There is a surprising population explosion of Antarctic glass sponges over the past several years, according to a new study published today (July 11) in the journal Current Biology. Scientists, who had long assumed that these marine creatures grew very slowly, were amazed to find that the sponges doubled in biomass and tripled in number between 2007 and 2013. The research team, made up of biologists from the Alfred Wegener Institute (AWI), Helmholtz Center for Polar and Marine Research, conclude that the startling increase in glass sponges is directly linked to the 1995 collapse of the Larsen A ice shelf, when an area of about 770 square miles (2,000 square kilometers) fell into the sea.
“To the organisms living on the sea bed, the disappearance of the hundred-meter-thick Larsen A ice shelf must have been like the heavens opening up above them,” said project leader Dr. Claudio Richter. In surface waters now awash in sunlight, plankton can flourish, creating a constant rain of food onto the sea floor.
Glass sponges (Hexactinellida) are archaic organisms with skeletons made of silica that dominate the shallow Antarctic sea floor. They feed on the tiniest of plankton, which they filter from the water. Like corals, they create their own habitats, said Richter, comparing the sponge communities to cities on the sea bed, attracting other sea dwellers to them.
Prior to the AWI study, many biologists thought these creatures grew so slowly that they would have to live 10,000 years or more to reach two meters (six-and-a-half feet) in size, according to an AWI press release. Now it appears that glass sponges can grow very quickly and in a short period of time, despite frigid water temperatures.
Richter, along with AWI biologist Laura Fillinger and colleagues from the University of Gothenburg and the Senkenberg Research Institute and Natural History Museum, based their findings on data collected during a 2011 Polarstern expedition to the region of the former Larsen A ice shelf. Using a remotely operated vehicle lowered onto the sea bed at a depth of about 140 meters, the team was able to compare what they saw on their video screens with observations from an earlier expedition in 2007 with Polarstern, an ice-breaking research vessel.
In 2007, scientists had observed the presence of large numbers of ascidians–or sea squirts–and only a few glass sponges. Four years later, the team found that the ascidian population had entirely disappeared and were replaced by three times the number of glass sponges, including some juveniles, according to lead author Fillinger.
As the ice shelves in Antarctica retreat or break up, new environments are being created for marine creatures. However, whether glass sponges are benefitting from global warming is a question that cannot yet be definitively answered, according to the research team.
“There are still too many unknowns to make predictions,” cautioned Fillinger. For instance, the influence of competitors remains unclear, as does the role of predators, she said. In 2011, the team saw very few snails and starfish, which feed on glass sponges. However, Fillinger says, it remains to be seen whether these predator populations will also explode and cause widespread devastation among glass sponge communities.
AWI biologists plan to continue monitoring changes in the biological communities that inhabit the western Weddell Sea.