Scientists have long debated how coral reefs - one of the worldâ€™s most productive and diverse ecosystems - can thrive in what is the marine equivalent of a desert. It turns out their success is all down to sponges...
Charles Darwin described tropical reefs as being like an oasis in a desert, living and growing in waters lacking important nutrients such as nitrogen and phosphorus — a phenomenon known as Darwin's Paradox.
Now scientists have found that sponges keep the reefs alive by turning waste products from corals and algae into a food source for other reef inhabitants.
The research suggests that sponges recycle nearly ten times as much matter as bacteria, and produce nearly as much nutrition as all the corals and algae on an entire tropical reef combined.
This recycling pathway, termed the 'sponge loop' by the research team, explains how energy and nutrients are conserved within the coral reef ecosystem and plays a pivotal role in the food web of coral reef ecosystems.
Lead author Jasper de Goeij, an aquatic ecologist at the University of Amsterdam, told BBC News that sponges are the "unsung heroes" of the reef community.
"Up until now no-one has really paid sponges much attention. They look nice, but everybody was more interested in corals and fish," he said.
Sponges are known to be filter feeders, feeding on small particles like bacteria, planktonic algae and even virus particles. The majority of their daily diet, however, consists of invisible dissolved organic substances, such as sugars. In fact, these dissolved substances form the largest source of energy and nutrients on coral reefs and are produced by corals and algae.
The research team fed four common species of sponge with labelled sugars, and then traced them as they travelled through the ecosystem. The sponges took up the food and rapidly turned it into detritus, which then rained down on the reef where it formed an important food source for small reef inhabitants such as crabs, snails and worms. These animals were then eaten by larger species, and the food was looped back into the food web.
The study was published in the journal Science.
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