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Ocean Acidification Excites Boring Sponges

This orange boring sponge (Cliona varians) overgrows several coral species at Panama's Smithsonian Tropical Studies Institute.


Amber Stubler

Boring sponges get a bad rap. Their own name betrays them, announcing to the world that they are unexciting, ordinary and quite frankly, boring. However, if ever a misnomer existed, this is it.

More flatteringly referred to as excavating or bioeroding sponges, these animals play the important yet thankless role of breaking down and recycling calcium carbonate (the main component of eggshells, corals and shelled marine organisms). Using a combination of chemical and mechanical erosion, these sponges dissolve and slowly chip away at dead, diseased, and occasionally healthy corals. In doing so, they recycle and refurbish coral material, create new space for settlement and add to the complexity of the environment. Although they are often an afterthought when understanding coral reef functioning, these antagonistic characters are really the stars of the show.

Or at least they will be.

As coral reef environments degrade, boring sponges actually increase in abundance. Corals and other reef organisms tend to be negatively affected by temperature increases and poor water quality—but boring sponges don’t seem to mind all that much. And herein lies the danger: the result could be a reduction in coral reef growth or, even worse, a permanent shift in the balance of growth and erosion in favor of erosion. In fact, Caribbean reefs have already become flatter over the last few decades: more than one-third are already eroding faster than new coral is growing. It’s like a forest where the trees are turning to sawdust faster than new wood is being produced.

Ocean acidification presents another potential advantage for boring sponges because it weakens and chemically dissolves coral skeletons, making the actual act of boring easier. As seawater pH decreases (a result of increasing carbon dioxide), corals are unable to create new layers of calcium carbonate as efficiently. Surprisingly, there has been relatively little research investigating how ocean acidification affects the interaction between coral and bioeroders such as boring sponges.

This is where my interests lie. I am currently in my second year of running experiments at the Smithsonian Tropical Research Institute’s Bocas del Toro, Panama lab to understand how future changes in pH will affect the relationship between the boring sponge Cliona varians and the common finger coral Porites porites. My early results have shown what I expected: that as pH decreases, sponge bioerosion increases significantly. Below is a scanning electron microscope image of sponge erosion scars (the small circular chips, like small bite marks) on coral in each of 3 different pH levels: current ("Ambient"), that projected for mid-century (a little lower, called "Moderate"), and the likely end-of-century value (much lower, called "High"). Under the lowest pH (highest acidity), you can see that the amount of small circular chips worn off the coral skeleton increases noticeably.

What does this mean? It means that over an 8-week period, boring sponges were able to remove more calcium carbonate in low pH seawater—a likely condition in the future as more carbon dioxide builds up in the atmosphere—than those sponges in seawater more like today’s pH. So an already bad situation could get much worse.

Obviously this is only part of the puzzle, which is why this year I will continue to conduct this research with the addition of temperature treatments to see how the combination of pH and temperature rise affects the bioerosion efficiency of sponges. Hopefully this research will provide insight into how climate change and ocean acidification may affect largely-unconsidered aspects of coral reef ecosystems and prove once and for all that boring sponges are, well, anything but boring.

Boring Sponge on Coral