Coral reefs comprise arguably some of the most diverse, fragile and beautiful ecosystems on the planet. Their structure, composed mainly of calcium carbonate secreted by living coral polyps, provides shelter for fish and marine invertebrates and helps protect nearby shorelines from erosion. Their critical role, however, cannot save them from destruction — other less hospitable ecosystems are replacing reefs from the Caribbean to the Australian coast at a steady pace. Although scientists have witnessed overtaken reefs’ rebound, they remain unsure about exactly what causes these rare events.To study coral reef recovery, an Australian team first had to induce macroalgal takeover on sections of the Great Barrier Reef. A researcher removes mesh from the cage around an experimental plot ( left) to permit fish to feed on the heavy thickets of macroalgae that grew there when larger herbivores were kept out (right).Researchers led by David Bellwood of James Cook University in Townsville, Australia, have used imaging to shed light on one factor potentially responsible for such positive phenomena on the Great Barrier Reef. The group knew before its investigation that local and roving herbivores help prevent phase shifts on the reef and wanted to see if the same fish could help revive a sample area overrun by vegetation.To begin the study, the scientists used cages of mesh and tubing to exclude all fish more than 10 cm in length from two 25-m2 plots of seafloor for three years. This exclusion, which simulated overfishing, allowed macroalgal cover to increase more than fiftyfold where coral had once dominated. Before nightfall on the last day of the simulation, they removed the mesh from the cages to grant all organisms in the area access to the plots. They recorded feeding activity at random 1-m2 areas with paired remote Sony digital video cameras in waterproof housings from dawn until dusk on the following five days. Observers analyzed the footage and recorded the location, species and size of the fish that were captured on camera, as well as the number of bites each animal took. Batfish (Platax pinnatus) — comparatively rare fish that normally feed on benthic invertebrates and plankton — removed and consumed large chunks of the dominant macroalgae at the site on a regular basis. Surprisingly, just two of the 43 indigenous herbivorous species fed on the algae, and even these seemed to eat only small amounts of superficial growth. Within eight weeks of cage disassembly, the fish had brought algal density at the plots down to control-site levels.Dusky batfish (Platax pinnatus) consumed more algal matter than any other species and were frequently caught munching on the macroalgal stands (left). The fish cleared much of the vegetation from the plots after only 10 days of feeding (right). Orbiculate batfish also are indigenous to the reef (center).On the basis of these results, outlined in the Dec. 19 issue of Current Biology, Bellwood and his colleagues describe the batfish as a sleeping functional group — one that assumes an alternative role only under exceptional circumstances. In this case, the critical species enjoys no legislative protection and faces increasing human encroachment on its breeding grounds. These pressures could cause the easily hunted batfish to face the same fate as that of the dugong and the green turtle, other major macroalgal eaters whose populations have decreased dramatically on reefs around the world. Although the team has yet to observe how well the coral population itself heals from the impact, its findings stress the need for governments and the scientific community to search for the forces that reverse as well as prevent phase shifts in all vulnerable ecosystems.