As coral reefs die, sponge vs. seaweed battle heats up

There's a turf war going on under the warm waters off the Florida Keys, a battle for no less than dominance of dying coral reef tracts.

It's sponge vs. seaweed, a match-up that for obvious reasons hasn't generated much attention. With the competitors lacking charisma, claws, teeth, spines, fins, legs or any mobility whatsoever, this struggle is slow, painfully so. But scientists running a long-term monitoring program call its outcome crucial to an array of fish, lobster and other reef denizens.

So far, sponges, particularly the Caribbean barrel variety that can grow larger than a backyard hot tub, are edging out macroalgae, commonly known as seaweed. That's about the only bright side to the dismal decline of corals from Biscayne Bay to the Bahamas and throughout the broader Caribbean basin.

"If you can't have coral, better that you should have sponges rather than macroalgae," said Joseph Pawlik, a marine biologist and co-leader of a team from the University of North Carolina-Wilmington that wrapped up a 10-day underwater research mission to Conch Reef off Islamorada on Wednesday.

Macroalgae — comprised of a variety of marine plants — isn't good for much more than grazing for parrotfish and other algae-eating species, said Pawlik. "They're food for fish but otherwise they flop around" and provide no habitat for reef dwellers.

Large barrel sponges, on the other hand, offer food and some of the shelter that elk horn, brain and other large hard coral once provided and also help keep those famously crystal waters of the Keys clear.

Barrels, which range in size from thimbles to garbage cans on Conch Reef but can reach small swimming pool dimensions in deeper waters, feed by filtering nutrients, plankton and other things. They do it relentlessly, pumping 100 times their volume every hour — thousands of gallons a day for a garbage-can-sized specimen, said Christopher Finelli, a UNC-Wilmington marine biologist and research team co-leader.

"They're really cool little pumps, really cool energy machines," said Finelli. "That's what they do, 24 hours a day, seven days a week."

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