 This robotic fish could pave the way to making underwater drones more efficient. As reported in the Journal of Bionic Engineering, the key to its design is the inclusion of an anal fin on its underside. Like in real fish, this fin balances the side-to-side motion produced by the flapping tail fin, which in robotic fish tends to create a disorienting shaking of the head. In fact, without the anal fin, this fish experienced more than 10 degrees of lateral swaying, which was reduced when the anal fin was added, and reduced even further when the anal fin was allowed to move on its own. Timing the movements of the anal and tail fins just right minimized head shaking by up to 65%. Future studies will examine how the anal fin's shape, position, and stiffness affect swimming metrics like speed, thrust, and efficiency, and could lead to improved underwater robotics for environmental monitoring and patrol missions.