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Department of Biology, CB#3280 Coker Hall, University of North Carolina, Chapel Hill, North Carolina 27599-3280
* To whom correspondence should be addressed. E-mail: joethomp{at}email.unc.edu
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Abstract |
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 TOP Abstract IntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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Abbreviations: DML, dorsal mantle length • IM-1, intramuscular fiber system 1 • IM-2, intramuscular fiber system 2 • IM-3, intramuscular fiber system 3
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Introduction |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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Cephalopods depend upon a hydrostatic skeleton for support during locomotion and movement. In the mantle of loliginid squid, skeletal support for locomotion is provided by a complex arrangement of fibers of muscle and of collagenous connective tissue (Ward and Wainwright, 1972; Bone et al., 1981). The connective tissue fibers are arranged in five highly organized networks: the inner and outer tunics, and three distinct systems of intramuscular fibers (Ward and Wainwright, 1972; Bone et al., 1981; for review, see Gosline and DeMont, 1985). These networks of collagen fibers help control changes in mantle shape during contraction of the muscles that power locomotion. In addition, the intramuscular collagen fibers store elastic energy during the exhalant phase of the jet and return the energy to help restore mantle shape and refill the mantle cavity (Ward and Wainwright, 1972; Bone et al., 1981; Gosline et al., 1983; Gosline and Shadwick, 1983a; Shadwick and Gosline, 1985; MacGillivray et al., 1999).
The organization of mantle connective tissue changes significantly during ontogeny in Sepioteuthis lessoniana, the oval squid. In hatchlings, the arrangement of outer tunic and intramuscular collagen fibers is hypothesized to permit large-amplitude movements of the mantle (Thompson, 2000; Thompson and Kier, 2001). In early ontogeny, the fiber angle of the collagen fiber networks changes exponentially, potentially limiting the amplitude of movement as the squid grow (Thompson, 2000; Thompson and Kier, 2001). Although these changes in connective tissue organization do not constitute a discrete metamorphosis, their influence on the mechanical properties of the mantle and the mechanics of jet locomotion may be considerable.
To explore the implications of changes in the organization of mantle connective tissue for the mechanics of jet locomotion, we studied the kinematics of the escape jet in an ontogenetic series of S. lessoniana. The escape jet is a distinct form of locomotion that typically involves a brief initial hyperinflation of the mantle (i.e., the mantle is expanded radially beyond its resting diameter; see Gosline et al., 1983) followed by a rapid contraction that expels water from the mantle cavity via the muscular funnel. In tethered S. lessoniana, we measured ontogenetic changes in the following kinematic parameters during the escape jet: the amplitude of mantle hyperinflation and mantle contraction, the rate of mantle contraction, and the frequency of escape jetting. In addition, we used measurements of mantle radius, mantle wall thickness, and mantle cavity volume to calculate the relative mass flux produced during the escape jet. Finally, we examined the relationship between mantle connective tissue morphology and mantle kinematics during the escape jet.
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Materials and Methods |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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We used S. lessoniana embryos that were collected from three locations (Gulf of Thailand; Okinawa Island, Japan; Tokyo Region, East Central Japan) over a 2-year period and reared (Lee et al., 1994) by the National Resource Center for Cephalopods (NRCC) at the University of Texas Medical Branch (Galveston, TX). Each of the three cohorts consisted of thousands of embryos from six to eight different egg mops. Thus, it is likely that the sample populations were not the offspring of a few closely related individuals, but were representative of the natural population at each collection site.
Commencing at hatching, and at weekly intervals thereafter, live squid were sent via overnight express shipping from the NRCC to the University of North Carolina. Animals from each of the following eight age classes were used in the experiments: newly hatched and 1, 2, 3, 4, 5, 6, and 9 weeks after hatching. These age classes correspond to the early life history stages defined by Segawa (1987), in which the squid achieve external adult morphology at a dorsal mantle length (DML) of about 40 mm (age 
6 week) and begin to mature sexually at 150 mm DML (age >9 weeks).
Prior to the start of the experiments, the animals were allowed about 30 min to equilibrate in an 80-l circular holding tank. The temperature (23 °C) and salinity (35 ppt) of the water in the holding tanks matched the temperature and salinity of the water in which the squid were raised. Circular water flow in the tank helped keep the squid swimming parallel to the sides of the tank to prevent injury. There were never more than seven squid in the holding tank at one time, and the maximum time an individual spent in the tank was 4 h.
Tethering
Initially, we attempted to measure mantle kinematics in free-swimming squid. The small size of the hatchling squid, combined with their inability to maintain position in flow, made it difficult to videotape at high magnification and thus obtain adequate spatial resolution for the kinematic measurements. To allow videotaping at high magnification and to increase the spatial resolution of the edges of the mantle, and thus the accuracy of the kinematic measurements, the squid were tethered.
Individual squid were removed from the holding tank with a glass beaker and anesthetized lightly in a 1:1 solution of 7.5% MgCl2: artificial seawater (Messenger et al., 1985). Anesthesia durations varied with the size of the animal (longer times for larger animals) but were never longer than 2 min. While anesthetized, the squid were tethered (Fig. 1). A needle (0.3-mm-diameter insect pin for smaller animals or 0.7-mm-diameter hypodermic needle for larger animals) was inserted through the brachial web of the squid, anterior to the brain cartilage and posterior to the buccal mass. The needle was positioned between these two rigid structures to prevent it from tearing the soft tissue of the squid. The needle was inserted into a hollow stainless steel post (hypodermic tubing) attached to a sheet of acrylic plastic. The needle fit tightly in the hollow post to prevent movement. Flat, polyethylene washers on the post and needle were positioned above and below the head to prevent vertical movement.
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Tethered squid were transferred to the video arena (0.4 m long by 0.2 m wide by 0.15 m deep) filled with aerated 23 °C artificial seawater and were allowed to recover. Tethered squid remained alive and in apparent good health for up to several hours, though most squid were tethered for fewer than 15 min.
Critique of tethering
Although tethering is an invasive technique, there were several indications that it was not unduly traumatic to the squid. First, tethered squid behaved similarly to the animals in the holding tank. Both the tethered and free-swimming squid spent most of the time hovering using the fins and low-amplitude jets. Second, unlike squid that are in distress or startled, more than 90% of the tethered animals did not eject ink. Third, the chromatophore patterns of tethered squid did not differ qualitatively from the patterns exhibited by the free-swimming squid in the holding tank. Finally, squid that were untethered and returned to the holding tank swam normally and could survive for several hours. It is not known how long these animals could have survived, because all the animals were killed for histological analysis after the day’s experiments were completed.
Tethering did, however, affect two aspects of swimming behavior. Tethered squid (1) performed escape jets with higher frequency and (2) performed more consecutive escape jets than the free-swimming squid in the holding tank. It is possible that the tethering apparatus may have affected mantle kinematics by restricting the flow of water out of the funnel. This is unlikely because the post was between 30% and 50% of the minimum funnel aperture in hatchlings and less than 20% of the minimum funnel diameter in the largest animals studied. In addition, the tethering apparatus did not contact the funnel during the experiments.
Mantle kinematics
Escape-jet behavior was recorded from above with a Panasonic AG-450 S-VHS professional video camera. The camera was adjusted so that the squid filled as much of the field of view as possible. To maximize the measurement resolution, the animal was oriented with the long axis of the mantle vertical in the video field (i.e., perpendicular to the video scan lines). Though the animals were free to rotate around the tether during the experiments, most remained near the original orientation. The frame rate of the camera (60 video fields per second) was more than 10 times faster than the observed frequency of the mantle jetting cycle. To reduce image blur, the high-speed shutter of the camera was set at 1/1000 s. Illumination was adjusted by means of a variac to the minimum level necessary to provide good contrast between the squid and the background.
Videotapes were analyzed using a Panasonic AG-1980P professional S-VHS videocassette recorder to identify escape-jet sequences suitable for digitizing. Only those sequences in which the mantle remained in the same orientation (i.e., the mantle remained nearly horizontal and did not twist relative to the head) were digitized. Individual video fields were digitized using an Imagenation (Beaverton, OR) PXC200 frame-grabber card in a microcomputer.
Mantle diameter changes during vigorous escape jets were measured from digitized video fields using morphometrics software (SigmaScan Pro 4.0, SPSS Science, Chicago, IL). Diameter at 
of the dorsal mantle length (DML) was measured in each video field prior to the start of and throughout the duration of an escape jet. The mantle diameter at DML (from dorsal mantle edge, Fig. 1) was selected because the greatest amplitude mantle movements occurred at that location in all squid examined. We normalized the data by dividing the mantle diameter measured in each video field by the resting diameter (=diameter of the anesthetized squid at DML) of the squid. Normalization by the resting mantle diameter standardized the analysis of mantle hyperinflation and mantle contraction data among the squid and allowed for comparisons between animals of different size. More than five escape-jet sequences were analyzed from each animal. Only the sequences that yielded the greatest mantle hyperinflation and the greatest mantle contraction were reported.
For many of the escape-jet sequences, the mantle diameter data were plotted against time. Time was estimated from the video camera frame rate (approximately 0.017 s per video field). To correct for differences in animal size, the diameter change between consecutive video fields was divided by the resting mantle diameter. The rate of mantle contraction was determined by dividing the mantle diameter change between successive video fields by 0.017 s. This calculation yielded a set of incremental rates of mantle contraction. The highest incremental rate was reported as the maximum rate of mantle contraction for that animal.
The frequency of escape jets was calculated by dividing the number of complete escape-jet cycles (the exhalant plus the inhalant phases) by the time required to perform the behavior. Time was estimated from the frame rate of the video camera as above. Measurements were made only from video sequences of squid that performed two or more escape jets in rapid succession. Multiple measurements were made for each squid, but only the highest calculated escape-jet frequency was reported.
Morphometrics
The dorsal mantle length of anesthetized squid was measured to the nearest 0.1 mm using calipers. We chose dorsal mantle length as a measure of squid size because it is simple to measure accurately and it correlates strongly with squid wet weight (Segawa, 1987).
The volume of the mantle cavity was measured for most animals after videotaping. Each squid was anesthetized (Messenger et al., 1985) at 20 °C for 15 min to relax the mantle musculature. The animal was then lifted from the anesthetic by the arms so that the mantle cavity remained filled with water. The exterior of the squid was gently blotted dry and the animal weighed on an electronic balance to the nearest 0.0001 g. The squid was returned to the water and then lifted by the tip of the mantle so that water emptied from the mantle cavity. The mantle was squeezed gently, in the posterior to anterior direction, to aid draining of the mantle cavity. The outside of the squid was blotted dry and the animal weighed again. We calculated the volume of the mantle cavity by dividing the difference in weight between the two measurements by the density of seawater at 20 °C (1.024 x 103 kg m-3). This procedure was repeated three to five times for each squid, and the average mantle cavity volume was recorded. We normalized the volume measurements by dividing the mantle cavity volume by the wet weight of the squid.
Mantle radius was measured in all the squid. Resting mantle diameter was measured from digitized video frames of the dorsal mantle of anesthetized animals. The mantle was assumed to be cylindrical, and the diameter was measured at DML. Mantle radius was then calculated from the diameter data.
The thickness of the mantle wall was measured in 21 specimens. Anesthetized animals were decapitated and a transverse slice of the mantle was made at one-third DML. A digital image of the slice was captured using a dissecting microscope, and the thickness of the mantle wall at the ventral midline was measured using morphometrics software.
The mantle wall thickness and radius were used to calculate mantle circumferential strain during the escape jet. The circumferential strain experienced during jet locomotion at the midpoint in the thickness of the mantle wall was calculated using the following equation from MacGillivray et al. (1999):
 | (1) |
c is the circumferential strain, Ri is the initial ("resting") outer radius of the mantle, Rf is the final outer radius of the contracted mantle, ti is the initial thickness of the mantle wall, and tf is the thickness of the contracted mantle wall. The resting outer radius and mantle wall thickness, Ri and ti, were measured from the digital images. The final outer radius of the contracted mantle, Rf, was measured from the videotapes, and tf was then calculated using the following equation from MacGillivray et al. (1999):
 | (2) |
Statistics
All correlations were made using the Spearman rank order correlation (Sokal and Rohlf, 1981). This nonparametric statistical test was used because the data for dorsal mantle length were not distributed normally (Kolmogorov-Smirnov goodness of fit test, P < 0.01; Zar, 1996) due to a sample bias toward smaller squid.
The mantle kinematics data were subdivided into the life-history stages identified by Segawa (1987). This scheme separates S. lessoniana into seven size classes based on morphological and ecological characteristics. The squid used in the experiments include four of Segawa’s (1987) life-history stages: hatchling (5 mm to 10 mm DML), juvenile 1 (11 mm to 25 mm DML), juvenile 2 (26 mm to 40 mm DML), and young 2 (60 mm to 100 mm DML). After subdivision into the appropriate life-history stage, the data in each stage were compared with a one-way ANOVA. Pairwise comparisons were made using the Student-Newman-Keuls method of comparison (Zar, 1996). This analysis was appropriate because the data in each stage were distributed normally (Kolmogorov-Smirnov goodness of fit test, P > 0.4 for each stage; Zar, 1996).
The mantle wall thickness and mantle radius data were log transformed and regressed against dorsal mantle length using a least-squares technique (Zar, 1996). Student’s t distribution was used to test the slopes against the null hypothesis slope of 1.0 (Zar, 1996).
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Results |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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of the mantle.
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The maximum rate of mantle contraction during the escape jet was highest in newly hatched squid and declined during ontogeny (Fig. 6; Spearman rank order correlation coefficient, -0.76, P << 0.001, n = 49). The maximum rate of mantle contraction varied from 7 to 13 mantle circumference lengths per second in newly hatched squid and from 3 to 5 lengths per second in the largest squid (Fig. 6). A one-way ANOVA among the life-history stages (Segawa, 1987) indicated that hatchling stage S. lessoniana had a significantly greater maximum rate of mantle contraction during the escape jet than all other life history stages (Student-Newman-Keuls test, P < 0.05; Table 1).
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Mantle radius also increased during ontogeny (Fig. 7C). The slope of the regression, 0.85, was significantly less than 1 (Student’s t test, P < 0.01).
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Discussion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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Fatigue
The relative proportions of circumferential muscle fiber types in the mantle of S. lessoniana change during ontogeny (Thompson, 2000; Thompson and Kier, 2001). Newly hatched individuals of S. lessoniana have a larger proportion of mitochondria-rich circumferential muscle fibers (analogous to vertebrate red muscle fibers, see Bone et al., 1981, and Mommsen et al., 1981) than older, larger squid. Preuss et al. (1997) reported a similar change during growth in the relative proportion of circumferential muscle fiber types in the mantle of another loliginid squid, Loligo opalescens. Preuss et al. (1997) suggested that the greater proportion of mitochondria-rich circumferential mantle muscle fibers made the hatchling squid more resistant to fatigue than older, larger animals. The data from the present study support their hypothesis. Small, young specimens of S. lessoniana were able to perform more consecutive escape jets and seemed to tire much less readily than their larger, older counterparts. However, motivational differences between small and large squid may also affect jetting behavior.
Newly hatched squid seem to rely more heavily on frequent jet locomotion than do larger squid (Fields, 1965; Hoar et al., 1994; Preuss et al., 1997). Two reasons have been suggested for this tendency. First, the fins of newly hatched squid are rudimentary relative to the adult fins (Boletzky, 1974; Okutani, 1987; Hoar et al., 1994), and it has been proposed that these diminutive fins may not generate sufficient thrust for locomotion or hovering (Boletzky, 1987; Hoar et al., 1994). Second, most newly hatched squid live in a fluid regime that is characterized by an intermediate Reynolds number (estimated from data in Packard, 1969, and O’Dor et al., 1986; see Jordan, 1992, and Daniel et al., 1992, for further discussion of intermediate Re) and in which the near parity of viscous and inertial forces inhibits coasting after a jet. Unlike large squid that can perform a single jet and then coast for a considerable distance, small squid must jet continuously to locomote. Hence, there may be an advantage in having a large proportion of the locomotor musculature specialized for fatigue resistance, particularly if jetting is the primary mode of locomotion. The price for such specialization, however, may be a reduction in the peak force produced by the mantle musculature during contraction.
Mantle kinematics
The mantle cavity of a hatchling of S. lessoniana holds a proportionately greater volume of water than the mantle cavity of a larger squid (Fig. 7A). In addition, a larger proportion of this volume is ejected from a hatchling during an escape jet (Fig. 5A). Finally, the maximum rate of mantle contraction during an escape jet is highest in a newly hatched squid (Fig. 6). Taken together, these data imply that mass flux (i.e., the product of the density of water in the mantle cavity and the volume rate of water flow out of the mantle cavity) during the escape jet is proportionately greater in hatchling than in larger, older individuals of S. lessoniana.
We used the mantle kinematics and morphometric data to calculate the relative mass flux during the escape jet in two life stages of S. lessoniana: a 5.5-mm-DML hatchling stage and a 65-mm-DML young 2 stage. We modeled the mantle as a cylinder with the "resting" wall thickness and radius calculated from the regressions of the mantle wall thickness (Fig. 7B) and mantle radius (Fig. 7C) data. To simplify the calculations, we based them on a transverse slice of the cylinder at DML. We assumed that both the length of the cylinder and the volume of the cylinder wall were constant; thus, the cylinder-wall area of the slice was held constant during the calculations. The initial mass-specific mantle cavity volume for each squid was obtained from the data in Figure 7A. We used the data for average mantle contraction and the maximum rate of mantle contraction from Table 1 to calculate the amplitude and rate of changes in mantle radius. We used equation (2) to calculate the increase in mantle wall thickness during the simulated jet. Finally, we calculated the relative mass (i.e., mass of water divided by mass of squid) of water remaining in the mantle cavity during the simulated jet at 25-ms intervals.
The calculations predict greater relative mass flux during the escape jet in the hatchling stage squid than in the young 2 stage (Fig. 8A). The average mass flux over the duration of the exhalant phase of the escape jet in a hatchling stage squid is about 2 times greater than that of an animal in the young 2 stage (Fig. 8A).
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Does the predicted ontogenetic decline in relative mass flux imply that the mass-specific thrust produced the escape jet is highest early in ontogeny? Mass flux constitutes only a portion of the total jet thrust. Under steady-state conditions, the instantaneous thrust produced during a jet is proportional to the product of the instantaneous mass flux and the instantaneous velocity of the water exiting the funnel (averaged over the funnel aperture; Vogel, 1994). Because it is likely that unsteady effects are important in jet locomotion (Anderson and DeMont, 2000), an unsteady term must also be included in an equation used to calculate jet thrust (see Anderson and DeMont, 2000).
Under both steady-state and unsteady conditions, the velocity of water exiting the funnel depends, in large part, on the diameter of the funnel aperture. The funnel complex is largest in newly hatched squid and decreases in relative size during ontogeny (Boletzky, 1974; unpubl. obs. of S. lessoniana and Loligo pealei). Unfortunately, funnel aperture cannot be determined simply from the size of the funnel of an anesthetized squid because it is a muscular structure that changes shape during a single jet (Zuev, 1966; O’Dor, 1988; Anderson and DeMont, 2000). Furthermore, measuring funnel aperture accurately during escape-jet locomotion in small hatchling and juvenile squid is not currently feasible. Without data on the scaling of the funnel aperture and dynamic changes in the aperture during a jet cycle, it is not possible to make precise predictions about the mass-specific thrust produced during the escape jet.
Whether jet thrust is generated by means of steady or unsteady mechanisms, the greater relative mass flux predicted for hatchling-stage individuals of S. lessoniana could allow a given thrust to be achieved with a relatively low jet velocity. This may result in a hatchling stage squid having a higher propulsion efficiency than an older, larger squid. Anderson and DeMont (2000) calculated the hydrodynamic propulsion efficiency (
) of the exhalant phase of the jet stroke using the following equation of rocket motor propulsion efficiency:
 | (3) |
In the intermediate Reynolds number fluid regime of the newly hatched and juvenile squid, the generation of jet thrust may not be represented accurately by existing equations. Previous theoretical treatments consider jet propulsion at high Reynolds numbers. In the absence of a mathematical model of jet locomotion at these intermediate Reynolds numbers, measurements of the actual thrust produced are required. Therefore, direct measurements of the thrust produced during an escape jet are needed to understand how the ontogenetic changes in mantle kinematics affect thrust.
Skeletal support and mantle kinematics
In many vermiform animals, the arrangement of connective tissue fibers in the body wall helps to control the shape of the animal during locomotion and movement (e.g., Harris and Crofton, 1957; Clark and Cowey, 1958). Similarly, the ontogenetic changes in mantle kinematics during escape-jet locomotion may result from ontogenetic alterations in the organization and the mechanical properties of the skeletal support system of the squid mantle.
In the mantle, skeletal support for locomotion is provided by a complex arrangement of fibers of muscle and connective tissue (Ward and Wainwright, 1972; Bone et al., 1981). As described earlier, the connective tissue fibers are arranged in distinct networks: the inner tunic, the outer tunic, and three distinct systems of intramuscular (IM) fibers—IM-1, IM-2, and IM-3 (Ward and Wainwright, 1972; Bone et al., 1981; for review, see Gosline and DeMont, 1985). The fibers in all the IM systems are collagenous (Ward and Wainwright, 1972; Gosline and Shadwick, 1983a; MacGillivray et al., 1999), and the collagen fibers in IM-1 and IM-2 are hypothesized to antagonize the circumferential muscles that provide power for locomotion.
The organization of collagen fibers in the outer tunic and IM fiber networks of the mantle changes dramatically during the ontogeny of S. lessoniana (Thompson, 2000; Thompson and Kier, 2001). The IM-1 collagen fiber angle (i.e., the angle of the collagen fiber relative to the long axis of the mantle) is lowest in newly hatched squid and increases exponentially during growth in squid up to 15 mm DML. In squid larger than about 15 mm DML, IM-1 fiber angle does not change substantially. IM-2 collagen fiber angle (i.e., the angle of the collagen fiber relative to the outer curvature of the mantle) is lowest in hatchlings and rises exponentially until the squid reach 15 mm DML. In animals larger than 15 mm DML, IM-2 fiber angle increases only slightly with size. The correlation between these ontogenetic alterations in connective tissue organization and the mantle kinematics measured in this study is striking (Fig. 9). Maximum mantle contraction (Fig. 5A), maximum mantle hyperinflation (Fig. 5B), and maximum mantle contraction rate (Fig. 6) all change exponentially up to a dorsal mantle length of about 15 mm.
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Simple mathematical models (Thompson, 2000; Thompson and Kier, 2001) of the ontogenetic changes in IM-1 and IM-2 fiber angle predict significantly greater amplitude of mantle movements during escape-jet locomotion in newly hatched squid than in older, larger animals. The models, which consider only the fiber angle and probable mechanical properties of the IM collagen fibers (see Gosline and Shadwick, 1983b), predict that mantle circumference changes up to -45% are possible in hatchling stage squid, whereas changes of only -25% to -30% are possible in squid at the young 2 stage (Thompson, 2000; Thompson and Kier, 2001). The present study supports the predictions of the models. Maximum mantle contraction during the escape jet in hatchlings of S. lessoniana ranged from -41% to -49% and from -27% to -32% in animals at the young 2 stage. Additional work on the ontogeny of the mechanical properties of squid mantle collagen is necessary to understand better the relationship among connective tissue organization, mantle mechanical properties, and mantle function.
Muscle mechanics
The maximum rate of mantle contraction was significantly higher in newly hatched individuals of S. lessoniana than in the larger animals (Fig. 6; Table 1). The shortening velocity of muscle fibers depends on the load of the muscle, the length of the thick filaments and sarcomeres, and the rate of cross-bridge cycling (Schmidt-Nielsen, 1997). The loading of the muscle fibers during jet propulsion is difficult to measure. It should be possible, however, to measure the contractile properties and myofilament dimensions of the circumferential muscle from an ontogenetic series of squid to examine the possibility of a change in performance of the muscle during ontogeny.
Comparison of the shortening speed of the circumferential muscles calculated here for S. lessoniana with previous measurements in adults of Alloteuthis subulata and Sepia officinalis are complicated by differences in temperature. The unloaded shortening speed at 11 °C of the circumferential musculature in A. subulata and S. officinalis was measured to be 2.0 lengths per second and 1.5 lengths per second, respectively (Milligan et al., 1997). In S. lessoniana, the maximum rate of mantle contraction at 23 °C ranged from a high of 13 lengths per second in the hatchlings to 4 in the young 2 stage squid.
Although the Q10 for cephalopod muscles has not been measured, previous work on type 1 and type 2 iliofibularis muscle fibers from Xenopus laevis (Lännergren et al., 1982) revealed a Q10 of approximately 2 over this temperature range. Thus, although the difference in measured shortening velocity between the young 2 stage of S. lessoniana and the adult of A. subulata may be due to temperature, it is unlikely that the much higher velocities measured in the hatchlings are simply an effect of temperature. In addition, the circumferential muscles are contracting against a load during an escape jet, and thus the unloaded shortening velocity of circumferential muscle in S. lessoniana will be higher than the values reported here.
In conclusion, we have described significant ontogenetic changes in the mantle kinematics of the escape jet in tethered squid. These kinematic changes are correlated strongly with alterations in the organization of the connective tissue fibers in the mantle; furthermore, they may affect the mass flux of the escape jet. An analysis of the mechanics of escape-jet locomotion in an ontogenetic series of squid is needed to better comprehend the implications of growth-related changes in mantle kinematics. Such an analysis will help us to understand the functional consequences of ontogenetic changes in morphology and will provide insight into the evolution of the form and function of hydrostatic skeletons.
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Acknowledgments |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionLiterature Cited |
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