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

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MIAMI — There’s a turf war going under the waters off the Florida Keys, a battle for no less than dominance of dying coral-reef tracts.

It’s sponge versus seaweed, a matchup that for obvious reasons hasn’t generated much attention. With the competitors lacking charisma, claws, teeth, spines, fins, legs or mobility, 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 inhabitants.

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 recently finished a 10-day underwater research mission to Conch Reef off Islamorada.

Macroalgae — consisting of a variety of marine plants — isn’t good for much more than grazing for parrotfish and other algae-eating species, Pawlik said. “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 help keep those famously crystal waters of the Keys clear.

Barrel sponges, 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-size specimen, said Christopher Finelli, a UNC-Wilmington marine biologist and research team co-leader.

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

Finelli spoke from the Aquarius Reef Base, the world’s only undersea research station, which scientists and graduate students from UNC-Wilmington have used to study sponges on nearby Conch Reef since 1997. The facility, permanently moored some 60 feet down off Islamorada, is maintained by the National Oceanic and Atmospheric Administration (NOAA).

During those 13 years, the changes to corals the scientists have observed near the base have been substantial and largely discouraging. Like other reefs in the Keys, Conch’s once-impressive pillars of corals have either died or been stunted under the siege of diseases, bleaching, rising sea temperatures, anchor damage and other stressors. Scientists estimate coral coverage has declined by 90 percent in less than 50 years.

Sponges, meanwhile, have expanded their territory and numbers, with the population on Conch growing by 40 percent since 2000, Pawlik said. They have easily supplanted corals as the prime habitat for reef dwellers in the Keys and many areas, Pawlik said. “Most of our research sites look like gardens of sponges,” he said.

The spread may be another hurdle for efforts to restore coral reefs, because sponges sometimes move in at the expense of surviving corals, which must compete for space and sunlight on reefs. Some sponge species can overgrow corals, and others can break down the limestone of dead corals, the ancient bones that living coral reefs are built on.

But the spread of the large barrel sponge, which can live 2,000 years and grow to a massive size that has earned it the title “redwood of the deep,” does offer hope that an alternative habitat will survive on the reef tracts, the UNC-Wilmington team said.

For instance, the phenomenon of ocean acidification, which many scientists consider a ripple effect of climate change as seas absorb more and more carbon dioxide, represents the next and potentially fatal threat to corals. Research suggests the chemical changes can cause corals, shellfish and other organisms to calcify, leaving their protective shells brittle and weak.

Sponges are less likely to be affected by those changes, Pawlik said. “Sponges may be in an advantageous situation.”

That appears so on Conch Reef and has been supported by other research, he said. This year, divers — who can spend up to six hours in forays from the Aquarius — noted a surge in baby barrel sponges. They also noted a number of other species of dead and dying sponges, possibly victims of a brutal winter that triggered massive fish kills across South Florida.

Sponges also face an array of threats, many of them poorly understood, from diseases to fishing lines, which can slice them like “a knife through butter,” Pawlik said. Some fish also consume some sponge species.

For now, the sponges rule but, he warned, the reign remains tenuous.

“This is clearly a battle for dominance on Florida’s coral reef,” he said.