Conventional wisdom holds that life in Antarctica moves at a glacial pace. Marine creatures called sponges, which live on the seafloor, have been known to go a decade without any measurable growth in the Antarctic. But that thinking has changed, in part because of a startling discovery off the eastern coast of the Antarctic Peninsula.
Researchers have found a "boomtown" of sponges. In a study published July 11 in the journal Current Biology, they report on the explosion of a community of glass sponges—organisms with skeletons made of silica, a mineral component of glass—on the seafloor below where an enormous ice shelf used to be.
These sponges—filter feeders not known for their rapid development—doubled in biomass and tripled in number over the course of two growing seasons.
Everyone believed that glass sponges in this area wouldn't grow and expand so rapidly, said Claudio Richter, a marine biologist at the Alfred-Wegener Institute in Bremerhaven, Germany, and a study co-author.
Antarctic sponges are famous for being the slowest of the slow-growing, said David Barnes, a benthic ecologist with the British Antarctic Survey in Cambridge who was not involved in the project.
"Now you've got these guys who [are] growing at the rate of tropical sponges," he said. No one thought those rates of change happened in the Antarctic.
Scientists are eager to solve the mystery—not just because they want a better understanding of sponge ecology in the Antarctic, but also because these seafloor animals have the potential to store carbon. This could be a boon when it comes to figuring out ways to address climate change.
Here Comes the Sun
This startling population boom is partly due to the collapse of the Larsen A ice shelf in 1995, Richter and colleagues write. In late January of that year, an area about 770 square miles (2,000 square kilometers) disintegrated. (Related: "Ice Shelf Collapses Reveal New Species, Ecosystem Changes.")
Remnants of the ice shelf covered the area until January 2006, when they too collapsed. (Explore an interactive map of Antarctica.)
Areas once enclosed in darkness were bathed in sunlight for part of the year, boosting the growth of microscopic algae—the base of the marine food chain.
Researchers went down to the area in 2007 to see what was happening to the animals living on the benthos, or seabed. Thanks to cooperative sea ice and weather conditions, they were able to go back in 2011 to see how things had changed.
Richter and colleagues took video of these glass sponge communities with a remotely operated vehicle and collected samples using a trawl net.
But experts are at a loss to explain exactly why these glass sponge populations grew so quickly.
"That is the million-dollar question," wrote Paul Dayton, a marine ecologist at the Scripps Institute of Oceanography in San Diego, California, who was not involved in the study, in an email.
Dayton has seen rapid growth in Antarctic sponges before in McMurdo Sound, but the circumstances were different. He speculates that a large iceberg blocked production of larger species of plant plankton, allowing smaller species—the favored food of glass sponges—to grow. The sponges took advantage and grew rapidly. (Watch a live feed of McMurdo Station.)
Richter and colleagues speculate that algae blooms injected pulses of food into the benthic community, contributing to the growth of glass sponges near the Antarctic Peninsula, Dayton noted. That wasn't the case in McMurdo Sound when Dayton did his glass sponge study.
The answer probably lies in figuring out exactly which species of plant plankton the different glass sponge species need, he said.
Study co-author Richter speculates that there may have been some unknown factor that previously limited the sponges' growth, and somehow that factor may have been eliminated.
The British Antarctic Survey's Barnes also warns that this is just a snapshot of a dynamic seafloor community in a specific area. Once things settle down, the area could resemble a "normal" Antarctic benthic habitat.
A Sinking Feeling
Whatever the reasons for this boom in glass sponges, "the repercussions of all this are quite far-reaching," Barnes noted.
When plant plankton produce food, they take carbon out of the seawater, he said. And when glass sponges feed on that plankton, the carbon contained in their prey becomes locked away on the seafloor. When the sponges die, they're buried in the sediment, sequestering their carbon.
"This is a brand-new carbon sink, really," Barnes said.
And although, in the grand scheme of planetary carbon budgets, a patch of seafloor in the Southern Ocean isn't going to make a huge dent, it could affect the models researchers develop to study Earth's climate, he added.
Richter plans to delve into this boomtown mystery with his graduate students. Not much is known about the ecology of these sponges, so he plans to figure out what exactly the sponges are eating and how much energy they expend in their daily lives.
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