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First published online December 14, 2006
Journal of Experimental Biology 210, 118-128 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02600

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Electromyography of the buccal musculature of octopus (Octopus bimaculoides): a test of the function of the muscle articulation in support and movement

Theodore A. Uyeno^* and William M. Kier

Department of Biology, CB# 3280 Coker Hall, University of North Carolina at Chapel Hill, NC 27599-3280, USA

^* Author for correspondence (e-mail: uyeno{at}bio.unc.edu)

Accepted 17 October 2006

The buccal mass musculature of the octopus (Octopus bimaculoides) was studied with electromyography to test the predictions of a previous morphological study in which we suggested that the muscles of the buccal mass serve as both the effectors of movement and as the joint itself, forming a new category of flexible joint termed a `muscle articulation'. The predictions of muscle function were tested by correlating muscle electrical activity in isolated buccal masses with spontaneous beak movements. Bipolar electromyography electrodes were implanted in the various beak muscles and beak position was recorded simultaneously with an electronic movement monitor (N=14). The results are consistent with the hypothesis that the lateral mandibular muscles produce opening movements of the beaks and provide the first definitive explanation of the opening mechanism. The results are also consistent with the hypothesis that the superior mandibular muscle functions primarily in closing. Co-contraction of the lateral mandibular muscles and the superior mandibular muscles was also observed, suggesting that these muscles may also stabilize the beaks during movement or provide a means of controlling the location of the pivot between the beaks. This study provides an important first test of the predictions of the role of the complex musculature found in muscle articulations such as the cephalopod buccal mass.

Key words: biomechanics, Cephalopoda, electromyography, muscle articulation, Octopus bimaculoides