THOMPSON, J.T.; KIER, W.M.; St. Joseph’s University; Univ. of North Carolina: Ontogeny of circular muscle ultrastructure and implications for jet locomotion in oval squid (Sepioteuthis lessoniana)

We investigated growth-related changes in the morphology and movement of the mantle of the Oval Squid, Sepioteuthis lessoniana. The maximum rate of mantle contraction during escape-jet locomotion was highest in newly hatched squid and declined through ontogeny (P<0.001; Thompson & Kier, 2001, Biol. Bull.201:154). The maximum rate of mantle contraction varied from 7 to 13 mantle circumference lengths per second (l/s) in newly hatched squid and from 3 to 5 l/s in the largest squid studied. Although we do not have information on the loading of the circular muscle fibers that provide the power for mantle contraction, the difference suggests that the contractile properties of the circular muscles change with growth. Because the shortening velocity of a muscle varies inversely with the length of its thick myofilaments, the ontogenetic difference in mantle contraction rate may reflect a change in the ultrastructure of the circular muscles. Indeed, the lengths of the thick filaments of the circular muscles increased significantly during ontogeny (P<0.001). The lengths of the thick filaments of hatchling squid, measured from transmission electron micrographs, averaged 1.01 µm ± 0.05 µm SD (n=5 animals; 563 filaments); the thick filament lengths of juvenile squid averaged 1.40 µm ± 0.13 µm SD (n=4 animals; 1127 filaments). Within an individual hatchling, we noted no significant differences between the thick filament lengths of the superficial mitochondria rich circular muscle fibers (SMR; analogous to vertebrate red muscle fibers) and the central mitochondria poor fibers (CMP; analogous to vertebrate white muscle fibers). Within an individual juvenile there was a trend for the thick filaments of the SMR fibers to be longer than the CMP fibers.