According to Gray's paradox, fish just don't have the muscle power to swim at the speeds that they do. Or to put it another way, they appear to require only half the power to reach the same speeds as a propeller-driven or towed model of the same fish. Once a fish starts swimming, the drag around its body mysteriously drops and it transfers momentum to its wake with minimum energy loss. For both fish and bees, vortex control appears to be the secret of efficient propulsion. And, of course, you must add the fish's ability to hover effortlessly and to turn in less than a body length.
Submarine commanders can only turn green with envy.
In the case of fish, scientists outside of Mitsubishi have made some progress. At MIT Lycra-clad robot tuna and robot pike are already swimming in tanks. The tuna is a good model as it is a big fish with a relatively simple swimming pattern in which all the propulsion comes from the final section of its tail. The pike is particularly interesting because of its astounding ability to go from 0 to 20 kilometers per hour with acceleration of up to 12 g . Power sources also present difficulties, as even the robot tuna which is 2.5 meters long could eventually end up motionless with its batteries flat.