On the face of it, coral reefs in tropical waters shouldn’t exist at all, because the waters that surround them are nutrient-poor – lacking the nitrogen and phosphorus that all living things need. Yet they teem with all sorts of marine life. Charles Darwin was the first to observe this phenomenon, likening tropical reefs to oases in a desert.
This phenomenon has long remained a mystery, but a recent study has revealed that, of all a coral reef’s spectacular creatures, the humble sponge may be the key player in enabling reef ecosystems to thrive. A team of Dutch researchers working in the Caribbean island of Curaçao and led by biologist Jasper de Goeij, discovered that sponges digest and recycle the vast quantities of organic matter that corals expel with extraordinary efficiency.
Most organisms can’t digest this material, but sponges circulate water at a rate of up to 100 litres a day to filter out the organic matter, expelling their old cells and using the energy from the organic matter to generate new ones. The sponges’ discarded cells are easily digested by other reef organisms, including snails, crustaceans and worms.
The researchers identified one sponge in particular, Halisarca caerulea, as an exceptionally efficient nutrient recycler. It takes in up to two-thirds of its own body weight in dissolved carbon daily, but hardly grows because its old cells are discarded almost as fast as it generates new ones. The scientists estimate that this “sponge loop” generates almost as many nutrients as all the corals and algae in a reef. This discovery opens the door to further insights into how to protect coral ecosystems, but de Goeij is also interested in promoting sponge cultivation for such uses as water purification and as a potential source of medicine. It’s a wonderful world!