Another version of this course (I think it was an OCW version on YouTube.) had a video of a fish holding position in a stream. There was an obstacle in the stream, and the fish swam up into its wake to save energy. Neat. Then they showed a video of a dead fish being towed through a simulated stream by a line. It exhibited "swimming" behavior that could be dismissed as just flapping in the current right up until the dead fish swam up into the lee of the nearby obstacle for shelter.

It's amazing how much adaptation and complex behavior can be embedded into an underactuated system. A fish's dead body "knows" how to save energy when swimming against a current!