The ontogeny of muscle structure and locomotory function in the long-finned squidDoryteuthis pealeii

Abstract

SUMMARYUnderstanding the extent to which changes in muscle form and function underlie ontogenetic changes in locomotory behaviors and performance is important in understanding the evolution of musculoskeletal systems and also the ecology of different life stages. We explored ontogenetic changes in the structure, myosin heavy chain (MHC) expression and contractile properties of the circular muscles that provide power for jet locomotion in the long-finned squid Doryteuthis pealeii. The circular muscle fibers of newly hatched paralarvae had different sizes, shapes, thick filament lengths, thin:thick filament ratio, myofilament organization and sarcoplasmic reticulum (SR) distribution than those of adults. Viewed in cross section, most circular muscle cells were roughly triangular or ovoid in shape with a core of mitochondria; however, numerous muscle cells with crescent or other unusual cross-sectional shapes and muscle cells with unequal distributions of mitochondria were present in the paralarvae. The frequency of these muscle cells relative to 'normal' circular muscle cells ranged from 1:6 to 1:10 among the 19 paralarvae we surveyed. The thick filaments of the two types of circular fibers, superficial mitochondria-rich (SMR) and central mitochondria-poor (CMP), differed slightly in length among paralarvae with thick filament lengths of 0.83±0.15 mm and 0.71±0.1 mm for the SMR and CMP fibers, respectively (P 0.05; ANOVA). During ontogeny the thick filament lengths of both the CMP and SMR fibers increased significantly to 1.78±0.27 mm and 3.12±0.56 mm, respectively, in adults (P<0.0001 for both comparisons; ANOVA with Tukey's highly significant difference post hoc tests). When sectioned parallel to their long axes, the SMR and CMP fibers of both paralarvae and adults exhibited the myofilament arrangements typical of obliquely striated muscle cells but the angle of obliquity of the dense bodies was 22.8±2.4 deg. and 4.6±0.87 deg. for paralarvae and adults, respectively. There were also differences in the distribution of the anastomosing network of SR. In paralarvae, the outer and central zones of SR were well developed but the intramyoplasmic zone was greatly reduced in some cells or was scattered non-uniformly across the myoplasm. Whereas in adults the intramyoplasmic SR region was composed primarily of flattened tubules, it was composed primarily of rounded vesicles or tubules when present in the paralarvae. The ontogenetic differences in circular muscle structure were correlated with significant differences in their contractile properties. In brief tetanus at 20°C, the mean unloaded shortening velocity of the paralarval circular muscle preparations was 9.1 L 0 s -1 (where L 0 was the preparation length that generated the peak isometric stress), nearly twice that measured in other studies for the CMP fibers of adults. The mean peak isometric stress was 119±15 mN mm -2 physiological cross section, nearly half that measured for the CMP fibers of adults. Reverse transcriptase-polymerase chain reaction analysis of pa...

Aperture effects in squid jet propulsion

“…In at least one species, the muscles of paralarvae have been shown to have a tolerance of half the 2.5×10 5 Pa value: 1.2×10 5 Pa in D. pealeii (Thompson et al, 2010a). However, this is still quite a bit larger than the maximal stress our model predicts for this size of squid.…”

“…Using simple allometric regressions (Stevenson, 1996) to calculate a model squid's jet frequency and mantle aperture from its mantle length, we found that small squid do not achieve the maximal output of mantle muscle, whereas large squid exceed the stress their muscle can withstand. However, Thompson et al showed that peak stress in the circular muscles of paralarvae is significantly lower than in adults, a point that is addressed in more detail in the discussion (Thompson et al, 2010a).…”

The length-force behavior and operating length range of squid muscle varies as a function of position in the mantle wall

“…In many striated muscles, long thin and thick filaments and higher thin:thick filament ratios are correlated with higher force output (Jahromi and Atwood, 1969;Josephson, 1975;Granzier et al, 1991). Previous ultrastructural investigations of the circular mantle muscles of squid (Ward and Wainwright, 1972;Moon and Hulbert, 1975;Bone et al, 1981;Thompson and Kier, 2006;Thompson et al, 2008;Thompson et al, 2010b) did not identify transmural differences in myofilament dimensions or their arrangement, but this has not, to our knowledge, been explored in a systematic way.…”

“…Squid modulate isometric force and shortening velocity by varying thick filament length, not by expressing different isoforms of myosin heavy chain (Kier and Schachat, 1992;Kier and Schachat, 2008;Thompson et al, 2010b;Shaffer and Kier, 2012). Although other myofilament lattice proteins have the potential to affect the length-force relationship as well (e.g.…”

“…We used the vibratome to cut 300 μm thick sheets of mantle tissue. The vibratome sliced through the radial muscle fibers, thereby inactivating them (see Milligan et al, 1997;Thompson et al, 2008;Thompson et al, 2010b). We selected a sheet that was 300 μm away from the appropriate surface of the mantle (i.e.…”