Sponges and algae are choking the Caribbean’s coral reefs

[Mexico] Descending into the clear blue water of the Colombia Reef, off Cozumel in Mexico, there are two ways to look at what greets you. One is as a fantastic, calming world of peace and harmony, so different from our hectic lives on shore. The soft corals sway in the current, colorful fish swim lazily by, and turtles float happily above.

But this is a mirage. The reality is more like a war zone. It’s every critter for itself: The angelfish eat the corals, some of which strike back with tiny harpoons; the barracudas eat the angelfish, which hide behind sponges. And the sponges, says Joseph Pawlik, are secretly plotting to take over the whole reef.

Pawlik, an ecologist and leading sponge expert at the University of North Carolina in Wilmington, has surveyed Caribbean reefs for decades. The recent evidence, he says, suggests the battle of the reefs has become somewhat lopsided. The sponges have banded together with another major player, the algae, to push out the corals and achieve reef domination—thereby adding another grave threat to one of the world’s most important ecosystems.

“They are taking the space,” Pawlik says. “That’s the most important commodity. Once they have the real estate it’s not going to go backwards.”

But reef science, like the reefs themselves, is a bit of a war zone these days. Everyone agrees that Caribbean reefs, like reefs all over the warming and acidifying ocean, are in trouble. Other experts say, however, that Pawlik’s vision of a Caribbean sponge apocalypse is itself a mirage.

The importance of predators

On a normal, healthy Caribbean coral reef, anywhere from 30 to 100 percent of the surface should be covered by coral. Seaweed and other algae should be a fraction of that, and sponges should be lucky to achieve 10 percent coverage.

The algae and sponges would happily expand, of course—but they’re constantly trimmed back by sea urchins and sea turtles, respectively, and limited by a lack of resources. Turtles are so voracious that many sponges have developed complex chemical defenses against them; sponges that aren’t so equipped must hide between stinging corals for protection.

Photo: A barrel sponge grows on a coral reef off Belize. Human impacts on the Caribbean may be favoring the growth of sponges over corals. PHOTOGRAPH BY BRIAN SKERRY, NATIONAL GEOGRAPHIC CREATIVE

View original article at: Sponges and algae are choking the Caribbean’s coral reefs





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