In horror flicks, creepy soundtracks are used to help scare the living daylights out of people. In a lab, two researchers used a similar device to freak out mud crabs in an effort to prove that they can hear.
The researchers placed the crabs in a big tank and piped in sounds commonly made by fish that eat them. The crabs were scared stiff. When mating calls and nest defense grunts of hardhead catfish and black drum fish played through an underwater speaker, the crabs didn’t dare venture out to dine on the juicy, defenseless juvenile clams that the researchers set out for them.
Just like in a movie theater, it was all an illusion. There were no fish; not even any swishing water. But for the crabs, it might as well have been the heart-pounding theme from “Jaws” or the shrieking violins in the “Psycho” shower scene, said A. Randall Hughes, an assistant professor of marine and environmental science at Northeastern University in Boston and a co-author of the study.
The findings are the first to show that crabs respond to noises made by predators, she said, and they serve as a reminder that humans know little about life in the oceans and could not answer a question as simple as whether crustaceans and organisms like them can hear.
“I was pretty surprised that we found they would do this at all,” Hughes said. “By having a human bias that because these crabs and other invertebrates don’t have ears, we think sound doesn’t play a role in their biology and their reactions to other species.”
The researchers had one worry — that the mud crabs would tremble at almost any sound. So they played noises emitted by snapping shrimp. The crabs kept eating with no reaction. They also didn’t hesitate much when the calls of oyster toadfish were played.
The study, published last week in the journal Proceedings of the Royal Society B, adds to the work in an area of research called the ecology of fear, studying how prey trembles in the presence of predators, Hughes said.
David Kimbro, also an assistant professor at Northeastern, and David Mann, a marine acoustics expert at Loggerhead Instruments in Sarasota, Fla., are the other co-authors of the study.
Last year, three scientists — Matthew A. Wale, Stephen D. Simpson and Andrew N. Radford — showed that ship noise interrupted the foraging of shore crabs. They gulped more oxygen, “indicating a higher metabolic rate and potentially greater stress,” the study said.
Mud crabs, about the length of the average adult’s thumb, are intermediate predators of critters with shells, banging on baby oysters and clams to crack them open with oversize claws like tiny mallets.
But in the ocean, there’s no such thing as a free lunch. On the reefs, when hardhead catfish and their ilk are swimming about in search of a meal, mud crabs are paralyzed with fear and feed less, giving juvenile clams and other bivalves a break and possibly contributing to an increase in their populations.
There’s no reason to think that hearing is unique to mud crabs, Hughes said. Their behavior could be common to Chesapeake Bay blue crabs and related species throughout the world. The next step is to conduct science to support that, she said.
“What we really want to do is learn more about what an oyster reef really sounds like,” she said. “Does a reef sound differ from Florida to the Chesapeake? Do crabs respond differently?”
The idea for the study came to Hughes and Kimbro as they observed the behaviors of crabs and other marine life on a reef off the Florida panhandle near Tallahassee in the summer of 2012.
A television producer who visited them one day noticed that fish on the reef made a lot of noise. “You think they can hear these fish?” Hughes recalled the producer asking. She and Kimbro looked at each other.
“We said, ‘I don’t know,’ ” Hughes said. “We looked into the papers and saw no research.”
For the two-year study, funded by the National Science Foundation and starting in 2012, the researchers caught about 200 crabs and tested their reactions to sounds at two university labs in Florida.
At the University of South Florida, several adult mud crabs were placed in tanks, and sensors were attached to the base of their antennae. The sensors were hooked up to computers that measured the crabs’ neurological reactions to sound pulses from a speaker near their bodies. The measurements jumped up and down on a graph, similar to the way lines rose and fell in experiments with mammals such as dolphins.
At the Florida State University Coastal and Marine Laboratory, adult mud crabs were placed in separate tanks with a food they love, juvenile clams. Some tanks had speakers, and some didn’t. The crabs in the tanks with no speakers were never reluctant to eat their fill. Crabs in the tanks with some predator acoustics were jittery and fed decidedly less often.
Mud crabs are the most abundant crabs in Gulf of Mexico reefs near the Florida panhandle, and they are prolific feeders — when they’re not under threat. And some of their fear, the study said, comes from sounds.
Humans hear by detecting changes in air pressure, but crabs detect another component, Hughes said: particle acceleration, through a body part called a statocyst at the base of the antennae. It’s a small sac with thousands of sensory hairs, important for the animal’s balance but also, the study found, for responding to sounds.
In the FSU tanks, Kimbro and Hughes piped in acoustics for five minutes and watched as the crabs then behaved timidly for two to four hours. “It had an effect. It slowed down their eating compared to crabs that didn’t hear the noises,” Hughes said.
“We were really excited; we were surprised. I don’t think anyone would think that they would respond to their predators in that way,” she said.