whatA school of fish and one predator. When the predator strikes, the school bursts outward and re-knits; when it circles slowly, the fish hold a hollow ball around it, an empty ring it cannot close.

whyFlash expansion and the hollow ball are real and filmed, and no fish plans them. The piece shows how a shape appears at the scale of the whole school from three habits each fish keeps for itself.

howEach fish keeps clear of its nearest neighbours, matches heading with those in view, and drifts toward their centre. The predator hunts the densest part of the school: creeps in, strikes through, then turns on the spot. Fish flee hard when it is fast and keep a wary distance when it is slow. The sparkline is the clearing, how far the nearest fish keeps. Click to drop the predator at the cursor.