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First published online September 16, 2005
Journal of Experimental Biology 208, 3655-3664 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01831

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Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp Odontodactylus scyllarus

S. N. Patek^* and R. L. Caldwell

Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA

View larger version (106K): [in a new window]   Fig. 1. Peacock mantis shrimp use a pair of large raptorial appendages (A, white arrow) to strike hard objects with such high speeds that cavitation bubbles form between the appendage and striking surface (Patek et al., 2004). (B–I) The dactyl heel (h) of the raptorial appendage strikes a snail (s) that is loosely wired to a stick. Images recorded at 0.2 msintervals. Scale bar, 1 cm. Cavitation (yellow arrow) is visible between the dactyl heel and snail (D–G).

View larger version (14K): [in a new window]   Fig. 2. Limb movement and force generation. The heel of the raptorial appendage (purple, right y axis) approaches the force sensor (located at distance 0, right y axis) and generates force (black, left y axis) during impact (peak 1) and when cavitation bubbles collapse (peak 2). Negative pressure as the limb rebounds from the sensor surface is indicated by the slight negative excursion of the force trace between the first and second force peaks. Distance data was digitized from high-speed video (100,000 frames s-1) and smoothed using the negative exponential function (polynomial regression and Gaussian density function; SigmaPlot v.9.0, Systat). The one-axis force sensor was sampled simultaneously at 100,000 samples s-1.

View larger version (31K): [in a new window]   Fig. 3. The formation, collapse and rebound of a cavitation bubble between a mantis shrimp's dactyl heel and a force sensor. The left trace (blue) indicates force output from a force sensor that was recorded synchronously with high-speed images at 100,000 samples s-1. The series of photographs on the right are recorded at 0.1 ms intervals (from the top down) and temporally aligned with the horizontal lines in the force trace. The two images on the left correspond with the two maximal force peaks. The formation of a cavitation bubble begins when the limb strikes the force sensor (1). The cavitation bubble collapses at the onset of the second peak (2), and then rebounds (3) until the last shown image. This sequence of cavitation bubble formation, collapse and rebound is typical of cavitation occurring near a boundary, in which peak force occurs during cavitation bubble collapse (Brennen, 1995; Tomita and Shima, 1990). Termed the rebound phase, a small cloud of bubbles is typically formed after the initial collapse of the primary cavitation bubble. These smaller bubbles will continue to collapse, but with smaller resultant forces than the collapse of the first large cavitation bubble (Brennen, 1995; Tomita and Shima, 1990). Videos of simultaneous force and video traces are available online as supplementary material.

View larger version (21K): [in a new window]   Fig. 4. Typical strike force (A) and sound (B) of a single limb striking a three-axis force sensor (only the z-axis data are shown here). Note that there is a slight offset in timing between the force and acoustic data; this offset is due to the approximately 62 µs necessary for the sound waves to reach the hydrophone, which was located several cm from the force sensor.

View larger version (49K): [in a new window]   Fig. 5. Comparison of relative power spectra between the first (blue) and second (green) acoustic peaks recorded as mantis shrimp struck a three-axis force sensor. The curved surface (A) and flat surface (B) yielded similar spectral distributions. Second peaks (green) typically contained more energy at higher frequencies in the ultrasonic range (above 20 kHz) than first peaks, which is consistent with cavitation being the source of the second peak (Brennen, 1995; Lush and Angell, 1984; Martin et al., 1981). The peak amplitudes of the acoustic recordings were scaled to 1.0 prior to conducting Fast Fourier Transforms, thereby allowing comparisons of relative power/frequency of first and second peaks within a given strike and across strikes.

View larger version (16K): [in a new window]   Fig. 6. Force generated when a mantis shrimp strikes with both raptorial appendages. Four force peaks are detected by a one-axis force sensor when struck by two raptorial appendages. Based on the high-speed video and acoustic analyses above, the two higher peak forces were generated by limb impact and the two lower peaks were generated during cavitation bubble collapse.