The musculature of squid arms and tentacles: Ultrastructural evidence for functional differences

The transverse muscle mass of the arm and the transverse and circular muscle masses of the tentacle of squid (Loligo pealei and Illex illecebrosus) were examined by transmission electron microscopy. Previous work had indicated that although similar in gross arrangement, the transverse muscle mass of the tentacle creates rapid elongation during prey capture while the transverse muscle mass of the arm is involved in creating bending movements. The difference in function between the transverse muscle masses of the arms and tentacles is reflected in differences in ultrastructure. The transverse muscle mass of the arm is made up of regular, obliquely striated muscle fibers 1-6 μm in diameter. The transverse and associated circular muscle masses of the tentacle are made up of cross-striated muscle fibers 1-3 μm in diameter. The cross-striated muscle fibers have A bands approximately 0.5 μm (I. illecebrosus) and 0.9 μm (L. pealei) in length and a resting sarcomere length of 0.9 μm (I. illecebrosus) and 1.6 μm (L. pealei), suggesting a relatively high shortening speed for this muscle type. The cross-striated cells are not divided up into myofibrils, and the sarcoplasmic reticulum is located beneath the sarcolemma. Vernier displacements of the sarcomeres were observed. The myofilaments of the obliquely striated muscle fibers of the arm surround a central core containing mitochondria and the cell nucleus. The sarcoplasmic reticulum of the obliquely striated cells is located beneath the sarcolemma, in the plane of the Z elements, and surrounding the mitochondrial core.