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Squeaking with a sliding joint: mechanics and motor control of sound production in palinurid lobsters

Sheila N. Patek

Duke University, Biology Department, Durham, NC 27708, USA

View larger version (31K): [in a new window]   Fig. 1. The sound-producing mechanism found in the Caribbean spiny lobster Panulirus argus. The plectrum is a medial extension of the base of each antenna that rubs over a file, located on each side of the antennular plate. The flap extends posteriorly from the edge of the plectrum and is not necessary for sound production (see Discussion). The plectrum ridges, part of the stridulatory membrane, are confined to the ventral surface of the plectrum. Image © Sally J. Bensusen/courtesy Natural History Magazine.

View larger version (16K): [in a new window]   Fig. 2. A comparison of stick-and-slip sound production in stringed instruments and spiny lobsters. (A) In stringed instruments, a bow rubs over a string to generate sound. The string is modeled as a mass attached to a fixed point with a spring (representing the elasticity of the string). As the bow rubs over the string, the bow sticks to the string because of static frictional forces. When the spring extension causes sliding friction to exceed static friction, the bow slips over the surface of the mass, and the spring returns to the resting position. (B) The proposed mechanism of sound production in spiny lobsters in which a plectrum rubs over a file to generate pulsed sounds. The plectrum is modeled as moveable unit consisting of a mass attached to two springs; the file is fixed in place. A series of sound pulses is produced when the static friction between the plectrum and file surfaces is periodically exceeded as the plectrum is pulled over the file. When the plectrum slips over the file, a sound pulse is produced. Adapted from Patek (2001a,b).

View larger version (72K): [in a new window]   Fig. 4. Muscles attaching to the base of each antenna in Panulirus argus. Anterior is towards the bottom of the page. (A) Dorsal view with half the dorsal carapace removed to show the promotor (P) and lateral levator (LL). (B) Sagittal view of the promotor medial lobe (Pm), promotor lateral lobe (Pl), lateral levator, depressor A (Da) and depressor D (Dd). The epistome (E) forms an internal skeleton with a prominent midline process (Ep) upon which depressor A attaches. (C) Dorsal view with dorsal carapace, promotor and lateral levator removed to show depressor A (Da), depressor C (Dc) and depressor D (Dd). Drawing based on specimen with 64 mm carapace length.

View larger version (16K): [in a new window]   Fig. 3. Models of the antennal joints found in sound-producing and non-sound-producing spiny lobsters. (A) In non-sound-producing palinurids, the joint between the antenna and cephalothorax is limited to one degree of freedom (left, adapted from Wainwright et al., 1976) and has an axis of rotation aligned between the two joint articulations (right). (B) In sound-producing palinurids, the same joint has multiple degrees of freedom (left), which is permitted by the loss of the medial joint articulation. With only one joint articulation, the medial process of the antenna (the plectrum) can generate translational motion over the file.

View larger version (17K): [in a new window]   Fig. 5. Sound and movement correlation. Each pulse of sound (A) is correlated with movement of the plectrum (B), illustrated by a stepped-like motion of the plectrum over the file. Sound is produced only during posteriorly directed movements of the plectrum (towards the top of the page).

View larger version (39K): [in a new window]   Fig. 6. Overall pattern of muscle activity during a rasp. Depressor muscles are active during anterior movement of the plectrum (0-200 ms on the graph). Promotor and levator muscles are active prior to (approximately 200 ms on the graph) and during (450-500 ms on the graph) sound production. Sound production is indicated by the stepped movement of the plectrum.

View larger version (26K): [in a new window]   Fig. 7. Single electrical units of activity occurred in some recordings of the antennal muscles. These are probably products of stretch receptor activation when the promotor muscle pulls upon the opposing depressor muscles during sound production.

View larger version (20K): [in a new window]   Fig. 8. Promotor muscle intensity during sound-producing and non-sound-producing movements. Mean values are indicated by horizontal bars. Lateral promotor lobe intensity (A), medial promotor lobe intensity (B) and the relative intensity of the two lobes (medial lobe intensity minus lateral lobe intensity) (C) are shown across individual lobsters (1-5) and three movement categories, no movement (F), posterior movement with no sound production (P) and sound production (R). See Materials and methods for the calculation of muscle intensity.

View larger version (65K): [in a new window]   Fig. 9. A comparison of plectrum and file morphology in Panulirus argus and Palinurus elephas. (A) Palinurus elephas (carapace length 145 mm); a dorsal view of the surface the lobster's right file (18 mm long); anterior is towards the bottom of the page, lateral is to the left of the page. Covering the file are microscopic shingles with anteriorly projecting edges (inset). Scale bar, 25 µm. On the lateral side of the file is a long groove. (B) Palinurus elephas; the ventral surface of the lobster's right plectrum (6 mm wide); lateral is to the right of the page. The plectrum consists of a hemisphere of soft tissue ridges, the stridulatory membrane, and the flap extends from the posterior edge of the stridulatory membrane. These ridges rub against the file; the long axis of the ridges is parallel to the long axis of the file and thus parallel to their motion over the file. A knob, adjacent to these ridges, fits into the groove on the file. The plectrum rubs posteriorly over the shingle edges (towards the top of the page). The scanning electron micrograph (inset) shows the posterior limit of the plectrum ridges on the stridulatory membrane. Scale bar, 50 µm. (C) The same view as in A for the file (14 mm long) and shingles of Panulirus argus (85 mm carapace length). Scale bar, 25 µm. (D) The same view as in B for the plectrum (4 mm wide) and plectrum ridges of Panulirus argus. Scale bar, 50 µm.

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