It sounds like the plot of a B-movie, but researchers have determined that sponges in over-fished areas are using deadly tactics - such as smothering and toxic mucus - to kill coral colonies so that the sponges can grow on what's left: skeletons.
The discovery shows how disruption of even just a few members of an ecosystem - in this case, tasty fish - can hurt many other organisms that live in the same area.
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The problem, highlighted in the latest issue of the journal PeerJ, is particularly evident at Caribbean coral reefs, where the combined effects of warming seawater temperatures, storms, and diseases have already decimated populations of corals.
While coral looks sort of like an undersea plant, groupings actually consist of tiny marine creatures that live together within compact colonies. They usually grow slowly. Sponges, on the other hand, grow either quickly or more slowly, depending on the species.
"If the goal is to save the corals that build Caribbean reefs, we have to protect the angelfishes and parrotfishes that eat sponges," lead author Tse-Lynn Loh, now a Postdoctoral Research Associate at the Haerther Center for Conservation and Research at Chicago's Shedd Aquarium, said in a press release.
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Loh and colleagues surveyed reefs from 12 countries across the Caribbean. The scientists compared 25 sites where fish abundance is very low (because of decades of intensive fish trapping) with 44 sites where fishes are plentiful (because they've been protected from fishermen).
Both fast and slow-growing sponges tended to dominate coral colonies at overfished sites, adversely impacting at least 25 percent of them. The damage was more than double the incidence of this seen at less-fished reefs. At the latter, angelfishes and parrotfishes munched on the sponges, helping to keep their dominant ways in check.
Even when the fish were around, slower-growing sponges still made a dent in coral colonies, since these sponges can unleash their arsenal of "weapons" that allow them to compete aggressively for reef real estate. In addition to the smothering tactics and toxic mucus, the sponges can kill coral by releasing other toxins and by shading out light.
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In past studies, seaweed was also found to hurt coral, but the new research led to the surprising discovery that seaweeds were more abundant on reefs with greater numbers of fish. The outcome is contrary to the conventional wisdom that fishes eat seaweeds and keep them in check. It now looks like the relationship between seaweeds and fishes is more complicated than previously imagined.
Conservationists in earlier years based some of their claims about needed regulation of fishing at reefs on the idea that fish kept damaging seaweed under control. Now, there is convincing evidence that sponges, and perhaps not so much seaweed, are heavily impacted by over-fishing, which sets the stage for the sponges to kill and take over coral reefs.
"Caribbean nations can now base their fishing policy decisions on the clear connection between overfishing and sponge-smothered corals," said co-author Joseph Pawlik of the University of North Carolina at Wilmington. "Coral conservation requires a healthy population of reef fishes."