Sponges are contributing to the collapse of coral reefs

When sponges bore holes into coral reefs to keep safe from predators, the reefs begin to erode from within. A new study by scientists at the Royal NIOZ Netherlands Institute for Sea Research describes how sponges contribute to the collapse of coral reefs and why this activity will speed up as atmospheric CO2 levels rise.

“An affected coral reef may seem intact on the outside, while on the inside looking like a complex of mining shafts with galleries and cavities that are completely filled with sponge tissue,” said study first author Alice Webb.

She explained that the sponges bore into coral skeletons to protect their own tissue from predators by hiding in the hard coral structure.

“To make the tunnels, they use a combination of chemical and mechanical demolition techniques. First the sponge pumps acidified fluid onto the coral to dissolve and chip off calcium carbonate pieces; then it removes the chips from the boring pit by contracting its tissue, resembling smooth muscle tissue,” said Webb.

“Sponges filter huge amounts of water constantly for feeding and breathing. They suck in water, take all the food particles and then spit the filtered water out. Chips of coral reef are expelled by the same route through these filtering canals, and out of the sponge through the so-called osculum.”

According to Webb, previous research showed that ocean acidification intensifies the bio-erosion of the corals, but the reason for this was unclear.

The bio-eroding sponges alter the chemical composition of the water by pumping protons from the surrounding seawater into the etching interface, which makes the surface of the skeleton more acidic and easier to dissolve.

“If more CO2 is dissolved in the seawater, the proton concentration rises and the sponge will simply have to spend less energy on lowering the pH at the sponge/coral interface.”

Webb said that these sponges are abundant on reefs in the Caribbean and also on the Great Barrier Reef. “Taking ocean acidification into account, our sponge findings make it possible to make better predictions of future coral reef conditions.”

The study is published in the journal Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer

Image Credit: Alice Webb