The pattern is hard to see at first because the movement seems to happen in the blink of an eye. The only thing that makes it visible at all is the fact that the bluegill sunfish in George Lauder’s experiment is swimming through water that is awash with tiny silvery glass beads that catch the light and reveal the fluid’s movement. “That’s the fish’s pectoral fin,” Lauder says, pointing to the grainy, black-and-white-picture. “It’s slowed down because the camera was taking 250 images per second. But do you see the way the water is moving in a sort of loop behind it?” Lauder is in the midst of conducting what may be the most thorough and technologically sophisticated study to date of how fish swim. It is a study that applies techniques borrowed from engineering to visualize the flow of water around the fish’s body.