Background
Sound Production:
A variety of crustacean species are known to be soniferous, including: white shrimp, snapping shrimp, crabs, spiny lobsters, rock lobsters, and red-banded lobsters. In all of these species sound is produced by exoskeletal movement, such as; stridulation (rubbing) of two body parts, percussion of two body parts, or percussion of a body part and the substrate.
American lobsters produce simultaneous waterborne sounds and shell vibrations, using internal musculature. This mechanism, which is unique amongst was first hypothesized when Fish (Fish, J. 1966 Sound Production in the American Lobster. Crustaceana 11:105) he observed differences in the spectral qualities of American lobster sounds versus sounds from stridulatory sound producers. While sound production by stridulation (such as in spiny lobsters and many insects) typically consists of wide band sounds with no harmonic component, Fish observed low frequency (100-130 Hz in 6 animals), tonal, short duration sounds in the American lobster that are, in fact, most similar to sounds produced by rattlesnakes, some insects, such as cicadas and some teleost fish, such as toadfish.
The remotor muscle at the base of the large (second) antennae has previously been implicated in lobster sound production. The remotor muscle has been studied on a cellular anatomical level and vibrations have been recorded in association with muscle contractions. Besides being a putative sonic muscle, the remotor muscle serves to move the antennae.
Sound Detection:
While the morphology of putative acoustic receptors in crustaceans has received considerable attention, few studies have examined the physiological and behavioral responses of crustaceans to sound stimuli. In American lobsters our understanding of how they respond to sound is limited to one brief report published by Offut (Offut, G.C. 1970 Acoustic stimulous perception by the American Lobster. Experientia 26:1276-1278). Offut used cardiac assays to record changes in the heart rate of five juveniles in response to waterborne sound waves of 10-150 Hz. He found that they responded to sounds between 18.7 and 150 Hz by briefly stopping or dramatically slowing their heart rate (bradycardia). Bradycardia is a common response of lobsters to novel stimuli and it has also been observed in lobsters exposed to small changes in temperature and salinity.
Lobsters lack the air-filled cavities necessary for pressure detection, so they must detect acoustic signals with particle displacement receptors. Particle displacement receptors are most effective in sensing both vibrations, which directly move the sensory hairs, and sounds produced close to the receptor. Many benthic inhabitants, such as sea robins, haddock, oyster toadfish, and lobsters themselves produce low frequency sounds that could be detected by lobsters using particle displacement receptors.
Two types of putative acoustic receptors in the American lobster are hair-fan organs and hair-peg organs. Hair-fan and hair-peg organs are sensory hairs that have been implicated in low frequency waterborne sounds and water current detection in the European lobster, Homarus vulgaris. These receptors are widely dispersed on the anterior carapace, but are absent from the antennae and antennules. However, statocyst receptors have been identified on the antennules of American lobsters that respond to vibrations but not waterborne sounds (Cohen M.J. 1955 The function of receptors in the statocysts of the lobster Homarus americanus. J. Physiology 130:9-31). The location of acoustic receptors on a movable appendage is advantageous for locating and orienting toward sound or vibration sources (phonotaxis), which would be especially advantageous for lobsters during nocturnal foraging and social interactions.