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amnhhealiz1The Lobster's Violin

It's enough to give a predator pause.

Story by Adam Summers - Illustrations by Sally J. Bensusan

Some lobsters and many eight-year-old violinists have a knack for making unpleasant noises; amazingly, crustaceans and humans use much the same mechanism to produce these awful sounds. The lobsters in question are not Maine's clawed variety but members of a family known as the Palinuridae, or spiny lobsters. These clawless marine invertebrates, found worldwide, often appear on menus as rock lobster or New Zealand lobster tail. Instead of a showy pair of claws, two long antennae are their most striking feature. The base of each antenna (where it joins the head) is thick and spiny-the reason for the lobster's common name.

Many invertebrates, such as crickets and cicadas, make noise by "plucking" a series of spikes or ridges (usually on their legs or wings)-much like a person drawing a thumbnail across a comb or a pick across guitar strings. But Sheila Patek, of Duke University, has discovered that spiny lobsters produce sound in a very different way: by drawing a bow across a vibrating surface. In this case the "bow," called the plectrum, is a flattened protuberance (actually a series of soft ridges) emerging from the basal segment of each antenna. (Earlier researchers thought the lobster's plectrum functioned like a pick; hence the confusing mix of terms.) The analogue of the violin string is the file-an oblong lump, or pad, one on either side of the lobster's head. By waggling an antenna, the lobster draws the plectrum across the file; the result is a surprisingly loud, rasping buzz. (One striking difference between lobster and violinist, of course, is that no amount of practice will turn this buzz into music.)

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To generate its loud, raspy buzz, the spiny lobster waggles one or both of its antennae, causing a flattened projection (the plectrum) on each antenna's spiky base to skid across an oblong lump (the file) located on either side of the animal's head, near the eye. Microscopic shingles on the file create friction, which is essential to sound production.

This type of mechanism is known as stick-and-slip motion. Imagine a box of rocks sitting on a conveyor belt, but instead of being able to move freely along the belt, the box is secured to a wall by a spring. As the belt moves, the box rides with it, stretching the spring. At some point, the tension of the spring becomes greater than the frictional force (the amount of resistance to movement that occurs between two moving objects in physical contact) between box and belt, and the box skitters along the belt toward the wall. This backward movement shortens (and thus reduces the tension of) the spring, permitting the box once again to ride the belt. Each time the box skips back across the belt, it makes an audible rumble; when the box rides smoothly, there is silence. The key to stick-and-slip sound production is friction. If the conveyor belt was greased, the box would move forward until the spring was stretched taut. Then the box would ride in place, with the belt sliding smoothly and soundlessly underneath it. No friction, no sound.

Violinists enhance the friction between the horsehair bow and the nylon or gut strings of their instrument by rubbing rosin on the bow. For lobsters, the friction comes from microscopic shingles on the otherwise smooth files. Each time the lobster's plectrum skids on the file, it produces a pulse of sound. As it travels the length of the file, the plectrum generates between two and twenty-four of these pulses, creating the characteristic raspy squeak. The duration of the sounds depends on the length of the file, which varies considerably from genus to genus. In fact, seven of the nine genera of spiny lobsters can be identified by the shape of their files and plectra. (The other two do not have files and thus make no noise at all.)

Patek believes that lobsters make these raucous sounds to deter predators. Think how you would react if a hot dog let out a loud squeak when you picked it up. However, the sound may do more than just startle potential predators. Spiny lobsters can do considerable damage with their stout antennal bases, which may be several inches long. In captivity they wield these spiky clubs aggressively and even catch the occasional fish dinner by slamming their antennae together. In the wild, lobsters may use sound to warn a predator that it is about to get clunked. Or the noise may simply inform a shady character that the element of surprise has been lost-the lobster version of "I've got my eye on you."

In any case, there is a very important biological reason a lobster would prefer a violin to a guitar. Lobsters, like all animals with exoskeletons, periodically shed their armor as they grow. As anyone who has appreciated soft-shell crabs can attest, naked crustaceans are both tasty and easy to eat. If a spiny lobster had to produce sound the way guitarists so often do-by plucking a hard plectrum across a series of hard ridges-the animal would be obliged to fall silent just when it would benefit most from an antipredator noisemaker: during the vulnerable few days it takes for the carapace to harden following a molt. The great advantage of the stick-and-slip approach is that a soft structure rubbing against another soft structure works just as well right after a molt as it did beforehand.

Many animals produce sounds to communicate with their own species-to issue warnings or invitations or to affirm their presence. Spiny lobsters appear to have developed this communication system solely to talk to other species. Their predators can certainly hear sounds in the range produced by the plectrum and file, but as far as we know, the lobsters themselves are completely deaf to their own playing.

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