Overfishing favours coral-killing sponges


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A lack of sponge-eating species such as angelfish and parrotfish leads to fast-growing sponge species taking over the reef.

We know from previous research that overfishing removes species that feed on sponges growing on coral reefs. Now, new research by the same team of ecologists suggests that it actually alters sponge communities across the Caribbean.

Biologists from the University of North Carolina Wilmington studied 109 species of sponges at 69 Caribbean sites and found that the 10 most common species made up 51% of the sponge cover on the reefs.

Joseph Pawlik, who co-authored the study says: "Sponges are now the main habitat-forming organisms on Caribbean coral reefs.

"Healthy coral reefs need predatory fish — they keep sponge growth down."

Reefs in the Cayman Islands and Bonaire — designated as off-limits to fishing — mostly have slow-growing sponges that manufacture chemicals that taste bad to predatory fish. Predatory fish there feast on the faster-growing, "chemically undefended" sponges, leaving only the bad-tasting but slow-growing, sponges.

But in overfished reefs, such as those off Jamaica and Martinique, the fast-growing, better-tasting sponges dominate. "The problem," says Pawlik, "is that there are too few fish around to eat them." So these fast-growing sponges quickly take over the reefs.

The results support the need for marine protected areas to aid in coral reef recovery, believes Pawlik.

"Overfishing of Caribbean coral reefs, particularly by fish trapping, removes sponge predators," the study says. "It's likely to result in greater competition for space between faster-growing palatable sponges and endangered reef-building corals."

The bad-tasting molecule used by the most common chemically-defended sponge species is a compound named fistularin 3. Similar chemical compounds defend some plants from insects or grazers in onshore ecosystems. "But the complexity of those ecosystems makes it difficult to detect the advantage of chemical defences across large areas," says Pawlik.

When it comes to sponges, the view of what's happening is more direct, he says. "The possibility of being eaten by a fish may be the only thing a reef sponge has to worry about."

And what happens to reef sponges may be critical to the future of the Caribbean's corals.

The results of the research are published in the journal Proceedings of the National Academy of Sciences (PNAS).

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