These orange sponges off Little Cayman Island would have eventually covered and smothered the corals beneath them, but were prevented from doing so by the grazing activity of predator fish such as angelfish and parrotfish. With coral cover on Caribbean reefs at historic lows, sponges are now the dominant, habitat-forming animals on most of them.
This decline in coral is due to disease, heat stress from warming waters and waves from storms. Additionally, scientists have discovered that overfishing has added to this decline by lessoning the numbers of predator fish that feed on some sponge species.
Joseph R. Pawlik, a marine scientist at the University of North Carolina Wilmington, and colleagues used the underwater research lab Aquarius, moored on Conch Reef off Key Largo, Fla., to study the chemical ecology of Caribbean reef sponges.
Some reef sponges produce chemical defenses, called metabolites, which make them distasteful to fish. These metabolite-producing sponges are largely left alone by predator fish, such as angelfish and parrotfish; those that don't produce metabolites are subject to being grazed upon by fish. "That being said, when overfishing by humans removes these predatory fish, reefs shift toward faster growing sponges that can out-compete reef corals for space," says Pawlik. "That further hinders corals' chances of recovery."
These findings regarding the relationship between sponges and predator fish in coral reef communities are important because previous research led scientists to believe that Caribbean sponge communities were built up in areas where plankton--tiny floating plants and animals--was available. But research by Pawlik and colleagues on sponge growth using a cage to exclude predators found the opposite was true: "Overfished reefs that lack spongivores (sponge-eating fish) soon become dominated by faster growing, chemically undefended sponge species, which better compete for space with reef-building corals," says Pawlik. This finding will be used to help guide fisheries management in the future throughout the Caribbean.
To learn more about this research, which was supported by a grant from the National Science Foundation (OCE 10-29515), see the NSF Discovery story Life on a coral reef: Insult is (sometimes) added to injury. (Date of Image: 2011-2012) [Image 3 of 6 related images. See Image 4.]