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First published online January 19, 2006
Journal of Experimental Biology 209, 433-443 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02017

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Ontogeny of mantle musculature and implications for jet locomotion in oval squid Sepioteuthis lessoniana

Joseph T. Thompson^* and William M. Kier

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

^* Author for correspondence at present address: Department of Biology, Saint Joseph's University, 5600 City Avenue, Philadelphia, PA 19131, USA (e-mail: joe.thompson{at}sju.edu)

Accepted 28 November 2005

We examined the relationship between mantle muscle structure and mantle kinematics in an ontogenetic series (5-85 mm dorsal mantle length) of oval squid, Sepioteuthis lessoniana. Thick filament length increased during growth in the mantle muscle fibres that power jet locomotion (i.e. the circular muscles). The thick filament length of both the superficial mitochondria-rich (SMR; analogous to vertebrate red muscle fibres) and central mitochondria-poor (CMP; analogous to vertebrate white muscle fibres) circular muscles increased significantly during ontogeny. Thick filaments in the SMR circular muscle fibres of newly hatched squid (N=5) ranged from 0.7 to 1.4 µm and averaged 1.0 µm, while the thick filaments of the SMR fibres of the largest squids (N=4) studied ranged from 1.2 to 3.4 µm and averaged 1.9 µm. The ontogeny of thick filament length in the CMP circular muscle fibres showed a similar trend. The range for hatchling CMP circular muscles was 0.7-1.4 µm, with an average of 1.0 µm, whereas the range and average for the largest squids studied were 0.9-2.2 µm and 1.5 µm, respectively. Within an individual hatchling, we noted no significant differences between the thick filament lengths of the SMR and CMP fibres. Within an individual juvenile, the thick filaments of the SMR fibres were 25% longer than the CMP fibres. The change in thick filament length may alter the contractile properties of the circular muscles and may also result in a decrease in the rate of mantle contraction during jetting. In escape-jet locomotion, the maximum rate of mantle contraction was highest in newly hatched squid and declined during ontogeny. The maximum rate of mantle contraction varied from 7-13 muscle lengths per second in newly hatched squid (N=14) and from 3-5 muscle lengths per second in the largest squids (N=35) studied.

Key words: cephalopod, obliquely striated muscle, thick filament, ontogeny, jet locomotion

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