Now in 3-D: The shape of krill and fish schools

An interesting story in Scientific American, where I found the link to the video, about the geometry of bait balls.

When the fish are all riled up in the presence of a predator, they do form these dense bait balls as each fish scrambles to get to the center to hide. This centripetal motion literally forms a sphere: the shape that has the most interior volume and keeps the fewest number of fish on the surface and thus more vulnerable to predators. Of course, this is not the shape that a shoal traveling casually forms. Instead, we have "circles, discs, ellipses, triangles, wedges, crescents, and lines" (McFarland and Moss 1967). Thus predation alone cannot be the only factor shaping shoals; is there another central factor and what is it?

LINK