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Uncharismatic. Sponges like Halisarca caerulea (left) and Scopalina ruetzleri (right) grow in crusts on the reef—and process a lot of nutrients.

Jasper de Goeij

Coral Reefs Feast on Sponge Detritus

Coral reefs shouldn’t exist. Teeming with fish and a rainbow of other organisms, they somehow thrive despite the fact that the water surrounding them contains hardly any nutrients. Now, scientists think they’ve solved part of the puzzle: These communities are living off the waste of sponges.

It’s easy to forget that sponges are animals. They don’t have faces. They don’t move. They don’t even have circulatory or digestive systems. Instead, they draw water through little holes in their bodies, and then pump it past cells that extract nutrients. Some sponges grow in the shape of barrels or balls, but the ones that Jasper de Goeij studies are flat, colorful crusts, only a few millimeters thick. When De Goeij, a marine biologist at the University of Amsterdam, started keeping them in the lab several years ago, he noticed that they were always dirty, covered in fluffy little pyramids of brown detritus. At first he blamed the tanks he kept them in, but he soon realized that the sponges themselves were making the dirt by sloughing off dead cells and excreting waste. Because sponges fill the cracks and crevices that make up about two-thirds of a Caribbean coral reef, De Goeij wondered if this detritus might feed these ecosystems.

Researchers already knew that microbes helped feed coral reef communities. They take up dissolved organic matter like sugars and amino acids and turn it into complex molecules that animals can use. (Most marine animals can’t just suck sugar out of the water; they need it in a more complicated form.) But De Goeij found in earlier research that there aren’t enough bacteria in reef waters to account for all the activity. Could sponges be responsible for the rest?

In a lab on the Caribbean island of Curaçao, De Goeij and his colleagues set up tanks with four species of sponges. To trace the path of nutrients, they fed sugars and other molecules made with heavier-than-usual forms of carbon and nitrogen to the animals. De Goeij found that the gunk that fell off the sponges also had those heavier elements in it, showing that the sponges had taken up the nutrients and turned them into cells and waste.

Next, the researchers took their experiments to reefs near their lab. They used tightly woven cotton fabric to close off two crevices of the coral reef—sponges love crevices—and injected the labeled food. After 6 hours, they removed the cloth and took samples every 6 hours from the sponges in the crevice; the sediment nearby; the water; and the snails, worms, and other animals in the area. After the initial 6 hours, the labeled food had made it to the sponges. Over the next day, it showed up in the sponges’ castoff cells and waste products. Within 2 days, it was in other animals in the area, meaning they’d eaten the sponges’ detritus, the team reports online today in Science. “It’s a really fast system,” De Goeij says. “In a 100-liter cave underwater, you put a teaspoon of sugar and 2 days later you can find it in a snail that’s crawling around.”

It's not clear how big the sponges' role is. De Goeij calculates that on the Curaçao reef, the animals cycle about 10 times more nutrients than bacteria do, but scientists will need to study more sponges in more places to work out how important they are overall. Scientists have observed that sponges are becoming more common on Caribbean reefs. That might be partly because of nutrients pouring into the water from agriculture and other human activities; algae use them to grow wild, then leak sugars that sponges can suck up.

“I’m excited, because by and large sponges have been kind of ignored,” says Joseph Pawlik, a marine chemical ecologist at the University of North Carolina, Wilmington. Sponges are hard to identify, and the ones that grow in cracks and crevices are hard to get to. Pawlik studies big sponges that grow on top of reefs, and he looks forward to finding out whether they’re ditching cells at the same rate as De Goeij’s thin, crustlike sponges. The new study “is going to generate a lot of new science and a much better understanding of how important sponges are to the reef ecosystem,” Pawlik says.