Glass sponges take advantage of retreating Antarctic ice shelves

To the surprise of the scientists, the density of these archaic filter-feeders has increased threefold between 2007 and 2011 despite only low plankton food supply and water temperatures of minus 2 degrees Celsius. The sponges had also grown remarkably quickly and had completely supplanted competitors for food. The results show that communities at the bottom of the western Weddell Sea react considerably more quickly to climate-related changes than previously thought.

Glass sponges (Hexactinellida), an archaic group of animals at the basis of the animal kingdom, dominate the shallow seafloor in the Antarctic. Many biologists believed that glass sponges grew so slowly that two-metre behemoths would have to be around 10,000 years or older. These assumptions have now been challenged in a new study led by scientists from the Alfred Wegener Institute (AWI) and published in the current issue of the scientific journal Current Biology.

During a Polarstern expedition to the poorly accessible region of the former Larsen A ice shelf, AWI biologists Laura Fillinger and Claudio Richter, together with colleagues from the University of Gothenburg and the Senkenberg Research Institute and Natural History Museum; succeeded in demonstrating that glass sponges can grow rapidly within a short period of time. "We were surprised by what we saw on our video screens in 2011 when we lowered our remotely operated vehicle onto the sea bed at a depth of around 140 metres. In an area, which had revealed large numbers of ascidians and only occasional glass sponges during an earlier expedition with the ice-breaking research vessel 'Polarstern' in 2007, four years later we found no ascidians at all. These pioneer species had completely disappeared, replaced by three times the number of glass sponges, including several juvenile individuals," reports Laura Fillinger, lead author of the study.

Until this time scientists had assumed that communities on the Antarctic sea bed only change very slowly because of the very low temperature (minus two degrees Celsius) and patchy supply of food in pack-ice covered waters. "We now know that glass sponges may undergo boom-and-bust cycles, allowing them to quickly colonize new habitats in a short period of time," says project leader Prof. Dr. Claudio Richter.

"To the organisms living on the sea bed, the disappearance of the hundred-metre-thick Larsen A ice shelf must have been like the heavens opening up above them," he adds. Where cold, darkness and food shortages had previously reigned, sunlight now allows plankton growth in surface waters and, hence, a rain of food comes down to the sea bed.

Glass sponges feed on the smallest plankton, which they filter from the water. The animals grow to a size of up to two metres, and their vase-like bodies provide perfect hiding, spawning and shelter opportunities for fish, invertebrates and many other sea dwellers. "Like corals, sponges create their own habitats. To an extent they are like cities on the sea bed. There is something going on wherever they grow, and this attracts other sea dwellers to them," says Claudio Richter.

New spaces are being created for such underwater worlds wherever the ice shelves on the Antarctic Peninsula are retreating or breaking up. However, scientists cannot yet definitively say whether this means that glass sponges will be one of the beneficiaries of climate change. Laura Fillinger: "There are still too many unknowns to make predictions. One example is the question of the influence of competitors: currently we are witnessing a fierce competition for space on the sea bed. Another concerns predators: in our dive in 2011 we hardly saw any of the snails and starfish, which feed on glass sponges. However, it is possible that these voracious predators will follow suit and wreak havoc."

Marine biologists at the Alfred Wegener Institute will continue to monitor the changes to communities in the western Weddell Sea. In January 2013, the planned dives in the area of the former Larsen A ice shelf had to be called off due to the solid pack ice in the Weddell Sea. However, Claudio Richter and his team hope for better pack-ice conditions during future Polarstern trips to this part of the Antarctic so that they can use new investigative methods to discover more about the life cycle of the glass sponges.

What are ice shelves?

An ice shelf is the continuation of a glacier, which floats on the sea, i.e. the part of a glacier that no longer rests on land or the sea bed. The most famous ice shelves are in the Antarctic, where the two largest of their kind can be found in the Filchner-Ronne Ice Shelf (with an area of some 422,000 square kilometres) and the Ross Ice Shelf (with an area of approximately 473,000 square kilometres). The thickness of the ice sheets differs from ice shelf to ice shelf, and ranges between 50 and 600 metres.

Larsen A ice shelf

"Larsen A" was the name given by scientists to the smallest and most northerly of the three Larsen ice shelves which once extended from the eastern coast of the Antarctic Peninsula to the western Weddell Sea. In January 1995 the Larsen A ice sheet collapsed within a few days during a storm, together with the Prince Gustav ice shelf to the north, affecting an area of some 2000 square kilometres. Its remains then drifted as an armada of small icebergs into the western Weddell Sea. The spectacular collapse filled headlines worldwide, because scientists had never seen before an ice shelf disintegrate so quickly. It was the first time scientists realised that climate change can produce an almost immediate and complete loss of ice shelves. The former Larsen A ice shelf region is now often covered with pack ice and therefore difficult to reach for research ships like the "Polarstern."