Off the coast of tiny Sombrero Island at the Batok dive site, I was gazing at a school of red tooth triggerfish swirling about 5 meters above me in a clockwise direction. Their usually blue forms were silhouetted black against the bright sun with only their pitchforked tailfins and size being the indicators of their species.

Suddenly I began to see flashes of silver light blinking among them. Tiny silver deltas, grouped together. I had no idea what was going on, but it was very pretty. A couple of short kicks and everything became clear. A different, small school of fish had settled amongst the red tooths - but they were swimming in counterclockwise direction. Their gill flaps were covered with especially shiny silver scales that were glinting in the sun. I instantly recognized them as juvenile pacific mackerels. Black and blue vs silver in opposing concentric circles. An intriguing sight on the reef.