Watch a group of these over a coral head and the spacing does not change. Every fish holds the same distance from its neighbors, faces the same way into the current, and adjusts continuously without ever bumping or drifting out of formation.
That spacing is doing arithmetic. Each fish is filtering a column of water for zooplankton, and two fish too close together are competing for the same column while leaving another unworked. Too far apart and the group loses the protection that being in a group provides. The distance they settle on is roughly the distance at which the shoal covers the current arriving at the coral head without gaps, and it holds all day.
Drop below them and the whole formation slides sideways and down into the branches as one, without any obvious signal passing between them, and reassembles thirty seconds later in the same arrangement.
The fish itself is barely there: transparent with a blue-white sheen, fine dark lines on the head, and the first dorsal spines drawn out into threads that give it its name.
- almost transparent with a blue-white sheen and fine dark lines on the head
- first dorsal spines drawn out into fine threads
- hovers in loose groups a few centimeters above branching coral
- up to 6 cm
