Aquatic architects

A red grouper rests in its excavated hole.
Courtesy of NOAA Underwater Research Program / University of North Carolina, Wilmington

It was 2 o’clock in the morning, and marine scientist Felicia Coleman, floating 150 kilometers offshore of the gulf coast of Tampa, Fla., was growing weary of looking at a monitor. It was displaying live images being captured by the remotely operated vehicle (ROV) some 90 meters below, showing lots of rock clusters inside holes in the seafloor, but nothing to explain how the holes got there. Then Coleman saw something unexpected—red grouper sweeping rocks from the seafloor holes they call home with their tail fins.

“It was one of those moments [that] you see these cartoons where scientists are yelling, ‘Eureka! I’ve found it!’” recalls Coleman, based at Florida State University. “It woke us up.” Watching the video stream, the researchers started to get the sense that...

Coleman was anxious to learn more, but studying anything 90 meters below the surface was no easy task. Then she learned that juvenile red grouper live in just a few meters of water all along the Florida Keys.

First, Coleman and her team identified holes that were still full of sediment, and placed juvenile red groupers in open-bottomed cages over the filled-in holes. Donning her SCUBA gear, she dropped to the seafloor to observe the fish gulping up the sediment into their mouths and dumping it at the perimeter of the cage. Within 48 hours, each hole had been sufficiently excavated to allow the juvenile fish to fit comfortably inside of it (Open Fish Sci J, 3:1–15, 2010).

These excavations do more than simply hollow out a home for a single, predatory fish. Spiny lobster, for example, which draw tens of thousands of fishers to the Gulf each year, tend to rest inside the holes during the day before heading out on their nightly hunting escapades. Furthermore, many sessile invertebrate species settle on the rock substrate exposed by grouper excavation, attracting even more organisms, such as cleaner shrimp that nestle among the tentacles of the excavation-associated anemone. Thus conservation efforts that target the red grouper, which are currently the most commonly fished grouper species in the Gulf of Mexico, could protect other marine species as well.

In fact, Coleman found that biodiversity increased significantly with the size of the excavated area, and decreased considerably as she looked farther and farther away from the holes. “The wonderful thing about this study is it shows that red grouper are important in their ecosystems in a way that beavers are in their ecosystems,” says marine ecologist Elliott Norse of the Marine Conservation Biology Institute. “They’re keystone habitat modifiers or ecosystem engineers.”

“We know that biodiversity is really important for ecosystem function—the higher the biodiversity, the better the recovery, and the less collapse you get in a fishery” following large harvests, says marine ecologist Susan Williams of the University of California at Davis. Thus, such ecosystem engineers are “very good targets” for management strategies. “If you protect them, you’re protecting a [whole] suite of species.”

Red grouper aren’t the only piscine architects. Gobies, for example, dig little holes in the sand, which in turn attract cleaner shrimp and other fish. And on a larger scale, tile fish are famous for the pueblo-like structures they build along the slop of the continental shelf.

But red grouper are “another clear example of how ecosystem engineers are important components of marine ecosystems,” says Hunter Lenihan of University of California, Santa Barbara, “and how we should try to focus attention on their management and their sustainability.”

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