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Nahant marine study makes underwater sounds a little clearer

David Kimbro, now a Northeastern professor, was part of a team using a Florida oyster reef to study the hearing abilities of mud crabs.

Amy Diaz de Villegas/WFSU

David Kimbro, now a Northeastern professor, was part of a team using a Florida oyster reef to study the hearing abilities of mud crabs.

In proving that mud crabs can hear, Northeastern University researcher A. Randall Hughes answered a question most scientists in her field never thought to ask.

Triggered by a layman’s question, her findings, published in a scientific journal last month, are providing new insight into the ecological community found on coral reefs.

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“This project was initiated by a TV producer that we were working with who knew that some of these fish made a lot of noise,” explained Hughes, the lead researcher on the study published in the June 18 edition of the Proceedings of the Royal Society B, a journal that focuses on the biological sciences.

Rob Diaz de Villegas was documenting research that Hughes and her team were doing on the oyster reefs in Alligator Harbor, Fla., Hughes said. He became aware of the noises made by the black drum, oyster toadfish, catfish, croaker, and other fish that feed on mud crabs.

“He said, ‘Hey, can the crabs hear the fish?’ ” recalled Hughes, who works at Northeastern’s Marine Science Center in Nahant, but at that time was with Florida State University. “I said, ‘Well, I don’t know,’ and I looked through the literature and nobody seemed to know, and that’s what launched this project.

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“It drove home for me the benefits of talking about research to people outside of academia, because a bunch of us have been studying these crabs for years, and it never occurred to any of us to find out whether they were responding to the sounds that the fish made.”

A mud crab.

Amy Diaz de Villegas/WFSU

A mud crab.

Focusing on the thumb-sized mud crabs, Hughes and the team of researchers — which included her husband, fellow Northeastern assistant professor David Kimbro, and a marine acoustics specialist, David Mann — found that when exposed to the sound of various predators, the tiny invertebrates changed their eating behavior. They would forage less — presumably because they’d then be less likely to attract attention from species on the lookout for a crab dinner.

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The researchers played various sounds to the crabs, and found that when exposed to the noises made by black drum and catfish, the crabs ate significantly fewer juvenile clams than if the team played no sounds, or the sound of a nonpredator.

“If you’re on an oyster reef, the point of an oyster reef for most humans is to produce more oysters and small bivalves; then, the fact that these crabs aren’t eating as many of those could translate into real benefits for the reef itself,” Hughes said.

The information can be valuable in understanding oyster reef communities, a study that Hughes plans to expand geographically now that she has relocated to Nahant.

The Marine Science Center’s director, Geoffrey Trussell, whose work often emphasizes the evolutionary and ecological significance of predation risk, said there are various land-based studies, including one of his, that show the presence of a predator to have a positive effect on plant abundance in areas where there are also herds of herbivores.

“When organisms are out foraging, they are focused on foraging and therefore less vigilant,” Trussell said. “Think of gazelles foraging on the African Serengeti. They’re foraging and then they pop their heads up and look around, and then go back to foraging. So every time that their head’s down and they’re foraging, a cheetah can move that much closer. There’s a trade-off between acquiring the energy you need when feeding with the vulnerability that comes with the act of feeding, because you’re not paying attention to potential predators around you.”

One cheetah catching a gazelle would achieve the cheetah’s goal and eventually thin the herd, but its attack, if it scares the rest of the herd away, has an immediate benefit to the plants that otherwise would be eaten by the foraging animals.

“What Randall is showing is that just the simple fear of being eaten can have the exact same effect,” Trussell said.

Hughes played the sound of one species of predator at a time during the crab study. The next phase of her team’s research is to blend the noises made by several predators.

“There’s a medley of sounds going on out there,” Hughes said, “so we’re interested in getting a handle on what natural oyster reefs sound like and, in turn, how crabs respond to a more complex sound environment.”

How important a discovery it is remains to be seen, although Hughes noted that it offers scientists more knowledge toward understanding the entire ecosystem.

“It’s a pretty basic thing that we didn’t know, so for that reason I think it’s important. It expands our understanding of these organisms, and opens up a whole new world of possible experiments,’’ Hughes said.

“As a scientist, that’s exciting to me, when the answer to one question just generates more questions.”

David Rattigan can be reached at drattigan.globe@ gmail.com.
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