Surgical-style endoscopes have given scientists their first peek inside the nooks and crannies of coral reefs, showing that sponges dwelling there may be sustaining the teeming bounty of these ecosystems. This is the first time scientists have been able to measure the ecological importance of hidden filter feeders.
One of the many puzzles Darwin noted was the "coral reef paradox." He was curious how such a rich ecosystem, now known to contain at least 100,000 species, could exist in such clear--and therefore nutrient-poor--water. In the past, researchers had proposed that the missing nitrogen, phosphorous, and other essentials came from fish, nitrogen-fixing microbes, or unknown organisms living in reef cavities.
Now, portable endoscopes, traditionally used for medical procedures such as examining the colon, have allowed scientists to peer into reef cavities through cracks as narrow as 3 millimeters. Claudio Richter of the Center for Tropical Marine Ecology in Bremen, Germany, and his team examined five reefs in the northern Red Sea off the coasts of Jordan, Egypt, and Israel. A thriving community of filter-feeding sponges covers about 82% of cavity surfaces, they report in the 18 October issue of Nature.
The team also discovered that these voracious filter-feeders capture 60% of the available phytoplankton as it passes through reef cavities. The nitrogen and phosphorous excreted by the sponges and released when they die, says Mark Wunsch, a study author from Bremen, accounts for one-third of the nutrients reaching coral organisms, offering a possible solution to Darwin's paradox.
This is a new picture of coral reefs, says Katharina Fabricius of the Australian Institute of Marine Science in Townsville, Queensland. "For the last 200 years, we basically only dealt with the light exposed 'skin' of the coral reef, ignoring that a whole digestive system lies underneath," she says. Because small cavities dominate almost all reefs, the findings are likely to apply worldwide. Researchers are now scoping out reefs in the Caribbean and Indonesia to see exactly how they compare to the Red Sea.