Ice sponging off the Antarctic shelf.

Ice sponging off the Antarctic shelf

When rippled by Antarctic ocean currents, the bushy white stalks of the sponge Homaxinella balfourensis look like flowers bending in a breeze. Until recently, biologists assumed the creatures increased in number and size as slowly as other Antarctic sponges, many of which grow only 1 to 4 inches in 20 years. But ecologist Paul K. Dayton of the Scripps Institution of Oceanography in La Jolla, Calif., has documented dramatic population shifts -- probably triggered by climate changes -- that suggest H. balfourensis is among the fastest-growing sponge species, gaining 15 inches in a year or two.

In the Sept. 29 SCIENCE, Dayton describes research linking periods of rapid sponge growth to the absence of anchor ice, an accumulation of ice plates up to 2 meters wide in shallow water. The sponge's stalks break off when anchor ice forms around them and then floats away. During the 1960s, when anchor ice was plentiful, H. balfourensis populations increased very slowly. But during the mid-1970s, when anchor ice was rare, its population boomed, doubling in one area over a period of two years. The boom led to rapid growth in size and population of a sponge predator, the starfish Perknaster fuscus antarcticus. Although anchor ice also kills the starfish, for some reason the return of the ice in the mid-1980s did not diminish the starfish population, Dayton says.

Dayton suggests a major shift in ocean currents under the Antarctic ice shelf triggered the fluctuations in sponge population. When the current flowed south to north, it swept supercooled water under the shelf, spawning production of anchor ice. But when the current came from the north, it brought warmer water that didn't encourage the ice matrices to form. Dayton thinks the current shifts may be connected to the El Nino-Southern Oscillation weather phenomenon, which may affect polar winds.

He says his discovery underscores the value of multidecade ecological research.