THOMPSON, J.T.*; SZCZEPANSKI, J.A.; BRODY, J.H.; CRESCENTI, L.M.; St. Joseph’s University, Philadelphia, PA; St. Joseph’s University, Philadelphia, PA; St. Joseph’s University, Philadelphia, PA; St. Joseph’s University, Philadelphia, PA: Ontogeny of isometric force production by the circular mantle muscles of loliginid squids and cuttlefishes.

Thick filament length in the mantle muscles that provide power for jet locomotion (i.e., the circular muscles) increases significantly during the growth of loliginid squids and some cuttlefishes. This trend holds for both the superficial mitochondria rich (SMR � analogous to vertebrate red muscle fibers) and central mitochondria poor (CMP � analogous to vertebrate white muscle fibers) circular muscles. The ontogenetic increase in thick filament length may affect the mechanical properties of the circular muscles. The contractile properties of striated muscles, such as the obliquely striated circular muscles of the mantle, depend on the lengths of the thick filaments and sarcomeres, the load on the muscle, and the rate of cross-bridge cycling (e.g., Josephson, 1975). Thick filament length is inversely proportional to shortening velocity and is directly proportional to peak isometric tension (e.g., Josephson, 1975). Assuming all else is equal, we predict that the circular muscles of newly hatched animals will produce lower peak isometric tension than the circular muscles of juveniles and adults. We performed a set of preliminary muscle mechanics experiments at 20°C on three species of loliginid squids (Lolliguncula brevis, Loligo pealei, and Sepioteuthis lessoniana) and two species of cuttlefishes (Sepia officinalis and Sepia pharonis). We found peak isometric tension (brief tetani, 50Hz, 100ms) of between 230 and 300 mN/mm² (N=12) in juveniles and subadults but only 170 to 200 mN/mm² (N=8) in hatchlings. The preliminary results are consistent with the prediction that peak isometric tension should increase during ontogeny in the species we studied.