Coral reefs are demolished from within, by bio-eroding sponges.
Coral reefs are demolished from within, by bio-eroding sponges. Seeking refuge from predators, these sponges bore tunnels into the carbonate coral structures, thus weakening the reefs. Scientists from the Royal NIOZ Netherlands Institute for Sea Research have uncovered how the sponges hollow out and take over reef skeletons. This finding, published in Scientific Reports on Thursday January 24th, helps to explain why sponges erode reefs faster as atmospheric CO2 levels rise.
With their three-dimensional structures, coral reefs provide ecosystem building blocks, hotspots for living organisms and natural coastal protection against waves. Worldwide, coral reefs are under pressure of the consequences of climate change, such as ocean acidification. NIOZ-first author Alice Webb explains why and how sponges contribute to the collapse of coral reefs.
Why and how do sponges excavate coral reefs?
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. Bio-eroding sponges bore into coral skeletons, not to feed on the calcium carbonate, but 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. 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.
Read more at Royal Netherlands Institute for Sea Research
Photo: A yellow sponge extruding from coral skeleton. CREDIT: Alice Webb.