Life in the lagoon depends on a daily flushing of water from the open ocean. This flows in through channels formed by natural gaps in the reef. With each changing tide, a soupy river of debris and nutrients flows out of the lagoon and into the blue. This attracts all sorts of life. One regular visitor to the channels is the gently gliding manta ray. It filters out minute creatures floating in the currents. But there's more than enough to go around. Schools of snapper take the lead while a wall of fusiliers mops up the remains.