But that pales into insignificance, when compared to the daily vertical migration of these mircoscopic animals called zooplankton. At sunset, all of these tiny creatures swin upwards, and under cover of darkness, they grease on floating algae close to the surface.Many of them are fish lave. In fact, almost every fish species on the Great Barrier Reef starts life in the plankton. They are billions upon billions of them making the round trip, the greatest daily migration on earth. They all travel an extraordinary distance. Size for size, it would be like me running America twice a day. That's if they are not caught on the way up. On the reef wall at about 150 meters deep, a huge sea fans. They look like plants, but they are colonialism animal, whose branching arms capture the rising plankton. Hidden among the branches is the p* seahorse. It's a tiny fish that also feeds on plankton. At a little more than a centimetre long fully grown, it's one of the world's smallest fish.