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American Association for the Advancement of Science

Sponges recycle resources for the reef

This is stone fish hiding behind a sponge.
[Image courtesy of Benjamin Mueller.]

For years, researchers have tried to explain how coral reef communitiesósome of the most productive ecosystems in the worldócan thrive in waters that don't have any nutrients. Somehow, these diverse ecosystems can grow very well in the marine equivalents of a desertóa mystery that has become known as "Darwin's Paradox."

Previous research has shown that tiny microbes can take some of the organic matter that's dissolved into the ocean by coral and algae and convert it to food for larger organisms. But, those microbes have never been enough to explain how coral reefs are able to recycle all of the organic matter that's produced by the reefs without losing most of it to the ocean.

Now, Jasper de Goeij and colleagues say that sponges have been the missing piece of the puzzle this whole time. The researchers studied four species of coral reef sponges and found that each one of them absorbed carbon and nitrogen from their surroundings and converted it to nutrient-rich filter cells. These filter cells are then quickly released into the water to be eaten by other creatures, like snails and hermit crabs, they say.

This "sponge loop" helps to explain how biological hot spots, such as coral reefs, do so well in parts of the ocean that lack nutrients like carbon and nitrogen. Sponges have been saving nutrients from drifting out to sea and turning them into food for larger organisms to consume, according to the researchers.