My early research in bio-mimetic engineering and autonomous underwater robotics keeps me interested in staying abreast of the latest in robotic sea creatures.
GhostSwimmer is a robotic fish created by researchers at Boston's Franklin W. Olin College of Engineering and Boston Engineering. It is perhaps the most advanced robotic fish anyone's built yet. It's modeled after the tuna, an extremely fast fish. For robotics engineers, it's particularly attractive because the fish is submarine shaped and has a relatively rigid torso. Aside from the hydrodynamic shape, the major planned improvement of this design is to have a spine formed of electroactive polymers that will flex when voltage is applied. (via The Economist)
This is an idea I had explored, although at the time the technology wasn't available to build one. Intriguing features of the real-life tuna's physiology are the fact that when the fish goes fast, all the fins tuck into the slots in the body so the fish becomes a completely streamlined shape with nothing projecting into the flow. In the deep ocean where the tuna lives, getting from place to place quickly is a lot more important than maneuvering.
Earlier work on robotic fish has also focused on propulsion via fins, here's a more conventional design based on servos.
Gizmodo has an article on work at the University of Washington carried out in the lab of Kristi Morgansen. I am particularly keen on the design of their robot; the pectoral fins are actuated by microlite servos, the rear of the fish is activated by a high speed servo, and the tail fin is driven by a high-torque micro motor.
This sort of design should yield higher maneuverability, and greater mechanical efficiency through reduced cavitation and drag. Of course fish have already proven the superiority of this sort of an arrangement! A useful contribution of their research is that they've derived a general mathematical framework for controlling robots that propel themselves by changing their shape.
Their current tests involve trying to make the fish school using very simple communication, and very simple commands, such as "swim in the same direction as me." The theoretical and practical groundwork for this flocking, schooling, and herding was laid by Craig Reynolds back in 1987. It will be cool to see a physical embodiment of Boids!