Lobsters rely on their exoskeleton, a hard external structure, to defend themselves against attack by predators. But because exoskeletons are rigid, they become too small for growing lobsters and must be periodically shed in a process known as molting. After molting, there is a period of time before the new exoskeleton is fully formed. During that time, the spinly lobster has only a soft covering over its body and is particularly vulnerable to attack. This is when their raspy noises are most vital to their survival.
Most arthropods produce sound by grinding parts of their hard exoskeleton together, creating sound in the same way a spoon does when dragged over a washboard. But this "spoon-and-washboard" sound mechanism requires a hard exoskeleton; it won't work shortly after a lobster molts. Spiny lobsters have evolved a different mechanism of sound production, one that more closely resembles a bow sliding over a the strings of a violin. This sound production mechanism, also known as the "stick-and-slip" mechanism, enables the lobsters to emit sounds even before their exoskeleton has hardened.
About the experiments
Dr. Sheila Patek of Duke University closely investigated the method by which spiny lobsters produce sound. She attached small sensors to the muscles of the spiny lobster's antenna and then recorded the sound the lobster produced using an underwater microphone. Additionally, she examined the physiology of the lobster's sound-producing structures. She found that the lobster slides a soft, fleshy plectrum over a smooth file-like surface located just beneath its eyes. Due to friction, the plectrum repeatedly sticks, then slips, each time producing a burst of sound.