This suggests, said Richter, that the sponges are eating the picoplankton and excreting nutrients that the reef coral and algae need to survive.
"Our findings may therefore provide a general answer to Darwin's question of how coral reefs manage to thrive in oligotrophic [nutrient-poor] waters," the researchers conclude in their scientific paper in Nature.
John Ogden, director of the Florida Institute of Oceanography in St. Petersburg, agrees that the cavity-dwelling sponges are a source of nutrients for the reef coral and algae, but he says Richter and his colleagues are overstating Darwin's paradox to make their point.
"This is a false conundrum," he said.
Researchers have known for a long time that a variety of processes, such as nitrogen fixation, groundwater seepage, and the influx of nutrients from the oceans, enable reefs to thrive in nutrient-poor water.
Nutrients are also thought to come from within the cracks and crevices that riddle the reef framework, and the main value of the CaveCam research is that it has revealed more detailed information on this source, Ogden said.
"They elaborated on one of the mechanisms in which nitrogen is contributed to reefs," he said.
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