Squid Fast and Slow Muscle: An Integrative Analysis of Fiber Evolution and Specialization
William M. Kier, Department of Biology, University of North Carolina, Chapel Hill
9 Nov 2007
Comparison of striated muscle from diverse animals reveals a remarkable range of performance. This modulation and specialization depends both on variation in the chemical composition (protein isoforms) of the fibers and on variation in the arrangement and dimensions of components of the fibers. Previous research on this issue has focused on vertebrate muscle, in which specialization occurs primarily through variation in chemical composition. Muscle specialization in invertebrates has received less attention, even though a greater range of striated muscle structure and performance is observed in these animals. Over the past few years, I have used an integrative approach to examine muscle specialization in squid, including analyses of evolution, muscle mechanics, behavior, ontogeny, histology, ultrastructure, biochemistry, and mRNA. I have focused on the cross-striated muscle mass of squid tentacles, which generates the remarkably rapid (15-25 msec) prey strike, and the obliquely striated muscle mass of the more slowly moving squid arms. This work has revealed that evolution of specialization in the tentacle fibers occurred not by changes in the chemical composition of the cells, but by dramatic changes in the ultrastructural arrangement and dimensions of the myofilaments of the fibers. Since the cross-striated tentacle cells are derived, through both development and evolution, from an obliquely striated muscle cell type, a comparison of these two fiber types provides an opportunity to document the physiological implications of the observed structural specializations.