Coral reefs are a paradise, both for divers and wildlife: They provide a home or a feeding ground for nearly one-quarter of all life in the ocean. But exactly how coral nourishes itself has long been a mystery. Scientists have struggled to understand how coral, which is made up of thousands of immobile, genetically identical polyps, shares and regulates resources among its many parts.
To investigate, researchers coated 14 species of coral—10 in the lab and four in the wild—with tiny fluorescent beads, which are each smaller than a pinhead and glow under light-emitting diode light. The illuminated beads helped the scientists track how water and particles circulated around the polyps (see video, above). The researchers performed the same experiment on dead corals as controls.
Next, the scientists analyzed their recordings with a computer program that tracked how the polyps transported the glowing beads. The fluorescent tracers showed that corals, including the purple Echinopora lamellosa and the brain-shaped Diploria labyrinthiformis, work together like laborers on an assembly line to move water through branching currents on their surfaces and within the folds that connect individual polyps. Groups of polyps form subnetworks that move water and nutrients along; these groups are slicked in varying concentrations of mucus, which pull particles, like conveyer belts, into hungry corals’ mouths, the researchers report today in Current Biology. The channels also keep the coral hydrated at low tide; when the researchers let one part of a coral dry out, its polyps pulled more fluorescent beads toward the parched section.
Previous studies have shown highly connected polyps are more resistant to bleaching. This new knowledge might help scientists better protect corals—and give them a chance in an ever-warming ocean.