How do invertebrates produce sounds?
Sound production by marine invertebrates has not been investigated to the same extent as it has been for fishes and marine mammals. However, the European spiny lobster and snapping shrimp are two species of invertebrates whose unique methods and purposes of sound production have been well documented. Barnacles and other mollusks also produce sound; however, their sound production mechanisms have not been studied in great detail.
Most marine invertebrates known to produce sounds do so by stridulation or rubbing two parts of their bodies together. The snapping shrimp, Alpheus heterochaelis, a common sound producer in the ocean, is an exception to this. Snapping shrimp have two claws, or cheliped, one of which is greatly enlarged and can grow up to half the size of the entire body. It was once thought that sounds produced by snapping shrimp occurred as a result of the top and bottom parts of the cheliped striking each other when the claw was snapped shut. However, it was later discovered that the sound is actually caused by the popping of a bubble that is produced when the cheliped opens and closes rapidly. The enlarged claw is usually slightly opened but during muscle contraction, the claw closes at a very high speed. This causes the water to cavitate and form a bubble of vapor. The sound that is heard from the shrimp is produced upon collapse of this bubble. Light is also produced when the bubble collapses and has been referred to as 'shrimpoluminescence' by scientists.
Spiny lobsters are clawless lobsters that produce a rasping sound using their antennae. The method a spiny lobster uses to produce sound has been compared to that of a violin. In a violin, the bow "sticks and slips" over the strings due to friction, generating acoustic vibrations. Spiny lobsters also produce sound using "stick and slip" friction. When the lobster moves its antennae in certain ways, a piece of soft tissue called the plectrum rubs against a smooth, stiff file located near the eye resulting in the production of sound. Because the "stick and slip" method does not require hard parts, spiny lobsters can still make sound when their exoskeleton is softened during molting. Most marine invertebrates that use stridulation to produce sound rely on hard surfaces. However, the spiny lobsters are unique in that they produce stridulatory sounds using a soft-tissue plectrum. This reduces their vulnerability during molting because they are still able to produce sound to ward off predators.
The New England mussel produces sound with its byssal threads, which are used to attach themselves to hard substrates. At temperatures above 10 degrees Centigrade, mussels can produce snapping sounds by stretching and breaking the byssal threads, these sounds are not intentionally produced. Sound production attributed to the New England mussel provided the first understanding of background noise levels produced by marine organisms in areas other than the southern and tropical coasts.
Male fiddler crabs use their enlarged claw, or cheliped, to produce sound by striking various parts of their body or the substrate which they are on. A variety of sounds produced this way have been described as drumming, honking, rasping, hissing, and rapping. Species specific sounds have been identified in recordings based on different frequencies and time intervals. For example, the sand fiddler crab produces rapping sounds between 600 and 2400 Hz and the mudflat fiddler crab produce sounds between 300 and 600 Hz.
The long-spined sea urchin has been observed to produce crackling sounds through stridulation of its stiff spines during movement. Stridulatory sounds may also be produced by the Aristotle's lantern and the test. The Aristotle's lantern is found on the ventral side of the urchin and is used for breathing and eating. The test is the body cavity of the sea urchin. These two anatomical features rub against one another during feeding and breathing producing a crackling sound.