The noisy world of mud crabs: Predatory fish sounds can alter crab behavior

But sex is just one of the many things that get fish mouthing off: they also use their watery voices to relay dis­tress, find prey, defend their nests, and attract mates.

All this noise got Hughes and her col­leagues thinking. If fish are vocal crea­tures, can their prey hear them? And if so, how do they react? Fear is an impor­tant part of eco­log­ical com­mu­ni­ties, and Hughes is one of a number of researchers at Northeastern's Marine Sci­ence Center studying how the phe­nom­enon drives predator-​​prey interactions.

Their work -- as well as that of researchers around the globe -- has shown that the visual and chem­ical cues that fish dis­patch into their envi­ron­ment can cause prey, such as mud crabs and shrimp-​​like crus­taceans called amphipods, to go into hiding. But, until now, no one had ever studied the way prey species react to fishes' audi­tory cues.

In a new paper pub­lished in the journal Pro­ceed­ings of the Royal Society B, Hughes and her team show that sound plays at least as much of a role in mud crabs' reac­tion to fish behavior as other widely studied cues -- and pos­sibly more.

"We showed that these crabs change their behavior in response to acoustic sig­nals," she said. "They're just as strong as chem­ical cues."

In the first step of the exper­i­ment, the team -- which also includes North­eastern assis­tant pro­fessor of marine and envi­ron­mental sci­ence David Kimbro and David Mann, an expert in marine acoustics based at Log­ger­head Instru­ments in Sara­sota, Florida -- looked at whether mud crabs respond to fish sounds. They put the crabs into mesocosms -- experimental envi­ron­ments designed to mimic the nat­ural world -- containing food in the form of juve­nile clams. They then sub­merged a micro­phone into the tank and trans­mitted var­ious types of sound record­ings of oyster toad­fish, hard­head cat­fish, and black drum fish.

"We pretty quickly saw that the crabs weren't feeding as much in response to the predator sounds," Hughes said.

The cat­fish and black drum had the most pro­nounced effect on the crabs' behavior, likely because they move on and off the reef during feeding times whereas the toad­fish stick around all the time. "Prey usu­ally respond dif­fer­ently if the cue is con­stant versus vari­able," Hughes said. "It makes sense -- if a cue is con­stant, you're going to have to eat some­time, so you become desen­si­tized to it."

Once the researchers deter­mined that the prey do indeed change their behavior in response to predator sounds, they decided to con­firm that this was due to the crabs' ability to actu­ally hear them, rather than some other hidden vari­able. Other researchers have exam­ined ter­res­trial crabs' ability to hear, but no one has looked at the capacity among marine crabs, which are very dif­ferent animals.

To per­form this exper­i­ment, the team implanted elec­trodes into the "sta­to­cyst" at the base of the mud crabs' antennae. This is a tiny sac con­taining a min­eral mass and thou­sands of sen­sory hairs. It's typ­i­cally thought to be impor­tant for marine ani­mals' bal­ance, but, Hughes said, "If they're going to respond to sound pres­sure or par­ticle accel­er­a­tion, that's where it would happen."

And indeed it did happen. The elec­trode sig­nals showed a strong cor­re­la­tion with par­ticle accel­er­a­tion when the crabs were stim­u­lated with fast pulses of noise. They didn't hear the same way we do -- through the impo­si­tion of sound waves on our audi­tory machinery -- but rather through bil­lions of dis­placed par­ti­cles knocking against the tiny hairs inside their statocysts.

The study is the first to show that marine crabs are able to hear and opens up a wide range of ques­tions for the team to probe in the future. The researchers have already col­lected sound­scapes from reefs up and down the eastern seaboard and hope to use that data to examine ques­tions such as whether mud crabs on all reefs show the same behav­iors, or if they're only sen­si­tive to locally dom­i­nant predator sounds.