waves travel along the pectoral margin from front to back, pushing water backwards while the animal hugs the seabed. It is precise, slow and well suited to manoeuvring in centimetres of water.
In oscillation, fewer than half a wave is present at any moment: the wing moves as a single unit, beating like a bird's. Eagle rays, cownose rays and mantas swim this way, and the associated body plan follows, with a stiffer, more pointed disc, a narrow high-aspect wing and a slender tail that no longer contributes to propulsion.
The modes are ends of a continuum rather than a strict dichotomy, and several groups sit between them. But the split explains a functional divide: undulators are bottom animals, and oscillators are the rays that cross open ocean.