National Geographic News
A red grouper is half-buried in the seafloor.
Red groupers (such as the one pictured) dig up enticing homes for other sea creatures, a new study says.

Photograph courtesy NOAA Undersea Research Program, University of North Carolina, Wilmington

Ker Than

for National Geographic News

Published January 25, 2010

Grouper "landlords" that carve out their rocky homes in the seafloor inadvertently give other marine animals free housing, a new study says.

Young red groupers, abundant along Florida's western coast, remove sand from hidden seabed holes. The resulting recesses act both as a habitat for the grouper and inviting homes for corals, lobsters, and other sea creatures.

The surprising find suggests that, like beavers, red groupers are "ecosystem engineers," modifying their environments to suit their needs while creating habitats for other animals.

(Related: "Predator Fish Help Coral Reefs Rebound, Study Shows.")

"The digging is phenomenal," said study leader Felicia Coleman, director of Florida State University's Coastal and Marine Laboratory.

"It creates opportunities for organisms to settle in places they normally couldn't."

One-Stop Shop

For the new study, divers witnessed juvenile red groupers excavating sand in relatively shallow water off the West Florida Shelf.

The young fish somehow ferret out buried "solution holes," which formed thousands of years ago when sea levels were lower and fresh water dissolved openings in the rock surface. As sea levels rose, these solution holes filled with sediment.

Using their mouths and fins, the fish remove the sand, forming pits nicknamed "grouper holes." The openings become dream properties for corals, sponges, and sea anemones, as well as cleaner fish, crabs, and spiny lobsters seeking refuge from predators. (See pictures of ocean wildlife.)

The fish meticulously maintain their holes, clearing away any stray sand or debris.

In return, the groupers get regular grooming sessions from the cleaner fish, a safe haven to attract mates, and, occasionally, an easy meal, the study says.

Your House or Mine?

After a few years living in shallow waters, the mature groupers move out to deeper waters, where they excavate larger solution holes that can reach up to 16 feet (5 meters) across and several feet deep.

"When you're out on the water and you come upon one of these holes, you just see a cloud of fish above it. It's just remarkable," Coleman said.

Once a mature grouper sets up house, it's generally for life, Coleman said. Males and females live in holes near one another, and females will pop over to the males' homes to mate.

"They don't move from their spot. And if you think about it, they don't have to," Coleman said.

"Their food comes to them, cleaning species come to them, and [at least for males] their mates come to them. It's the perfect existence in some respects."

Research published online January 9 in The Open Fish Science Journal.

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