Lobsters Play Biological Violins

"Lots of people have tried to explain how these lobsters make sounds, and most of them were wrong," said Sheila Patek, whose research is reported in the May 10 issue of Nature. "We've never seen this before."

Using an underwater microphone and tiny sensors attached to the lobster's antennal muscles, Patek showed that when a lobster moves its antennae in a certain way, a nubbin of tissue called a plectrum rubs over a file near its eyes, creating frictional pulses of sound. Unlike crickets and other animals that produce sound by scraping a hard "pick" over a ridged "file," a lobster's plectrum is made of soft tissue, and the file's surface is macroscopically smooth. So, although the sound they produce is hardly musical - it resembles a cross between a stick dragged across a washboard and a moist finger rubbed on a balloon - the underlying mechanism is similar to a violinist drawing a bow across the strings of her instrument.

Since lobsters cannot hear except at very close range, the sounds they make are probably not used to communicate with each other, Patek said. Instead, she said the sounds serve as a defense against predators, which may be startled long enough for the lobster to escape.

"If you were reaching down to pick up a sandwich, and it squeaked, you might pause," Patek explained.

Sound-based defense mechanisms are relatively common in nature, Patek said, but the lobster's is unusual from an evolutionary as well as a structural standpoint. Not all lobsters are noisy, only certain species in the Palinuridae, or spiny lobster, family. These lobsters bear little resemblance to the docile creatures found in supermarket tanks; aside from their mottled coloring, their most striking characteristic is a pair of long, stiff, spine-encrusted antennae, and several faded scratches on Patek's arms bear witness to the antennae's effectiveness as defensive tools.

During the molting period, however, the spiny lobsters' antennae and shell are too soft to protect them against predators. Instead, the lobsters must rely on scare tactics - sound - to drive predators like sharks, grouper, and triggerfish away. A sound-producing mechanism that relied upon hard surfaces would be of little use during this vulnerable stage. This suggests that the lobsters' soft-tissue-based sound structures are an evolutionary response to predation, Patek said.

"Organisms face many mechanical problems," Patek said in an interview. "In this case, lobsters are able to make sound without relying on hard parts, and therefore they can make sound when their exoskeleton is softened and they are most vulnerable to predation."

Patek said future research might turn up other examples of animals using the same violin-like "stick-and-slip" method to produce sound. She added that she hopes her research will spur others to investigate sound-producing mechanisms and their evolutionary history. Patek's own future includes a three-year postdoctoral Miller Fellowship at UC-Berkeley, where she intends to study the evolution of signals and communication in mantis shrimp.