Image: Courtesy of Center for Tropical Marine Ecology/IRIS KOETTER

Scientists have long wondered how corals manage to eke out a living in the nutrient-poor waters they tend to occupy. Indeed, this coral reef paradox has been under investigation since Darwin's time. Now new findings, published today in the journal Nature, may help to resolve this mystery. According to the report, creatures that dwell in reef interiors help feed their hosts.

Using an underwater endoscopic camera and an accompanying light projector (see image), Claudio Richter of the Center for Tropical Marine Ecology in Bremen, Germany and colleagues were able to examine cavities up to four meters deep in coral reefs located in the Red Sea. Because crevice openings measure a mere 20 centimeters across on average, they had previously been inaccessible to divers inspecting the reefs. When the scientists got a glimpse inside, they found a plethora of filter-feeding organisms lining the cavities. Sponges accounted for more than 50 percent of the newfound creatures, which also included sea squirts, bivalves and lugworms.

By filtering phytoplankton out of ocean water flowing through the coral's labyrinth structure and then excreting nutrients such as ammonia and phosphate, these cave-dwelling communities provide a major food source for the coral, the researchers say. In fact, their calculations indicate that the amount of nutrients generated in this manner exceeds that of any other known pathway. These findings, the authors conclude, "may therefore provide a general answer to Darwin's question of how coral reefs manage to thrive [in waters lacking plant nutrients]."