Field study suggests sponges creating food for coral reef organisms

(Phys.org) —A team of researchers from the Netherlands has found that material sloughed off by sponges is eaten by organisms that live on or near coral reefs. In their paper published in the journal Science, the team describes experiments they conducted in their lab, then in the field to find out if sponges contribute to the survival of coral reef organisms by providing them with something to eat.

For many years scientists have wondered how it is that there is so much life on and around coral reefs—such areas are a virtual desert which begs the question, what are all the organisms eating? Prior research has found that bacteria in the water produce some nutrients, but not nearly enough to feed the wide variety of organisms found around coral reefs. Suspecting that sponges might provide some answers, the research team ran some experiments to find out if they are actually feeding those around them.

First, the team placed several sponges in a tank full of water, then fed them some sugar and a mixture of other material that held carbon and nitrogen in heavier than normal forms. Then they waited—sponges slough off detritus left over from meals they have consumed. Examining the detritus that covered the sponges, the researchers found evidence of the heavy nitrogen and carbon, indicating that the material the sponges were sloughing off was residue from food they had eaten.

Next, the researchers moved to the ocean, specifically the coral reef off the island of Curaçao in the Caribbean. There they created a trap for several sponges living in cracks in the coral. Doing so allowed for keeping food in and also for collecting and measuring what the sponges were sloughing off. They fed the sponges the same kind of food as in the tank experiment then waited again. And once again, they found that the material sloughed off by the sponges had heavy carbon and nitrogen in it. They then waited a few more days before gathering some specimens of aquatic life living in the crack and found that they had measurable amounts of the heavy nitrogen and carbon in their tissues as well. This meant, of course, that they had eaten the material sloughed off by the sponges, and in so doing revealed at least one source of coral reef sustenance.