How do featherback fishes use their long anal fins for propulsion?

Featherback fishes (family Notopteridae) utilize their long anal fins to generate propulsion while keeping their bodies rigid. This method of locomotion is distinct from the typical side-to-side body and tail movements seen in many other fish.1

The rigidity of the featherback's body is likely correlated with the presence of a long, gas-filled swim bladder that extends into the tail region. By maintaining a straight body posture and relying on the undulations of the anal fin, these fishes achieve movement without the lateral oscillations that would otherwise disrupt their stability.1