By Jane Sutton

MIAMI (Reuters) - Giant barrel sponges that can live for thousands of years have proliferated in the waters around the Florida Keys, the apparent winner in a recent battle for dominance among corals, seaweed and sponges.

That's a good thing, according to researchers wrapping up a 10-day stint at the Aquarius Underwater Laboratory off Key Largo on Wednesday, because the sponges filter the water and provide a habitat for valued fish species.

Corals have been in decline for decades along the reef tracts of the Caribbean and the Florida Keys for a variety of reasons, leaving free space for sponges and macroalgae -- seaweed -- to move in. Barrel sponges are hollow and come in a range of colors including red and purple.

"If you can't have corals, better that you should have sponges than macroalgaes. And right now it appears the sponges are doing OK," said Joseph Pawlik, one of the University of North Carolina-Wilmington researchers studying the sponges.

The barrel sponge population has increased by about 40 percent since 2000 in the reef alongside the bus-sized Aquarius lab, which sits in 60 feet of water off the coast of Key Largo in the Florida Keys, the researchers said.

"We had lots of baby sponges on the reef this year," Chris Finelli said in a teleconference from inside Aquarius.

Anecdotal evidence suggests the same thing is occurring throughout the region, as barrel sponges and seaweed battle for dominance on reefs once covered with coral.

Seaweed is food for some fish species but it doesn't live long, the researchers said.

"They don't seem to make space or homes for any of the things that we value as far as fisheries go," Pawlik said from North Carolina. "Giant barrel sponges are the most important habitat-forming organism on Florida's coral reefs."

"COOL LITTLE PUMPS"

Baby barrel sponges are thimble-sized but they can grow to be as large as 55-gallon drums.

By observing the same sponges over the years and calculating their growth rate, the researchers determined that some are more than 2,000 years old, making them among Earth's oldest living creatures -- the Redwoods of the reefs.

"Those giant barrel sponges filter about 100 times their own volume every hour," Finelli said. "They're really cool little pumps."

Since 1997, teams from the University of North Carolina at Wilmington have made twice-yearly visits to the Aquarius lab, which is owned by the National Oceanic and Atmospheric Administration.

They live for 10 days at a time in the lab, which gives them the unique opportunity to spend up to nine hours a day measuring and photographing life on the reefs.

"The only way you can tell if the community is changing is research like this," Finelli said.

The lab also acts as a decompression chamber. Before returning to the surface, the researchers will spend 16 hours decompressing to bleed off the excess nitrogen that builds up in their bodies at undersea pressures.

(Editing by Pascal Fletcher and Cynthia Osterman)