Researchers have discovered that American lobsters can hear low frequencies, and that the mechanism for their hearing rests in external hairs on the surface of their exoskeleton, not in their antennae, as had been assumed.

The results, published March 25 in the Journal of Experimental Biology, support the hypothesis that lobsters produce and detect sounds to communicate with one another. The findings also hint that hearing may be possible in more invertebrates than previously considered.

"It has been known for decades that marine mammals and fish produce sounds to find food or navigate underwater, but most importantly to communicate between individuals," said first author Youenn Jézéquel, a marine biologist at the Université de Bretagne Occidentale, in France. "In marked contrast, sound communication in marine crustaceans is poorly known, [though] we know that some of them, including lobsters, produce sounds."

Lobsters have been known since the 1950s to produce buzzing sounds by rapidly contracting internal muscles located at the base of the antennae, causing the carapace, the shell around the body, to vibrate.

While the mechanism of sound production in lobsters is understood, the precise function of sound in their behavior is unknown. One suggested hypothesis is that lobsters use their sounds to deter predators. However, the researchers were interested in finding out whether lobsters were communicating with each other on an individual level.

But the researchers first needed to learn whether the lobsters could hear at all. According to the study, there is no published data confirming that lobsters can hear. This is due in part to difficulties in underwater monitoring of the type of low-frequency sounds that lobsters make and are likely to hear.

Lobsters lack the air-filled space and compressible tissue used for hearing by vertebrates on land. However, the team, after observing fighting lobsters buzzing at each other, had previously hypothesized that lobsters could hear one another.

In this study, the researchers measured something called auditory evoked potentials, or the time it takes for the nervous system to detect a sound and respond to it. They measured this brain activity in 16 lobsters by using an electronic meter that Jézéquel described as "similar to the [brain] electrodes used in medicine."

The researchers discovered that while lobsters can hear, they do so at a different frequency than humans.

In the experiments, the lobsters' brains responded to sounds up to a frequency of 250 hertz, just below middle C on a piano. However, the animals' hearing was most sensitive around 100 hertz, too low for some people to even hear.

This low-frequency hearing may make lobsters more vulnerable to the threat of noise pollution, which is known to disrupt animal feeding and mating activities.

"This low-frequency sound detection also falls in the same frequency bandwidth produced by man-made noise," Jézéquel said. "Hence, this study raises clear concerns about the potential impacts of anthropogenic noise on lobsters."

The fact that lobsters can detect sounds at such a low frequency also makes it more likely that they use the sounds they produce to communicate with one another.

"Interestingly, this sound detection frequency encompasses the same frequency bandwidth of the buzzing sounds they produce, which reinforce our initial hypothesis for sound communication in lobsters," Jézéquel said.

While the team was able to establish that the lobsters were able to hear, they also wanted to determine what organs lobsters use to hear.

In other crustaceans, such as crabs, sound detection has been linked to the statocyst, a fluid-filled organ found at the base of each antenna that acts as an internal sensory receptor. Another possible detector of sound in lobsters are the hairfans, branched sensory hairs that are abundant on the surfaces of lobsters and other crustaceans, but whose function is unknown.

The team tested its subjects' hearing both before and after removal of the statocyst and found that the statocyst contributed little to the lobsters' hearing. This suggests that it is actually the lobster's hairfans that detect sounds.

While the results of this study are specific to lobsters, the researchers are optimistic about the avenues of research into other crustaceans it opens.

"The fact that hairfans are likely responsible for lobster sound detection [implies] that hearing is mechanistically possible in a wider array of marine invertebrates than previously considered," said Jézéquel. "Indeed, hairfans are found in large numbers in most marine crustaceans."

The study, "Sound detection by the American lobster (Homarus americanus)," published March 25 in the Journal of Experimental Biology, was authored by Youenn Jézéquel and Laurent Chauvaud, Université de Bretagne Occidentale; Ian T. Jones, Julien Bonne and T. Aran Mooney, Woods Hole Oceanographic Institution; and Jelle Atema, Boston University.