How does the continuous fin around an eel’s tail help it swim?

Eels utilize a specialized form of movement known as anguilliform locomotion, characterized by sinuous waves that travel along their entire flexible body from head to tail.1

While the continuous dorsal, anal, and caudal fin running around the tail tip is a distinct anatomical feature of the eel, the primary propulsive thrust is generated by the undulatory waves of the body musculature itself. As these waves pass backward, their amplitude and speed increase, allowing the tail region to contribute significantly to the overall thrust.21

The continuous fin structure does not function as the primary engine for propulsion; rather, in aquatic vertebrates, fins generally serve to provide stabilization and steering. Specifically, dorsal and ventral fins help prevent yawing (lateral seesawing) by providing surface area behind the fish's center of gravity, while fins extending at right angles help control rolling. Thus, the eel's continuous fin aids in maintaining balance and stability during its characteristic undulatory swimming, while the body segments themselves produce the necessary force for forward movement.31