mud crabs noisy world
June 19, 2014

Researchers Find That Marine Crabs Are Capable Of Hearing

Alan McStravick for - Your Universe Online

The overly amorous exhortations of a couple in the hotel room next to your own might be enough to mar a vacation experience. A polite knock or a phone call to the front desk could be effective in dampening the passion enough to allow you to return to sleep. But what does one do when the lovemaking is occurring just off the coastline, underwater, by freakishly loud fish?

This was the case in 2005 when the black drum fish, engaging in a bit of unprecedentedly fierce mating, disturbed land lubbers enough that the Miami Police were the recipients of a surprising amount of phone calls regarding their midnight mating calls. Many might be surprised that fish can be responsible for producing audibly noticeable sound to humans but, according to Randall Hughes, assistant professor of marine and environmental sciences at Northeastern University, the sea presents a veritable symphony of sound just like what we experience on land.

Despite the above example which bordered perhaps on being NSFW, sex is but one cause behind the production of noise by fish. Fish will call an audible when they are in distress, find prey and as a defense of their nests as well as attracting potential mates. Contrary to popular belief, however, fish will not use their watery voices to secure directions to Sydney Harbor or inquire of sea turtles exactly what their age is.

If fish communicate with one another using audible cues, Hughes and her colleagues began to wonder if their noise was also perceptible to potential prey. As important as determining if prey can hear the pescatarian intonations was learning what their reactions might be as a result. Because fear is an important motivator in ecological communities, Hughes, whose research focuses on fear in a predator-prey interaction, was eager to get to the bottom of this new research opportunity. She and other researchers at Northeastern's Marine Science Center set out to do just that.

Previous research by Hughes and colleagues from the Marine Science Center – as well as that of researchers around the globe – has already determined that both visual and chemical cues put off by fish are effective at causing prey, such as mud crabs and shrimp-like-crustaceans called amphipods, to go into hiding. This new research is novel due to the fact that it represents a first in the study of the reactions of prey species to auditory cues presented by fish predators.

Published this week in the journal Proceedings of the Royal Society B, Hughes team presents evidence that explains how the reactions of mud crabs to fish behavior relies as much on sound cues (if not more) as the other more widely studied visual and chemical cues.

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

The team, consisting also of David Kimbro, assistant professor of marine and environmental science at Northeastern, and David Mann, an expert in marine acoustics based at Loggerhead Instruments in Sarasota, Florida, began their research by observing mud crabs in a controlled setting to take note of whether or not there was a response to fish sounds.

The controlled environment noted above was what is referred to as a mesocosm. This is an experimental environment designed to mimic the natural world. Stocked in the mesocosm were juvenile clams meant to be a source of food for the mud crabs. Once the mud crabs were comfortable in the environment, the team submerged a speaker which broadcast recordings of several different sounds like 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 noted.

The oyster toadfish, which typically stays on the reef with the mud crab most of the time, elicited less of a response than did the hardhead catfish and black drum fish that move on and off of the reef during feeding times. “Prey usually respond differently if the cue is constant versus variable,” Hughes explained. “It makes sense – if a cue is constant, you're going to have to eat sometime, so you become desensitized to it.”

The change in the behavior of the mud crab was the first step. Once that fact was observed the team set about to make certain some other unseen variable wasn't the reason for the response. Previous research has established that terrestrial crabs possess the ability to hear. The marine crab, however, has not faced the same scrutiny and the differences between terrestrial and marine crabs are significant enough that study was warranted.

The second phase of the study involved the implantation of electrodes into the statocyst at the base of the crab's antennae. The statocyst is a tiny sac that contains a mineral mass and thousands of sensory hairs. Widely regarded as important for the marine animals' balance, Hughes stated, “If they're going to respond to sound pressure or particle acceleration, that's where it would happen.”

As it turns out, Hughes hypothesis was correct. The signal received from the implanted electrode showed a strong correlation with particle acceleration when the crabs were stimulated with fast pulses of noise. Crabs, it should be noted, don't “hear” the same way we humans do, via the imposition of sound waves on our auditory machinery. Crabs react to billions of displaced particles knocking against the tiny hairs inside their statocysts.

Being the first study to ever show that marine crabs are, in fact, able to hear is just the beginning for this research team. Future studies will be conducted with a wide range of soundscapes from up and down the eastern seaboard which the team has already collected. As they explain, collecting and presenting location specific sounds will help the team to understand if mud crabs on all reefs react uniformly on all reefs or if their response is contingent on having heard locally dominant predator sounds.

Unfortunately, with regard to the frisky couple in the room next to yours, might I suggest investing in a pair of ear plugs?