http://www.newscientist.com/news/news.jsp?id=ns99991445
Coral reef survival in nutrient impoverished seas is possible because of some hidden, but super-efficient filter feeders, say German scientists.
Their study has uncovered a variety of creatures that live in cracks between rocks, straining scarce nutrients from the water. Many of the 370 species of sponge and other
filter feeder are new to science.
To get at these unique communities, the scientists developed CaveCam - a sort of diver's endoscope - that allows them to take pictures up to four metres into rock crevices.
Reef paradox
The hidden sponges may answer Darwin's "coral reef paradox", claim the authors. The great naturalist was puzzled by the fact that while reefs are one of the most species-rich and productive ecosystems, they could thrive in nutrient-poor seas. In particular, nitrogen and phosphorus compounds are very limited and ought to constrain reef growth.
One possible answer to the paradox would be lots of so-called "active suspension feeders", says Claudio Richter of Bremen University, Germany, who led the study. These are creatures such as sponges that pump water through their bodies and sift out tiny floating organisms, such as bacteria.
However, previous studies had not identified enough of these super-efficient filter feeders to keep reefs going. "Scientists were also puzzled by the depletion of very small plankton," Richter says, because there did not seem to be many sponges about to eat them.
Super sieve
Richter's study of Red Sea reefs has found the missing piece in the coral jigsaw. The researchers estimate that rock crevices provide lots of surfaces for filter feeders to live on - 2.4 to 7.4 square metres for every square metre of reef.
Most impressive though, was the filtering ability of the crevice communities. The scientists measured plankton in the water before and after it had been through a crevice. They calculate that the plankton filtering performed by the sponge community is nearly half the gross metabolism of the reef itself.
Howard Lasker, a reef expert at the State University of New York in Buffalo, agrees that the sponge communities are extraordinarily efficient. "I'm sure this will turn out to be relevant to all reefs," he adds.
However, while Richter believes the main significance of the communities is in locking up nutrients that originated from outside the reef, Lasker is more cautious. "It is unclear at this point to what extent the material is really from off the reef," he says.
Journal reference: Nature (vol 413, p 726)
James Randerson
